WEBVTT 00:00:00.000 --> 00:00:02.940 align:middle line:90% [AUDIO LOGO] 00:00:02.940 --> 00:00:11.760 align:middle line:90% 00:00:11.760 --> 00:00:13.010 align:middle line:90% ANA BELL: All right, everyone. 00:00:13.010 --> 00:00:14.690 align:middle line:90% Let's get started. 00:00:14.690 --> 00:00:18.280 align:middle line:84% So today's lecture will be on this thing called dictionaries, 00:00:18.280 --> 00:00:20.530 align:middle line:84% and it's not the dictionaries that our parents 00:00:20.530 --> 00:00:22.240 align:middle line:90% and grandparents used. 00:00:22.240 --> 00:00:26.110 align:middle line:84% Notice, I never actually used regular book dictionaries 00:00:26.110 --> 00:00:29.330 align:middle line:84% either, maybe once in my entire life. 00:00:29.330 --> 00:00:32.090 align:middle line:84% But it's actually on a Python dictionary. 00:00:32.090 --> 00:00:35.500 align:middle line:84% So this is going to be a new data type that we have not 00:00:35.500 --> 00:00:36.800 align:middle line:90% worked with before. 00:00:36.800 --> 00:00:39.040 align:middle line:84% But it'll be a compound data type, 00:00:39.040 --> 00:00:41.612 align:middle line:84% much like we've seen lists and tuples to be. 00:00:41.612 --> 00:00:43.570 align:middle line:84% It's just going to be very different than lists 00:00:43.570 --> 00:00:45.850 align:middle line:90% and tuples. 00:00:45.850 --> 00:00:50.730 align:middle line:84% So before I introduce a bunch of syntax 00:00:50.730 --> 00:00:53.670 align:middle line:84% and what a Python dictionary is, let's 00:00:53.670 --> 00:00:57.990 align:middle line:84% try to just motivate the need for such a data structure. 00:00:57.990 --> 00:01:00.800 align:middle line:84% So suppose we have the following problem. 00:01:00.800 --> 00:01:02.450 align:middle line:84% We've been dealing with this problem 00:01:02.450 --> 00:01:04.310 align:middle line:90% in many of our lectures. 00:01:04.310 --> 00:01:08.970 align:middle line:84% But we, once again, want to store student information. 00:01:08.970 --> 00:01:12.650 align:middle line:84% So let's say we want to store grade information 00:01:12.650 --> 00:01:15.320 align:middle line:90% for a bunch of students. 00:01:15.320 --> 00:01:20.450 align:middle line:84% With what we know so far, we can store information using lists. 00:01:20.450 --> 00:01:22.430 align:middle line:84% It's a very reasonable data structure 00:01:22.430 --> 00:01:25.070 align:middle line:84% to use because we might get new students in the class. 00:01:25.070 --> 00:01:26.240 align:middle line:90% Students might drop. 00:01:26.240 --> 00:01:27.870 align:middle line:84% Grades might change, things like that. 00:01:27.870 --> 00:01:31.440 align:middle line:84% So let's use this mutable data structure a list. 00:01:31.440 --> 00:01:36.210 align:middle line:84% Let's say we want to store names of students and their grades 00:01:36.210 --> 00:01:38.190 align:middle line:84% in the class, like their final letter grade. 00:01:38.190 --> 00:01:41.040 align:middle line:84% Additionally, we can store things like micro quiz grades 00:01:41.040 --> 00:01:42.420 align:middle line:90% and psets grades. 00:01:42.420 --> 00:01:43.980 align:middle line:84% But for now, let's just assume we're 00:01:43.980 --> 00:01:46.450 align:middle line:84% storing just the names and the final grades in the class. 00:01:46.450 --> 00:01:49.140 align:middle line:84% So if we do this using lists, one reasonable way 00:01:49.140 --> 00:01:51.622 align:middle line:84% to store this information is by saying, 00:01:51.622 --> 00:01:53.580 align:middle line:84% well, I'm going to have a list of all the names 00:01:53.580 --> 00:01:55.070 align:middle line:90% of the students in my class. 00:01:55.070 --> 00:01:56.820 align:middle line:84% I'm going to have a list of all the grades 00:01:56.820 --> 00:01:58.470 align:middle line:90% of these students in the class. 00:01:58.470 --> 00:02:01.470 align:middle line:84% And I'm basically going to go index by index 00:02:01.470 --> 00:02:06.210 align:middle line:84% and make the rule that says, at a particular index, 00:02:06.210 --> 00:02:09.220 align:middle line:84% I'm storing all the information related to this one person. 00:02:09.220 --> 00:02:12.120 align:middle line:84% So at index 0 here, I'm storing the name 00:02:12.120 --> 00:02:14.160 align:middle line:90% of the student and their grade. 00:02:14.160 --> 00:02:17.250 align:middle line:84% At index 1, I'm storing the name of that student 00:02:17.250 --> 00:02:18.570 align:middle line:90% John and their grade. 00:02:18.570 --> 00:02:21.323 align:middle line:84% At index 2, I'm storing the name of that student 00:02:21.323 --> 00:02:21.990 align:middle line:90% and their grade. 00:02:21.990 --> 00:02:24.450 align:middle line:84% And at index 3, and so on and so on. 00:02:24.450 --> 00:02:26.580 align:middle line:84% So now I basically have to remember 00:02:26.580 --> 00:02:30.300 align:middle line:84% that, for a particular index, I am storing all the information 00:02:30.300 --> 00:02:34.170 align:middle line:90% related to that student, right? 00:02:34.170 --> 00:02:38.850 align:middle line:84% OK, so seems like a reasonable way to do this. 00:02:38.850 --> 00:02:41.610 align:middle line:84% Now, let's say that I wanted to look up 00:02:41.610 --> 00:02:44.110 align:middle line:84% the grade for a particular student. 00:02:44.110 --> 00:02:48.150 align:middle line:84% So I write this function called get_grade. 00:02:48.150 --> 00:02:49.780 align:middle line:90% It takes in some parameters. 00:02:49.780 --> 00:02:52.560 align:middle line:84% So the first thing it will take in is the name of the student, 00:02:52.560 --> 00:02:54.270 align:middle line:90% so Ana, for example. 00:02:54.270 --> 00:02:58.077 align:middle line:84% And I would pass in the list of all the names in my class 00:02:58.077 --> 00:02:59.910 align:middle line:84% and the list of all the grades in the class. 00:02:59.910 --> 00:03:03.480 align:middle line:84% So these two lists that I've previously created, right? 00:03:03.480 --> 00:03:05.980 align:middle line:84% So these get passed in to this function. 00:03:05.980 --> 00:03:08.550 align:middle line:84% So you can imagine if we have a list of everybody at MIT, 00:03:08.550 --> 00:03:11.100 align:middle line:84% these lists are going to be pretty large that we're 00:03:11.100 --> 00:03:12.870 align:middle line:90% passing in as parameters. 00:03:12.870 --> 00:03:15.810 align:middle line:84% How do we actually grab the letter grade 00:03:15.810 --> 00:03:17.220 align:middle line:90% associated with a student? 00:03:17.220 --> 00:03:18.600 align:middle line:84% Well, we're going to use the fact 00:03:18.600 --> 00:03:22.890 align:middle line:84% that the letter grade for the student at index 00:03:22.890 --> 00:03:31.350 align:middle line:84% I in the grades list is going to be grabbing the letter 00:03:31.350 --> 00:03:34.710 align:middle line:84% grade for the student at that same index in the name list. 00:03:34.710 --> 00:03:38.100 align:middle line:84% So we have to figure out this particular student being 00:03:38.100 --> 00:03:42.540 align:middle line:84% passed in here, what index they're at in the names list. 00:03:42.540 --> 00:03:44.610 align:middle line:84% So that's what this line of code is doing. 00:03:44.610 --> 00:03:48.120 align:middle line:84% It's using this index function on the name 00:03:48.120 --> 00:03:51.630 align:middle line:84% list with a parameter, for example, Ana. 00:03:51.630 --> 00:03:57.040 align:middle line:84% So this will return for us the index where Ana is in my list. 00:03:57.040 --> 00:04:01.350 align:middle line:84% So from the previous example, it's going to say that-- 00:04:01.350 --> 00:04:04.110 align:middle line:84% it's going to return the number 0 because Ana is stored 00:04:04.110 --> 00:04:06.880 align:middle line:90% in the name list at index 0. 00:04:06.880 --> 00:04:11.080 align:middle line:84% So now that I have that index in hand stored in variable I, 00:04:11.080 --> 00:04:15.550 align:middle line:84% can just index into the grade list at that same index, OK? 00:04:15.550 --> 00:04:18.130 align:middle line:84% So I can get grade list at index 0 00:04:18.130 --> 00:04:19.750 align:middle line:84% will return for me the grade that I 00:04:19.750 --> 00:04:22.300 align:middle line:84% got for that particular class or whatever 00:04:22.300 --> 00:04:24.910 align:middle line:90% we're storing here, OK? 00:04:24.910 --> 00:04:28.210 align:middle line:84% And then we just return the tuple student comma great. 00:04:28.210 --> 00:04:30.760 align:middle line:84% So this becomes really messy, right? 00:04:30.760 --> 00:04:32.290 align:middle line:84% I already mentioned that if I have 00:04:32.290 --> 00:04:33.970 align:middle line:84% a list of a whole bunch of students 00:04:33.970 --> 00:04:37.840 align:middle line:84% for a really large class or the entire university, then 00:04:37.840 --> 00:04:40.450 align:middle line:84% it becomes really unwieldy to just keep 00:04:40.450 --> 00:04:41.950 align:middle line:90% passing in all these lists. 00:04:41.950 --> 00:04:44.890 align:middle line:84% If I have in addition all these micro quiz 00:04:44.890 --> 00:04:46.780 align:middle line:84% lists and all these problem set lists 00:04:46.780 --> 00:04:49.510 align:middle line:84% that also store additional information for the student, 00:04:49.510 --> 00:04:53.320 align:middle line:84% I then have to pass those in for their respective functions. 00:04:53.320 --> 00:04:56.410 align:middle line:84% And so it gets really messy, right, 00:04:56.410 --> 00:04:59.380 align:middle line:84% writing these functions that retrieve this information. 00:04:59.380 --> 00:05:01.870 align:middle line:84% And additionally, if we're mutating these lists, 00:05:01.870 --> 00:05:03.580 align:middle line:84% like if a new student comes in and we 00:05:03.580 --> 00:05:05.210 align:middle line:84% need to add all their information, 00:05:05.210 --> 00:05:07.780 align:middle line:84% I need to make sure to update every single one of these lists 00:05:07.780 --> 00:05:08.710 align:middle line:90% that I'm maintaining. 00:05:08.710 --> 00:05:11.500 align:middle line:84% If a student leaves or drops the class, 00:05:11.500 --> 00:05:14.710 align:middle line:84% I need to remember to remove that index from all 00:05:14.710 --> 00:05:15.980 align:middle line:90% of these different lists. 00:05:15.980 --> 00:05:17.920 align:middle line:84% So really, really messy situation 00:05:17.920 --> 00:05:21.610 align:middle line:84% that we could get into by using this method to store 00:05:21.610 --> 00:05:24.830 align:middle line:90% information about students. 00:05:24.830 --> 00:05:27.230 align:middle line:84% So let's try a different approach. 00:05:27.230 --> 00:05:30.160 align:middle line:84% Instead of using all of these different lists, 00:05:30.160 --> 00:05:32.290 align:middle line:84% let's say that we're going to store everything 00:05:32.290 --> 00:05:34.570 align:middle line:90% in a master list. 00:05:34.570 --> 00:05:36.310 align:middle line:90% So we're not storing many lists. 00:05:36.310 --> 00:05:39.220 align:middle line:84% We'll just store one list for the grades in the class. 00:05:39.220 --> 00:05:42.220 align:middle line:84% And the way that it will be stored 00:05:42.220 --> 00:05:46.780 align:middle line:84% according to in this slide is going to be this grades list. 00:05:46.780 --> 00:05:49.780 align:middle line:84% OK, so this is one list with three elements in it. 00:05:49.780 --> 00:05:52.510 align:middle line:84% And you can imagine if we have more students we would just 00:05:52.510 --> 00:05:55.220 align:middle line:84% put all these students in this master list. 00:05:55.220 --> 00:06:00.310 align:middle line:84% So what is each one of these student elements? 00:06:00.310 --> 00:06:03.100 align:middle line:84% Well, each student element is itself a list. 00:06:03.100 --> 00:06:05.360 align:middle line:84% So already, I've got my master list. 00:06:05.360 --> 00:06:08.480 align:middle line:84% And each element within this list is also a list. 00:06:08.480 --> 00:06:11.080 align:middle line:84% So this is a list for Eric, a list for Ana, 00:06:11.080 --> 00:06:12.160 align:middle line:90% and a list for John. 00:06:12.160 --> 00:06:14.410 align:middle line:90% These are variable names. 00:06:14.410 --> 00:06:17.530 align:middle line:84% What are these lists going to be comprised of? 00:06:17.530 --> 00:06:20.090 align:middle line:84% Well, they will be comprised of three things. 00:06:20.090 --> 00:06:22.430 align:middle line:90% So notice two commas here. 00:06:22.430 --> 00:06:24.340 align:middle line:84% So the first thing is their name. 00:06:24.340 --> 00:06:28.150 align:middle line:84% The second thing is another list containing their problem set 00:06:28.150 --> 00:06:28.870 align:middle line:90% grades. 00:06:28.870 --> 00:06:33.280 align:middle line:84% And I'm using this element of that list 00:06:33.280 --> 00:06:36.760 align:middle line:84% to denote what that set of numbers represents. 00:06:36.760 --> 00:06:41.268 align:middle line:84% And then another list as my third element 00:06:41.268 --> 00:06:43.060 align:middle line:84% being the scores for the micro quiz grades. 00:06:43.060 --> 00:06:44.770 align:middle line:84% And again, I'm denoting the first element 00:06:44.770 --> 00:06:50.800 align:middle line:84% of that list telling me what this list contains. 00:06:50.800 --> 00:06:54.520 align:middle line:84% OK, so I've got lists, master list 00:06:54.520 --> 00:06:56.950 align:middle line:84% with three sublists for my three students. 00:06:56.950 --> 00:06:59.290 align:middle line:84% And each one of those lists contains three elements, 00:06:59.290 --> 00:07:00.940 align:middle line:84% a string, a list, and another list. 00:07:00.940 --> 00:07:05.410 align:middle line:84% And those two lists are then also comprised of a string 00:07:05.410 --> 00:07:06.700 align:middle line:90% and a list themselves. 00:07:06.700 --> 00:07:08.470 align:middle line:90% So super complex. 00:07:08.470 --> 00:07:11.800 align:middle line:84% A data structure-- or a sort of composition or design 00:07:11.800 --> 00:07:13.750 align:middle line:90% choice that I've made here. 00:07:13.750 --> 00:07:17.290 align:middle line:84% But it solves the problem of maintaining 00:07:17.290 --> 00:07:21.350 align:middle line:84% all these different lists in separate variables. 00:07:21.350 --> 00:07:23.980 align:middle line:84% So now, let's say I wanted to write a function that 00:07:23.980 --> 00:07:28.330 align:middle line:84% gets the grades for a particular student for either problem 00:07:28.330 --> 00:07:31.170 align:middle line:90% set or micro quizzes grades. 00:07:31.170 --> 00:07:33.100 align:middle line:84% This is the function that does that. 00:07:33.100 --> 00:07:36.990 align:middle line:84% So again, it's not looking super nice. 00:07:36.990 --> 00:07:39.210 align:middle line:84% So what is this function going to take in? 00:07:39.210 --> 00:07:41.950 align:middle line:84% The who is going to be a string representing the name, 00:07:41.950 --> 00:07:43.410 align:middle line:90% so for example, Ana. 00:07:43.410 --> 00:07:46.710 align:middle line:84% The what will be also a string representing 00:07:46.710 --> 00:07:51.630 align:middle line:84% what information I'd like to grab, either ps or mq. 00:07:51.630 --> 00:07:55.810 align:middle line:84% And the data is going to be my master list of all the grades. 00:07:55.810 --> 00:07:59.250 align:middle line:84% So this grades equals this list of every body. 00:07:59.250 --> 00:08:00.930 align:middle line:84% So what is this code going to do? 00:08:00.930 --> 00:08:04.820 align:middle line:84% Well, it has a for loop down here 00:08:04.820 --> 00:08:07.190 align:middle line:90% and a nested for loop inside it. 00:08:07.190 --> 00:08:09.920 align:middle line:84% The outer for loop basically looks 00:08:09.920 --> 00:08:12.680 align:middle line:84% through each one of these elements here 00:08:12.680 --> 00:08:16.070 align:middle line:84% and looks at the element at index 0, 00:08:16.070 --> 00:08:18.620 align:middle line:84% so either Eric, Ana, or John, and grabs 00:08:18.620 --> 00:08:23.690 align:middle line:84% only the list where that piece, this string here 00:08:23.690 --> 00:08:25.430 align:middle line:90% matches the who. 00:08:25.430 --> 00:08:29.030 align:middle line:84% So if student at index 0 equals who right here, 00:08:29.030 --> 00:08:31.220 align:middle line:84% then we found the student I'm interested in grabbing 00:08:31.220 --> 00:08:32.870 align:middle line:90% the information for. 00:08:32.870 --> 00:08:33.530 align:middle line:90% Cool. 00:08:33.530 --> 00:08:35.840 align:middle line:84% So now I've gotten I've grabbed the right piece, 00:08:35.840 --> 00:08:37.700 align:middle line:90% the right list. 00:08:37.700 --> 00:08:42.440 align:middle line:84% And now I'm interested in their grades for a particular what, 00:08:42.440 --> 00:08:44.059 align:middle line:90% so either mq or ps. 00:08:44.059 --> 00:08:47.730 align:middle line:84% So I do the exact same thing again for that list here, 00:08:47.730 --> 00:08:48.230 align:middle line:90% right? 00:08:48.230 --> 00:08:52.310 align:middle line:84% So if I'm interested in Ana's ps grades, 00:08:52.310 --> 00:08:56.300 align:middle line:90% I grab these lists here. 00:08:56.300 --> 00:08:59.344 align:middle line:84% And then I'm going to check if the-- 00:08:59.344 --> 00:09:03.080 align:middle line:84% info at index 0, so either this ps or this mq 00:09:03.080 --> 00:09:06.000 align:middle line:84% matches the what, so either ps or mq 00:09:06.000 --> 00:09:07.830 align:middle line:84% to match what I'm interested in grabbing, 00:09:07.830 --> 00:09:09.512 align:middle line:84% the information, what information I'm 00:09:09.512 --> 00:09:10.470 align:middle line:90% interested in grabbing. 00:09:10.470 --> 00:09:14.280 align:middle line:84% And then I'm going to go inside this if statement 00:09:14.280 --> 00:09:17.220 align:middle line:84% if they match, and then I return the who and the info. 00:09:17.220 --> 00:09:19.200 align:middle line:90% So again, super complex. 00:09:19.200 --> 00:09:22.410 align:middle line:84% No need to understand this that well because we're not going 00:09:22.410 --> 00:09:24.130 align:middle line:90% to use this method for long. 00:09:24.130 --> 00:09:27.890 align:middle line:84% So this get_grades here, for example, 00:09:27.890 --> 00:09:31.450 align:middle line:84% if I grab Eric's micro quiz grades and I run the code, 00:09:31.450 --> 00:09:34.840 align:middle line:84% it will return for me this tuple that 00:09:34.840 --> 00:09:36.770 align:middle line:84% returns for me the name of the student, 00:09:36.770 --> 00:09:39.820 align:middle line:84% and then just this sublist of the thing that I 00:09:39.820 --> 00:09:42.100 align:middle line:84% was interested, in this case, micro quiz. 00:09:42.100 --> 00:09:43.940 align:middle line:84% And it grabs for me all the grades. 00:09:43.940 --> 00:09:46.720 align:middle line:84% And then I can then index into this returned 00:09:46.720 --> 00:09:50.390 align:middle line:84% tuple to grab either the first quiz or the second quiz grades. 00:09:50.390 --> 00:09:54.360 align:middle line:90% OK, and same for Ana. 00:09:54.360 --> 00:09:56.130 align:middle line:84% In this particular case, it grabs 00:09:56.130 --> 00:09:58.830 align:middle line:84% for me just the tuple with my name 00:09:58.830 --> 00:10:01.890 align:middle line:84% and then that sublist with the problem set grades. 00:10:01.890 --> 00:10:04.460 align:middle line:90% 00:10:04.460 --> 00:10:08.090 align:middle line:90% OK, so again, really messy. 00:10:08.090 --> 00:10:09.860 align:middle line:84% I've made my design choice for how 00:10:09.860 --> 00:10:13.430 align:middle line:84% to create all these lists with sublists and sublists 00:10:13.430 --> 00:10:15.300 align:middle line:90% within those. 00:10:15.300 --> 00:10:17.720 align:middle line:84% And so I'd have to document that probably 00:10:17.720 --> 00:10:19.190 align:middle line:90% if I was using this method. 00:10:19.190 --> 00:10:22.520 align:middle line:84% And then this function to grab this information, again, 00:10:22.520 --> 00:10:25.100 align:middle line:90% super complex. 00:10:25.100 --> 00:10:27.110 align:middle line:90% Hard to read. 00:10:27.110 --> 00:10:29.810 align:middle line:84% So it's not really a great way to store information 00:10:29.810 --> 00:10:32.460 align:middle line:84% either, but the idea behind this, 00:10:32.460 --> 00:10:35.450 align:middle line:84% which is to try to store some data 00:10:35.450 --> 00:10:40.640 align:middle line:84% associated with some sort of key, right, the ps or mq, 00:10:40.640 --> 00:10:43.010 align:middle line:84% or in this case, I'm storing a bunch of grades 00:10:43.010 --> 00:10:44.120 align:middle line:90% for Eric or Ana or John. 00:10:44.120 --> 00:10:47.120 align:middle line:90% That idea we can explore. 00:10:47.120 --> 00:10:50.490 align:middle line:84% And that's basically what dictionaries will do for us. 00:10:50.490 --> 00:10:54.020 align:middle line:84% It will allow us to create data structures that 00:10:54.020 --> 00:10:59.130 align:middle line:84% map some sort of custom index, a key, to some value. 00:10:59.130 --> 00:11:01.460 align:middle line:84% So much like a book dictionary does, 00:11:01.460 --> 00:11:04.140 align:middle line:84% it maps a word to its definition. 00:11:04.140 --> 00:11:06.750 align:middle line:84% We'll be able to create our own dictionaries that map 00:11:06.750 --> 00:11:09.940 align:middle line:90% some object to another object. 00:11:09.940 --> 00:11:13.140 align:middle line:84% So when we create a dictionary, we 00:11:13.140 --> 00:11:15.540 align:middle line:84% call every quote unquote "element" 00:11:15.540 --> 00:11:17.370 align:middle line:90% in the dictionary an entry. 00:11:17.370 --> 00:11:22.600 align:middle line:84% And that entry is that mapping of a key to a value. 00:11:22.600 --> 00:11:25.710 align:middle line:84% So just to draw a parallel with the list, 00:11:25.710 --> 00:11:28.440 align:middle line:84% we can think of a list as mapping something 00:11:28.440 --> 00:11:30.390 align:middle line:90% to another something. 00:11:30.390 --> 00:11:36.780 align:middle line:84% The thing that a list maps is this index, numbers 0, 1, 2, 00:11:36.780 --> 00:11:38.370 align:middle line:90% 3 in that order, right? 00:11:38.370 --> 00:11:41.460 align:middle line:84% So it has to have an element at index 0, 00:11:41.460 --> 00:11:45.450 align:middle line:84% and then that index increases by 1 from thereon, OK? 00:11:45.450 --> 00:11:48.150 align:middle line:84% And for each one of these indices, 00:11:48.150 --> 00:11:53.370 align:middle line:84% I mapping that index to some element in my list, right? 00:11:53.370 --> 00:11:54.870 align:middle line:84% That's basically what the list does. 00:11:54.870 --> 00:11:57.180 align:middle line:84% There's something associated with index 0, something 00:11:57.180 --> 00:11:59.650 align:middle line:84% associated with index 1, and so on. 00:11:59.650 --> 00:12:04.080 align:middle line:84% So it's like a very restrictive dictionary, right? 00:12:04.080 --> 00:12:06.330 align:middle line:84% An actual Python dictionary works 00:12:06.330 --> 00:12:10.740 align:middle line:84% in a similar way except that now I am not putting 00:12:10.740 --> 00:12:13.800 align:middle line:90% any restrictions on my indices. 00:12:13.800 --> 00:12:18.640 align:middle line:84% My indices here become these custom indices called a key. 00:12:18.640 --> 00:12:24.220 align:middle line:84% And so now I'm able to associate a value, equivalent element 00:12:24.220 --> 00:12:26.540 align:middle line:90% in my list, with that key. 00:12:26.540 --> 00:12:32.780 align:middle line:84% So I can have an element associated with any object. 00:12:32.780 --> 00:12:39.020 align:middle line:84% So I am using the term value here, and in a dictionary 00:12:39.020 --> 00:12:41.580 align:middle line:84% the key is associated with a value. 00:12:41.580 --> 00:12:43.610 align:middle line:84% And that's one entry in the dictionary. 00:12:43.610 --> 00:12:45.770 align:middle line:84% Now, this is going to be a little bit confusing 00:12:45.770 --> 00:12:47.930 align:middle line:84% because we've been using the term 00:12:47.930 --> 00:12:52.130 align:middle line:84% value to refer to just some object's value, 00:12:52.130 --> 00:12:53.980 align:middle line:90% right, like int-- 00:12:53.980 --> 00:12:57.900 align:middle line:84% variable a has value five or something like that. 00:12:57.900 --> 00:13:00.300 align:middle line:84% But now I'm going to try to make a conscious effort, now 00:13:00.300 --> 00:13:02.300 align:middle line:84% that we're introducing dictionary and dictionary 00:13:02.300 --> 00:13:06.200 align:middle line:84% values associated with a key, to whenever 00:13:06.200 --> 00:13:08.540 align:middle line:84% I'm talking about the dictionaries value 00:13:08.540 --> 00:13:11.840 align:middle line:84% to say dictionary value just so it's not confusing. 00:13:11.840 --> 00:13:13.740 align:middle line:90% But just keep that in mind. 00:13:13.740 --> 00:13:15.588 align:middle line:84% It can be a little bit confusing at first, 00:13:15.588 --> 00:13:17.630 align:middle line:84% now that we're using the same terminology for two 00:13:17.630 --> 00:13:19.910 align:middle line:90% different things. 00:13:19.910 --> 00:13:21.520 align:middle line:90% So we're going to go through-- 00:13:21.520 --> 00:13:23.260 align:middle line:84% in this lecture, we're going to introduce 00:13:23.260 --> 00:13:26.170 align:middle line:84% a bunch of syntax and operations with dictionaries. 00:13:26.170 --> 00:13:28.150 align:middle line:84% And there will be lots of you-try-it exercises 00:13:28.150 --> 00:13:30.525 align:middle line:84% just to give you a little bit of practice with the syntax 00:13:30.525 --> 00:13:33.680 align:middle line:84% because this is kind of a syntax-heavy lecture. 00:13:33.680 --> 00:13:36.040 align:middle line:84% So hopefully it helps a little bit. 00:13:36.040 --> 00:13:40.810 align:middle line:84% But let's first see how to store data in a Python dictionary. 00:13:40.810 --> 00:13:45.120 align:middle line:84% So as I mentioned, a Python dictionary stores entries. 00:13:45.120 --> 00:13:48.730 align:middle line:84% And that entry is a key value pair. 00:13:48.730 --> 00:13:52.810 align:middle line:84% So you're mapping one key to its value. 00:13:52.810 --> 00:13:54.678 align:middle line:84% The key can be any immutable object, 00:13:54.678 --> 00:13:56.970 align:middle line:84% and we're going to see what this means in a little bit. 00:13:56.970 --> 00:13:59.310 align:middle line:84% And the value associated with that key, 00:13:59.310 --> 00:14:01.650 align:middle line:84% or the Python value associated with that key, 00:14:01.650 --> 00:14:04.380 align:middle line:84% can be any object you'd like, even 00:14:04.380 --> 00:14:07.140 align:middle line:90% lists or other dictionaries. 00:14:07.140 --> 00:14:09.440 align:middle line:84% So the way we create a Python dictionary 00:14:09.440 --> 00:14:14.100 align:middle line:84% is by using these open and closed curly braces. 00:14:14.100 --> 00:14:16.550 align:middle line:84% So tuples were open and closed parentheses. 00:14:16.550 --> 00:14:18.890 align:middle line:84% Lists were open and closed square brackets. 00:14:18.890 --> 00:14:21.780 align:middle line:84% Dictionaries are open and closed curly braces. 00:14:21.780 --> 00:14:24.860 align:middle line:84% And this creates inside memory an empty dictionary, 00:14:24.860 --> 00:14:27.450 align:middle line:84% so a dictionary with zero entries. 00:14:27.450 --> 00:14:31.430 align:middle line:84% So the length of that dictionary is 0. 00:14:31.430 --> 00:14:33.390 align:middle line:84% To create a dictionary with one entry in it, 00:14:33.390 --> 00:14:35.770 align:middle line:90% again, we have curly braces. 00:14:35.770 --> 00:14:37.510 align:middle line:90% And we add one entry in it. 00:14:37.510 --> 00:14:40.020 align:middle line:84% So this something colon something 00:14:40.020 --> 00:14:43.050 align:middle line:84% else is an entry in my dictionary, one entry. 00:14:43.050 --> 00:14:46.530 align:middle line:84% And the thing before the colon is the key. 00:14:46.530 --> 00:14:48.810 align:middle line:84% And the thing after the colon is the value 00:14:48.810 --> 00:14:50.070 align:middle line:90% associated with that key. 00:14:50.070 --> 00:14:53.580 align:middle line:90% 00:14:53.580 --> 00:14:56.270 align:middle line:84% So you can think of it, if we're drawing a parallel to lists, 00:14:56.270 --> 00:15:00.740 align:middle line:84% this is now mapping at this custom index 4, 00:15:00.740 --> 00:15:02.145 align:middle line:90% we're putting element 16. 00:15:02.145 --> 00:15:05.350 align:middle line:90% 00:15:05.350 --> 00:15:08.290 align:middle line:84% OK, so we can also create dictionaries that 00:15:08.290 --> 00:15:09.950 align:middle line:90% aren't just full of integers. 00:15:09.950 --> 00:15:12.400 align:middle line:84% And you can mix and match data types as you'd like. 00:15:12.400 --> 00:15:16.390 align:middle line:84% But usually, in dictionaries we have the keys all 00:15:16.390 --> 00:15:20.660 align:middle line:84% be the same type and the values all be the same type. 00:15:20.660 --> 00:15:22.810 align:middle line:84% But you can certainly mix and match types, just 00:15:22.810 --> 00:15:26.440 align:middle line:84% like you could create lists and tuples full of an integer 00:15:26.440 --> 00:15:31.200 align:middle line:84% and a float and another list and mix and match in that way. 00:15:31.200 --> 00:15:33.170 align:middle line:84% So here I'm creating a dictionary, 00:15:33.170 --> 00:15:36.470 align:middle line:84% again, open and closed curly braces starts my dictionary. 00:15:36.470 --> 00:15:41.640 align:middle line:84% And it has four elements in it, sorry, four entries in it. 00:15:41.640 --> 00:15:44.780 align:middle line:84% And each entry is separated by a comma. 00:15:44.780 --> 00:15:47.190 align:middle line:90% I've got here my first entry. 00:15:47.190 --> 00:15:54.350 align:middle line:84% So it is mapping the key Ana to the dictionary value B. 00:15:54.350 --> 00:15:58.910 align:middle line:84% My second entry key mapped to value A. Third entry maps 00:15:58.910 --> 00:16:03.680 align:middle line:84% key John to value B. And last entry maps key Katy to value A. 00:16:03.680 --> 00:16:07.280 align:middle line:84% So this is a dictionary that essentially maps strings 00:16:07.280 --> 00:16:09.720 align:middle line:90% to other strings. 00:16:09.720 --> 00:16:13.910 align:middle line:84% So you can see here I've visualized the dictionary 00:16:13.910 --> 00:16:15.200 align:middle line:90% that we just created. 00:16:15.200 --> 00:16:18.020 align:middle line:84% We've got these custom indices, right? 00:16:18.020 --> 00:16:25.150 align:middle line:84% So we're basically mapping names to letter grades, OK? 00:16:25.150 --> 00:16:26.350 align:middle line:90% Everything OK so far? 00:16:26.350 --> 00:16:29.050 align:middle line:84% Does it make sense, I guess, conceptually? 00:16:29.050 --> 00:16:32.010 align:middle line:90% OK, awesome. 00:16:32.010 --> 00:16:35.190 align:middle line:84% OK, so the first thing we'd like to do 00:16:35.190 --> 00:16:37.950 align:middle line:84% is, once we have a dictionary full of a bunch of entries, 00:16:37.950 --> 00:16:40.380 align:middle line:90% how do we grab an entry? 00:16:40.380 --> 00:16:44.930 align:middle line:84% How do we look up a value associated with a key? 00:16:44.930 --> 00:16:49.900 align:middle line:84% So the way we do that is in a very similar way to the way 00:16:49.900 --> 00:16:54.100 align:middle line:84% we look up an element in a list, right? 00:16:54.100 --> 00:16:58.370 align:middle line:84% A key in a dictionary is just a custom index. 00:16:58.370 --> 00:17:00.560 align:middle line:84% So how did we look up an element in a list? 00:17:00.560 --> 00:17:03.130 align:middle line:84% So if I wanted the element at index 3, 00:17:03.130 --> 00:17:05.770 align:middle line:84% I would basically say L squared brackets 3. 00:17:05.770 --> 00:17:09.020 align:middle line:84% And that grabs for me the value at that index. 00:17:09.020 --> 00:17:12.230 align:middle line:84% Well, now, I've got my custom indices, right? 00:17:12.230 --> 00:17:15.380 align:middle line:84% My custom indices are these strings. 00:17:15.380 --> 00:17:17.700 align:middle line:84% The syntax will be exactly the same. 00:17:17.700 --> 00:17:19.980 align:middle line:84% I've got this custom index I'd like to look up. 00:17:19.980 --> 00:17:26.490 align:middle line:84% So I say, dictionary, name, square bracket, custom index. 00:17:26.490 --> 00:17:29.540 align:middle line:84% So if I say grade square bracket John, 00:17:29.540 --> 00:17:32.990 align:middle line:84% Python will go in to my dictionary named grades. 00:17:32.990 --> 00:17:35.770 align:middle line:90% It'll look up the key John. 00:17:35.770 --> 00:17:37.750 align:middle line:84% And it'll return for me the value 00:17:37.750 --> 00:17:41.940 align:middle line:90% associated with that key, B. 00:17:41.940 --> 00:17:46.230 align:middle line:84% So this entire expression here evaluates 00:17:46.230 --> 00:17:48.150 align:middle line:84% or gets replaced with the string B. 00:17:48.150 --> 00:17:51.570 align:middle line:84% Just like when we indexed into a list, L1 square brackets 3, 00:17:51.570 --> 00:17:54.660 align:middle line:84% we replaced that entire indexing operation 00:17:54.660 --> 00:17:57.550 align:middle line:84% with the value of the element at that location. 00:17:57.550 --> 00:17:59.680 align:middle line:90% So similar here. 00:17:59.680 --> 00:18:02.580 align:middle line:84% If I try to index into a dictionary 00:18:02.580 --> 00:18:05.130 align:middle line:84% and that key doesn't exist, so notice 00:18:05.130 --> 00:18:07.350 align:middle line:84% my dictionary has no string Grace, 00:18:07.350 --> 00:18:09.130 align:middle line:90% Python will give me a key error. 00:18:09.130 --> 00:18:11.160 align:middle line:84% So if you run code with dictionaries 00:18:11.160 --> 00:18:14.250 align:middle line:84% and you get a key error exception being raised 00:18:14.250 --> 00:18:17.040 align:middle line:84% in the console, you will know that you're trying to index 00:18:17.040 --> 00:18:19.005 align:middle line:90% into a key that doesn't exist. 00:18:19.005 --> 00:18:22.090 align:middle line:90% 00:18:22.090 --> 00:18:25.620 align:middle line:84% So the question might be, yes, we're 00:18:25.620 --> 00:18:29.670 align:middle line:84% able to look up a value given a key. 00:18:29.670 --> 00:18:31.860 align:middle line:84% Can we do the same thing but backwards? 00:18:31.860 --> 00:18:36.810 align:middle line:84% Given a key, sorry, given a value, like A, B, C, whatever, 00:18:36.810 --> 00:18:39.820 align:middle line:84% can we look up a key associated with that value? 00:18:39.820 --> 00:18:40.950 align:middle line:90% And the answer is no. 00:18:40.950 --> 00:18:43.470 align:middle line:84% We'd have to write some sort of loop 00:18:43.470 --> 00:18:46.050 align:middle line:84% or some sort of code that goes through every item 00:18:46.050 --> 00:18:50.070 align:middle line:84% in my dictionary to check each value 00:18:50.070 --> 00:18:53.040 align:middle line:84% and see whether the key associated with that value 00:18:53.040 --> 00:18:54.850 align:middle line:84% is equivalent to the one I'm looking for. 00:18:54.850 --> 00:18:58.680 align:middle line:84% So there is no nice expression to do that backward operation. 00:18:58.680 --> 00:19:02.170 align:middle line:84% And that's because the values in my dictionary can be repeated. 00:19:02.170 --> 00:19:05.370 align:middle line:84% So if I look up the value B, and I 00:19:05.370 --> 00:19:07.830 align:middle line:84% want what's the key associated with B, 00:19:07.830 --> 00:19:09.580 align:middle line:84% well, there's actually two of them. 00:19:09.580 --> 00:19:11.940 align:middle line:84% So how does Python know I want both of them? 00:19:11.940 --> 00:19:13.950 align:middle line:84% How does it know I want only one of them? 00:19:13.950 --> 00:19:16.650 align:middle line:84% How does it know I want maybe a list of all these things? 00:19:16.650 --> 00:19:17.400 align:middle line:90% It doesn't, right? 00:19:17.400 --> 00:19:19.440 align:middle line:84% So you'd have to write code that does 00:19:19.440 --> 00:19:22.580 align:middle line:90% something for that operation. 00:19:22.580 --> 00:19:25.960 align:middle line:84% And we're going to see how to do that later. 00:19:25.960 --> 00:19:28.250 align:middle line:84% OK, so let's have you work on this You Try It. 00:19:28.250 --> 00:19:31.400 align:middle line:84% And this is just an exercise in looking up a value. 00:19:31.400 --> 00:19:34.570 align:middle line:84% So this is a function I'd like to write 00:19:34.570 --> 00:19:36.170 align:middle line:90% according to the specification. 00:19:36.170 --> 00:19:38.500 align:middle line:90% So it's called find_grades. 00:19:38.500 --> 00:19:41.860 align:middle line:84% Grades is a dictionary mapping student names to grades, 00:19:41.860 --> 00:19:44.110 align:middle line:84% so string-to-string exactly like we've 00:19:44.110 --> 00:19:46.450 align:middle line:90% seen in the previous slide. 00:19:46.450 --> 00:19:50.370 align:middle line:84% And students is going to be a list of student names. 00:19:50.370 --> 00:19:54.090 align:middle line:84% So in the example here, I've got my input dictionary, 00:19:54.090 --> 00:19:55.260 align:middle line:90% this thing we just saw. 00:19:55.260 --> 00:19:57.240 align:middle line:84% And then my list of student grades 00:19:57.240 --> 00:20:01.250 align:middle line:84% is, for example, these two strings Matt and Katy. 00:20:01.250 --> 00:20:04.880 align:middle line:84% For a bunch of these questions, especially in the micro quiz 00:20:04.880 --> 00:20:07.820 align:middle line:84% and things like that, if it gets a little confusing 00:20:07.820 --> 00:20:12.020 align:middle line:84% when I try to write the specification in a very 00:20:12.020 --> 00:20:14.000 align:middle line:84% detailed way to make it clear what I'd 00:20:14.000 --> 00:20:17.810 align:middle line:84% like from this function, it's important to try 00:20:17.810 --> 00:20:20.840 align:middle line:84% to use the example to help you figure out 00:20:20.840 --> 00:20:22.370 align:middle line:84% what we'd like because we're writing 00:20:22.370 --> 00:20:24.570 align:middle line:84% the specification in a general sense. 00:20:24.570 --> 00:20:27.020 align:middle line:84% But the example should hopefully make things really 00:20:27.020 --> 00:20:28.740 align:middle line:90% clear for what we'd like. 00:20:28.740 --> 00:20:32.360 align:middle line:84% So in this particular case, what we want the function to return 00:20:32.360 --> 00:20:37.470 align:middle line:84% is a list of the grades for the students being passed in, 00:20:37.470 --> 00:20:37.970 align:middle line:90% right? 00:20:37.970 --> 00:20:39.020 align:middle line:90% So we look up Matt. 00:20:39.020 --> 00:20:41.540 align:middle line:84% We see that their grade is a C. We look up Katy. 00:20:41.540 --> 00:20:44.390 align:middle line:84% Their grade is an A, so I want to return the list C comma 00:20:44.390 --> 00:20:47.742 align:middle line:84% A in that same order that I passed in my students. 00:20:47.742 --> 00:20:49.950 align:middle line:84% So I'll give you a couple of minutes to work on that, 00:20:49.950 --> 00:20:51.580 align:middle line:84% and then we can write it together. 00:20:51.580 --> 00:20:55.430 align:middle line:90% So that's line 94. 00:20:55.430 --> 00:20:58.100 align:middle line:90% 00:20:58.100 --> 00:21:00.100 align:middle line:84% So this is just an exercise on looking up 00:21:00.100 --> 00:21:02.587 align:middle line:90% values in the dictionary. 00:21:02.587 --> 00:21:04.420 align:middle line:84% All right, does anybody have a start for me? 00:21:04.420 --> 00:21:09.120 align:middle line:90% 00:21:09.120 --> 00:21:10.972 align:middle line:90% Yes, please. 00:21:10.972 --> 00:21:12.700 align:middle line:90% STUDENT: [INAUDIBLE] 00:21:12.700 --> 00:21:14.020 align:middle line:90% ANA BELL: Yep, Lnew. 00:21:14.020 --> 00:21:17.280 align:middle line:90% How about that? 00:21:17.280 --> 00:21:19.310 align:middle line:90% So this will be my results list. 00:21:19.310 --> 00:21:19.820 align:middle line:90% Yep. 00:21:19.820 --> 00:21:22.530 align:middle line:90% STUDENT: [INAUDIBLE] 00:21:22.530 --> 00:21:24.253 align:middle line:90% ANA BELL: Yep, for loop. 00:21:24.253 --> 00:21:27.211 align:middle line:90% STUDENT: [INAUDIBLE] 00:21:27.211 --> 00:21:36.580 align:middle line:90% 00:21:36.580 --> 00:21:39.550 align:middle line:84% ANA BELL: Yep, so grade square bracket lm looks up 00:21:39.550 --> 00:21:45.370 align:middle line:84% the value associated with my student named lm. 00:21:45.370 --> 00:21:49.580 align:middle line:84% And maybe we can save it like this grade equals this. 00:21:49.580 --> 00:21:51.370 align:middle line:90% And then you said append? 00:21:51.370 --> 00:21:57.440 align:middle line:84% Yep, so we can do Lnew.append the grade. 00:21:57.440 --> 00:21:58.370 align:middle line:90% Anything else? 00:21:58.370 --> 00:21:59.680 align:middle line:90% STUDENT: [INAUDIBLE] 00:21:59.680 --> 00:22:00.710 align:middle line:90% ANA BELL: Yep, a return. 00:22:00.710 --> 00:22:04.080 align:middle line:90% So we can return Lnew. 00:22:04.080 --> 00:22:07.050 align:middle line:90% Yep, so very reasonable code. 00:22:07.050 --> 00:22:07.950 align:middle line:90% I like it a lot. 00:22:07.950 --> 00:22:11.110 align:middle line:90% 00:22:11.110 --> 00:22:12.580 align:middle line:84% Besides the first lecture, I don't 00:22:12.580 --> 00:22:15.130 align:middle line:84% know that we've written any code that didn't involve a loop. 00:22:15.130 --> 00:22:18.550 align:middle line:84% So your best bet for writing code 00:22:18.550 --> 00:22:20.080 align:middle line:84% for any sort of thing in this class 00:22:20.080 --> 00:22:22.240 align:middle line:90% is to think, what loop can I do? 00:22:22.240 --> 00:22:26.350 align:middle line:84% So let's run the code, and it should return for me C comma A. 00:22:26.350 --> 00:22:26.935 align:middle line:90% And it does. 00:22:26.935 --> 00:22:31.220 align:middle line:90% 00:22:31.220 --> 00:22:34.520 align:middle line:84% Now that we can iterate-- so I mentioned this before, 00:22:34.520 --> 00:22:37.400 align:middle line:84% but once we're iterating over tuples and lists and things 00:22:37.400 --> 00:22:39.020 align:middle line:84% like that, one thing I would add, 00:22:39.020 --> 00:22:41.780 align:middle line:84% just for debugging purposes, is say something like lm 00:22:41.780 --> 00:22:46.910 align:middle line:84% is, and then you can say an example of what it could be, 00:22:46.910 --> 00:22:49.760 align:middle line:84% like Ana or Matt or whatever it could be, 00:22:49.760 --> 00:22:53.000 align:middle line:84% just to remind yourself that that thing, 00:22:53.000 --> 00:22:55.167 align:middle line:90% that loop variable is a string. 00:22:55.167 --> 00:22:56.750 align:middle line:84% And so it's one less thing to remember 00:22:56.750 --> 00:22:58.340 align:middle line:90% as you're writing further code. 00:22:58.340 --> 00:22:59.465 align:middle line:90% But this is really nice. 00:22:59.465 --> 00:23:03.940 align:middle line:90% 00:23:03.940 --> 00:23:06.970 align:middle line:84% OK, so dictionaries are already proving 00:23:06.970 --> 00:23:09.140 align:middle line:90% to be really, really useful. 00:23:09.140 --> 00:23:13.810 align:middle line:84% We can create values associated with custom indices. 00:23:13.810 --> 00:23:17.020 align:middle line:84% And if we want to grab the value associated with that custom 00:23:17.020 --> 00:23:21.400 align:middle line:84% index, it's really just a matter of indexing using a key, 00:23:21.400 --> 00:23:24.790 align:middle line:84% using that specific key, much like we did indexing 00:23:24.790 --> 00:23:26.260 align:middle line:90% into a list, OK? 00:23:26.260 --> 00:23:29.530 align:middle line:84% No need to loop, none of that iteration. 00:23:29.530 --> 00:23:34.950 align:middle line:84% It's just a single line of code that indexes into the list. 00:23:34.950 --> 00:23:36.980 align:middle line:84% So let's see a few more operations before we 00:23:36.980 --> 00:23:38.730 align:middle line:90% do the next You Try It. 00:23:38.730 --> 00:23:41.420 align:middle line:84% So I've got my list of grades that we've 00:23:41.420 --> 00:23:43.970 align:middle line:84% been working with in the past couple of slides. 00:23:43.970 --> 00:23:45.980 align:middle line:84% Let's say that we now want to add 00:23:45.980 --> 00:23:48.410 align:middle line:90% a new student and their grade. 00:23:48.410 --> 00:23:52.700 align:middle line:84% The way we do that is very similar to the way 00:23:52.700 --> 00:23:56.100 align:middle line:84% that we would add an element to a list 00:23:56.100 --> 00:23:59.870 align:middle line:84% once we already have an index for that list, right? 00:23:59.870 --> 00:24:03.740 align:middle line:84% Here, notice we don't actually have a slot for Grace. 00:24:03.740 --> 00:24:06.980 align:middle line:84% Yet, I'd like to add her to my dictionary. 00:24:06.980 --> 00:24:08.810 align:middle line:90% That's OK. 00:24:08.810 --> 00:24:13.760 align:middle line:84% With this particular syntax here, so grades at key Grace, 00:24:13.760 --> 00:24:18.530 align:middle line:84% if Python does not find Grace in my dictionary of keys, 00:24:18.530 --> 00:24:23.500 align:middle line:84% it will just add her, OK, which is really nice, right? 00:24:23.500 --> 00:24:26.080 align:middle line:84% I don't need to check if she's already in there. 00:24:26.080 --> 00:24:27.040 align:middle line:90% There's no looping. 00:24:27.040 --> 00:24:29.950 align:middle line:84% You just say grades at Grace equals A. Boom, 00:24:29.950 --> 00:24:32.560 align:middle line:90% it adds it for you. 00:24:32.560 --> 00:24:35.530 align:middle line:84% What if I want to change an entry in my dictionary? 00:24:35.530 --> 00:24:40.180 align:middle line:84% Well, let's say I want to change Grace's grade to a C. Grades 00:24:40.180 --> 00:24:46.450 align:middle line:84% at custom index grace equals C. We'll go in, look at my keys. 00:24:46.450 --> 00:24:50.860 align:middle line:84% When Grace didn't exist, Python added her with her value. 00:24:50.860 --> 00:24:53.320 align:middle line:84% But she already exists there, so Python will just 00:24:53.320 --> 00:24:56.760 align:middle line:90% overwrite her value. 00:24:56.760 --> 00:25:00.200 align:middle line:84% So really nice, something to look out for in case 00:25:00.200 --> 00:25:02.697 align:middle line:84% you already have values in the dictionary. 00:25:02.697 --> 00:25:04.280 align:middle line:84% You want to be careful if you actually 00:25:04.280 --> 00:25:05.480 align:middle line:90% do want to overwrite things. 00:25:05.480 --> 00:25:07.610 align:middle line:84% But it's really, really nice behavior. 00:25:07.610 --> 00:25:10.830 align:middle line:84% And it's different than lists, right, 00:25:10.830 --> 00:25:13.290 align:middle line:84% especially adding an entry to the dictionary. 00:25:13.290 --> 00:25:16.920 align:middle line:90% 00:25:16.920 --> 00:25:18.480 align:middle line:84% You can delete entries, much like we 00:25:18.480 --> 00:25:21.180 align:middle line:90% deleted entries from a list. 00:25:21.180 --> 00:25:24.350 align:middle line:90% We use the del function. 00:25:24.350 --> 00:25:27.860 align:middle line:84% And the del function says, what entry you'd 00:25:27.860 --> 00:25:29.900 align:middle line:90% like to delete from what list? 00:25:29.900 --> 00:25:34.790 align:middle line:84% So here we just say the name of our dictionary at index Ana. 00:25:34.790 --> 00:25:38.690 align:middle line:84% So this will completely remove Ana and her value 00:25:38.690 --> 00:25:42.020 align:middle line:84% and the value associated with Ana from the dictionary. 00:25:42.020 --> 00:25:46.200 align:middle line:90% 00:25:46.200 --> 00:25:49.640 align:middle line:84% So what I want to make a note of is that our dictionary is being 00:25:49.640 --> 00:25:52.280 align:middle line:84% mutated with all of these different methods 00:25:52.280 --> 00:25:54.590 align:middle line:84% or all of these different functions, right? 00:25:54.590 --> 00:25:56.967 align:middle line:84% So here, when I added Grace, I've 00:25:56.967 --> 00:25:58.550 align:middle line:84% mutated my original dictionary, right? 00:25:58.550 --> 00:26:02.090 align:middle line:84% The animation didn't make a copy of this dictionary 00:26:02.090 --> 00:26:04.970 align:middle line:84% with Grace added, leaving the original unchanged. 00:26:04.970 --> 00:26:08.480 align:middle line:84% I've literally gone in and mutated my original dictionary 00:26:08.480 --> 00:26:09.470 align:middle line:90% to add Grace. 00:26:09.470 --> 00:26:13.040 align:middle line:84% I've mutated the original dictionary to change her grade. 00:26:13.040 --> 00:26:15.050 align:middle line:84% I've mutated the original dictionary 00:26:15.050 --> 00:26:17.120 align:middle line:84% to remove Ana from the dictionary, right? 00:26:17.120 --> 00:26:19.910 align:middle line:84% So all these functions are actually 00:26:19.910 --> 00:26:23.140 align:middle line:90% mutating my dictionary. 00:26:23.140 --> 00:26:26.080 align:middle line:84% OK, one other very useful thing that you 00:26:26.080 --> 00:26:28.900 align:middle line:84% can do with dictionaries is to check 00:26:28.900 --> 00:26:31.750 align:middle line:90% if a key is in my dictionary. 00:26:31.750 --> 00:26:35.220 align:middle line:84% So we do this using in operator, this in keyword. 00:26:35.220 --> 00:26:39.900 align:middle line:84% We've seen in keyword being used to check if an element is 00:26:39.900 --> 00:26:43.830 align:middle line:84% in a list, to check if a substring or a character 00:26:43.830 --> 00:26:46.920 align:middle line:84% is in a string, to check if some element is in a tuple. 00:26:46.920 --> 00:26:50.310 align:middle line:84% We can also use it to check if an element or a key 00:26:50.310 --> 00:26:53.510 align:middle line:90% is in my dictionary. 00:26:53.510 --> 00:26:55.480 align:middle line:90% So I want to make a note. 00:26:55.480 --> 00:26:57.160 align:middle line:90% It's only checking the keys. 00:26:57.160 --> 00:27:00.310 align:middle line:84% It does not look for the values in the dictionary. 00:27:00.310 --> 00:27:02.860 align:middle line:84% We'll see how to check if some value is 00:27:02.860 --> 00:27:04.450 align:middle line:84% in the dictionary in a little bit. 00:27:04.450 --> 00:27:06.430 align:middle line:84% But the in keyword specifically only 00:27:06.430 --> 00:27:08.240 align:middle line:84% looks at the keys in the dictionary. 00:27:08.240 --> 00:27:11.260 align:middle line:90% So if I have the expression-- 00:27:11.260 --> 00:27:15.640 align:middle line:84% the string John is in grades, Python only looks at the keys 00:27:15.640 --> 00:27:17.515 align:middle line:90% and say, yep, there it is. 00:27:17.515 --> 00:27:19.390 align:middle line:84% I don't care what value's associated with it. 00:27:19.390 --> 00:27:22.070 align:middle line:84% I just care that it's in my keys. 00:27:22.070 --> 00:27:25.510 align:middle line:84% So this entire expression here, John in grades, 00:27:25.510 --> 00:27:28.270 align:middle line:84% will evaluate, so be replaced with true. 00:27:28.270 --> 00:27:31.310 align:middle line:84% Daniel, obviously, is not in my dictionary keys. 00:27:31.310 --> 00:27:32.410 align:middle line:90% So it returns false. 00:27:32.410 --> 00:27:37.900 align:middle line:84% B is not in my dictionary keys, even though it's in my values. 00:27:37.900 --> 00:27:40.960 align:middle line:84% It still returns false because it only looks at the keys. 00:27:40.960 --> 00:27:44.580 align:middle line:90% 00:27:44.580 --> 00:27:49.490 align:middle line:84% All right, let's have you try this exercise. 00:27:49.490 --> 00:27:53.470 align:middle line:84% So function is called find in L. Again, 00:27:53.470 --> 00:27:56.410 align:middle line:84% we can use the specifications and the example 00:27:56.410 --> 00:27:59.840 align:middle line:84% to help us figure out what we'd like from this function. 00:27:59.840 --> 00:28:04.700 align:middle line:84% So Ld is going to be a list of dictionaries. 00:28:04.700 --> 00:28:09.400 align:middle line:84% So in the example here I've got three dictionaries defined, 00:28:09.400 --> 00:28:13.900 align:middle line:84% and the first parameter here, the thing being passed as Ld 00:28:13.900 --> 00:28:17.710 align:middle line:84% is the list with D1, D2, D3 as my elements. 00:28:17.710 --> 00:28:21.440 align:middle line:90% And k is just an integer. 00:28:21.440 --> 00:28:25.970 align:middle line:84% What I'd like to do is return true from the function 00:28:25.970 --> 00:28:31.460 align:middle line:84% if that k is a key in any of these dictionaries 00:28:31.460 --> 00:28:32.640 align:middle line:90% and false otherwise. 00:28:32.640 --> 00:28:36.050 align:middle line:84% So as soon as I see a key that matches k, 00:28:36.050 --> 00:28:37.710 align:middle line:90% I want to return true. 00:28:37.710 --> 00:28:42.590 align:middle line:84% So in this example here, when I look for the k 2 00:28:42.590 --> 00:28:46.580 align:middle line:84% inside these dictionaries, D1 doesn't have it, but D2 has it. 00:28:46.580 --> 00:28:48.830 align:middle line:90% So I would return true. 00:28:48.830 --> 00:28:52.880 align:middle line:84% When I look for 25 in that same list of dictionaries, 00:28:52.880 --> 00:28:56.900 align:middle line:84% 25 is a value in one of these, in D3, 00:28:56.900 --> 00:28:59.495 align:middle line:84% but it's not a key in D1, D2 or D3. 00:28:59.495 --> 00:29:00.620 align:middle line:90% So that would return false. 00:29:00.620 --> 00:29:04.890 align:middle line:90% 00:29:04.890 --> 00:29:10.442 align:middle line:84% All right, so that's just a little lower, line 115. 00:29:10.442 --> 00:29:12.150 align:middle line:84% Give you a couple of moments, and then we 00:29:12.150 --> 00:29:14.068 align:middle line:84% can write it together like usual. 00:29:14.068 --> 00:29:16.110 align:middle line:84% All right, does anyone want to start me off here? 00:29:16.110 --> 00:29:20.916 align:middle line:90% So how can we do this? 00:29:20.916 --> 00:29:22.650 align:middle line:90% STUDENT: Create a loop. 00:29:22.650 --> 00:29:25.040 align:middle line:90% ANA BELL: Create a loop, yes. 00:29:25.040 --> 00:29:25.710 align:middle line:90% For? 00:29:25.710 --> 00:29:28.520 align:middle line:84% STUDENT: [INAUDIBLE] for d in Ld. 00:29:28.520 --> 00:29:29.300 align:middle line:90% ANA BELL: Yep. 00:29:29.300 --> 00:29:32.450 align:middle line:90% OK, so that means that D is-- 00:29:32.450 --> 00:29:36.650 align:middle line:84% I can say like K1 mapped to v1 or something like that, 00:29:36.650 --> 00:29:37.765 align:middle line:90% right, key to a value. 00:29:37.765 --> 00:29:43.940 align:middle line:90% 00:29:43.940 --> 00:29:48.930 align:middle line:90% If k in D? 00:29:48.930 --> 00:29:52.020 align:middle line:84% Yep, so that will check for me my keys 00:29:52.020 --> 00:29:56.970 align:middle line:84% in that particular dictionary that I'm looking at right now. 00:29:56.970 --> 00:29:59.923 align:middle line:84% Yep, we can immediately return true, right? 00:29:59.923 --> 00:30:01.590 align:middle line:84% As soon as we found it, no need to check 00:30:01.590 --> 00:30:02.550 align:middle line:90% the other dictionaries. 00:30:02.550 --> 00:30:08.125 align:middle line:84% Just pop out of the function and return true. 00:30:08.125 --> 00:30:11.480 align:middle line:90% STUDENT: [INAUDIBLE] 00:30:11.480 --> 00:30:13.715 align:middle line:84% ANA BELL: Same inside the if or inside 00:30:13.715 --> 00:30:15.200 align:middle line:90% the for or outside the 4. 00:30:15.200 --> 00:30:16.360 align:middle line:90% STUDENT: Outside the 4. 00:30:16.360 --> 00:30:19.150 align:middle line:84% ANA BELL: Outside the for, we can return false. 00:30:19.150 --> 00:30:20.820 align:middle line:90% Yep. 00:30:20.820 --> 00:30:24.450 align:middle line:90% I like this code a lot. 00:30:24.450 --> 00:30:28.630 align:middle line:84% Uses this in operator to do the task. 00:30:28.630 --> 00:30:30.930 align:middle line:84% So the return false outside of for loop 00:30:30.930 --> 00:30:32.940 align:middle line:84% works really well because if I've 00:30:32.940 --> 00:30:36.060 align:middle line:84% gone through every D inside Ld here, 00:30:36.060 --> 00:30:39.930 align:middle line:84% then I'm checking every single dictionary, right? 00:30:39.930 --> 00:30:43.740 align:middle line:84% As soon as I find one that has that key, 00:30:43.740 --> 00:30:49.290 align:middle line:84% this return true acts like a break and a return, right? 00:30:49.290 --> 00:30:52.410 align:middle line:84% So it breaks out of the loop and returns immediately. 00:30:52.410 --> 00:30:55.820 align:middle line:90% And it doesn't return false. 00:30:55.820 --> 00:30:57.940 align:middle line:84% But if I've gone through every dictionary 00:30:57.940 --> 00:31:03.760 align:middle line:84% and didn't find the key matching k, then I return false. 00:31:03.760 --> 00:31:06.390 align:middle line:90% Yeah. 00:31:06.390 --> 00:31:08.535 align:middle line:84% Did anybody try it a different way, or is this-- 00:31:08.535 --> 00:31:13.870 align:middle line:90% 00:31:13.870 --> 00:31:17.110 align:middle line:84% we could certainly try it with a Boolean flag, right? 00:31:17.110 --> 00:31:21.010 align:middle line:84% We could flag the fact that we found it through some loop. 00:31:21.010 --> 00:31:23.830 align:middle line:84% And keep track of it, and at the end, just return that flag. 00:31:23.830 --> 00:31:25.000 align:middle line:90% That's another way to do it. 00:31:25.000 --> 00:31:27.460 align:middle line:84% But this is probably the most Pythonic way. 00:31:27.460 --> 00:31:30.570 align:middle line:90% 00:31:30.570 --> 00:31:33.300 align:middle line:84% So we can run it on these two examples here, right? 00:31:33.300 --> 00:31:36.360 align:middle line:84% So I'm looking up 2 to return true 00:31:36.360 --> 00:31:38.310 align:middle line:84% and looking up 25 to return false. 00:31:38.310 --> 00:31:40.030 align:middle line:90% And it does. 00:31:40.030 --> 00:31:42.730 align:middle line:84% Questions about this code or dictionary so far. 00:31:42.730 --> 00:31:47.980 align:middle line:90% Is everything OK so far? 00:31:47.980 --> 00:31:50.600 align:middle line:90% OK. 00:31:50.600 --> 00:31:52.320 align:middle line:84% All right, a couple more operations. 00:31:52.320 --> 00:31:55.130 align:middle line:84% So, so far, we've looked up values in a dictionary. 00:31:55.130 --> 00:31:56.870 align:middle line:84% We've added stuff to the dictionary. 00:31:56.870 --> 00:31:59.300 align:middle line:84% We've deleted stuff from the dictionary. 00:31:59.300 --> 00:32:02.360 align:middle line:84% One really useful thing to do is to be 00:32:02.360 --> 00:32:06.620 align:middle line:84% able to look at every single entry in my dictionary. 00:32:06.620 --> 00:32:08.450 align:middle line:84% The reason why we'd want to do this 00:32:08.450 --> 00:32:13.130 align:middle line:84% is because we should assume that when we create our dictionaries 00:32:13.130 --> 00:32:15.650 align:middle line:90% there's no order to them, right? 00:32:15.650 --> 00:32:17.480 align:middle line:90% This is very much unlike lists. 00:32:17.480 --> 00:32:19.320 align:middle line:90% Lists had an order to them. 00:32:19.320 --> 00:32:23.120 align:middle line:84% We knew that the first element in our list was at index 0. 00:32:23.120 --> 00:32:25.310 align:middle line:84% The next one was at index 1, and so on, right? 00:32:25.310 --> 00:32:28.790 align:middle line:84% Lists were ordered sequences of elements. 00:32:28.790 --> 00:32:34.460 align:middle line:84% But dictionaries are not ordered sequences of elements. 00:32:34.460 --> 00:32:38.030 align:middle line:90% That's not super true. 00:32:38.030 --> 00:32:40.460 align:middle line:84% Up until a very recent version of Python, 00:32:40.460 --> 00:32:41.990 align:middle line:90% there was no guaranteed order. 00:32:41.990 --> 00:32:46.640 align:middle line:84% They were put in some order that I couldn't figure out 00:32:46.640 --> 00:32:48.470 align:middle line:90% how it was determined. 00:32:48.470 --> 00:32:53.220 align:middle line:84% But I forget which Python version, maybe 3.6 or something 00:32:53.220 --> 00:32:58.740 align:middle line:84% like that, started to guarantee an order for the dictionary 00:32:58.740 --> 00:32:59.310 align:middle line:90% elements. 00:32:59.310 --> 00:33:01.710 align:middle line:84% And that order was the same order 00:33:01.710 --> 00:33:04.680 align:middle line:84% that you inserted the elements, OK? 00:33:04.680 --> 00:33:07.410 align:middle line:84% But if you'd like to write robust code that 00:33:07.410 --> 00:33:11.070 align:middle line:84% could be run by people using an older version of Python, 00:33:11.070 --> 00:33:13.260 align:middle line:84% you should write the code assuming 00:33:13.260 --> 00:33:14.880 align:middle line:90% that no such order exists. 00:33:14.880 --> 00:33:15.480 align:middle line:90% And it's OK. 00:33:15.480 --> 00:33:19.360 align:middle line:84% It doesn't make the code that much harder to write. 00:33:19.360 --> 00:33:21.610 align:middle line:84% But if we're not assuming any order 00:33:21.610 --> 00:33:25.520 align:middle line:84% to Python entries in the dictionary, 00:33:25.520 --> 00:33:27.910 align:middle line:84% then that means a lot of times, we actually 00:33:27.910 --> 00:33:30.100 align:middle line:84% have to look at each entry in the dictionary 00:33:30.100 --> 00:33:33.170 align:middle line:90% to do some sort of task. 00:33:33.170 --> 00:33:35.950 align:middle line:84% So one of the first things you might want to do 00:33:35.950 --> 00:33:39.460 align:middle line:84% is to iterate through all the keys in the dictionary. 00:33:39.460 --> 00:33:44.320 align:middle line:84% To do that, we use a function called grades.keys. 00:33:44.320 --> 00:33:48.070 align:middle line:84% And this grades.keys function here 00:33:48.070 --> 00:33:49.870 align:middle line:84% doesn't mutate the dictionary at all. 00:33:49.870 --> 00:33:52.720 align:middle line:84% But instead, it returns for me an iterable, 00:33:52.720 --> 00:33:57.640 align:middle line:84% a sequence of values, which are all the keys in my dictionary. 00:33:57.640 --> 00:34:01.030 align:middle line:84% Now, the data type of this return value 00:34:01.030 --> 00:34:03.880 align:middle line:90% is called dict_keys. 00:34:03.880 --> 00:34:07.060 align:middle line:84% It's not a data type we've worked with before, OK? 00:34:07.060 --> 00:34:08.770 align:middle line:90% It looks really weird. 00:34:08.770 --> 00:34:11.230 align:middle line:84% But if you'd like, and you don't have to do this, 00:34:11.230 --> 00:34:14.409 align:middle line:84% you can always cast this sequence of values-- 00:34:14.409 --> 00:34:17.900 align:middle line:84% that's type dict keys-- to a list, like this. 00:34:17.900 --> 00:34:20.469 align:middle line:84% So if you cast to a list grades.keys, 00:34:20.469 --> 00:34:25.000 align:middle line:84% it gives for us this more recognizable list with each key 00:34:25.000 --> 00:34:28.440 align:middle line:90% being an element in the list. 00:34:28.440 --> 00:34:29.770 align:middle line:90% You don't have to do this. 00:34:29.770 --> 00:34:33.239 align:middle line:84% But if it makes it easier for you, you can, OK? 00:34:33.239 --> 00:34:37.060 align:middle line:84% So this line of code here, grades.keys returns for you-- 00:34:37.060 --> 00:34:40.335 align:middle line:84% you can think of it like this iterable, this list of all 00:34:40.335 --> 00:34:41.460 align:middle line:90% the keys in the dictionary. 00:34:41.460 --> 00:34:43.293 align:middle line:84% Again, they're not ordered, right? 00:34:43.293 --> 00:34:45.210 align:middle line:84% They're ordered in the order that I added them 00:34:45.210 --> 00:34:47.580 align:middle line:84% into the dictionary, right, Ana, then Matt, then John, then 00:34:47.580 --> 00:34:48.090 align:middle line:90% Katy. 00:34:48.090 --> 00:34:50.832 align:middle line:84% But they're not sorted in alphabetical order. 00:34:50.832 --> 00:34:52.290 align:middle line:84% If you have integers, they won't be 00:34:52.290 --> 00:34:53.969 align:middle line:84% sorted in ascending or descending order. 00:34:53.969 --> 00:34:58.370 align:middle line:84% So it's best to just not assume an order to begin with. 00:34:58.370 --> 00:35:01.160 align:middle line:84% Similarly, we can get an iterable of all the values 00:35:01.160 --> 00:35:03.050 align:middle line:90% in the dictionary. 00:35:03.050 --> 00:35:07.970 align:middle line:84% And to do this, no surprise there, we use grades.values. 00:35:07.970 --> 00:35:10.520 align:middle line:84% And this is, again, a function which doesn't 00:35:10.520 --> 00:35:11.630 align:middle line:90% mutate the grades at all. 00:35:11.630 --> 00:35:15.530 align:middle line:84% But instead, it gets replaced with this dict values data 00:35:15.530 --> 00:35:16.430 align:middle line:90% type. 00:35:16.430 --> 00:35:18.180 align:middle line:84% I've never seen it before either. 00:35:18.180 --> 00:35:20.600 align:middle line:84% And you can cast it to a list if you'd 00:35:20.600 --> 00:35:23.820 align:middle line:84% like because it makes more sense to us at this point in time, 00:35:23.820 --> 00:35:27.350 align:middle line:84% which just returns for us this list of every single value 00:35:27.350 --> 00:35:28.550 align:middle line:90% in my dictionary. 00:35:28.550 --> 00:35:30.050 align:middle line:90% Again, no order, right? 00:35:30.050 --> 00:35:33.230 align:middle line:84% We can see that there's no order except for the order 00:35:33.230 --> 00:35:36.600 align:middle line:84% that we actually added the elements in. 00:35:36.600 --> 00:35:37.397 align:middle line:90% Yeah. 00:35:37.397 --> 00:35:39.480 align:middle line:84% STUDENT: When you said like it acts like a couple, 00:35:39.480 --> 00:35:42.750 align:middle line:84% you mean like if I do grades that, at other times, 00:35:42.750 --> 00:35:44.130 align:middle line:90% it will print out the same list? 00:35:44.130 --> 00:35:46.940 align:middle line:90% 00:35:46.940 --> 00:35:50.720 align:middle line:84% ANA BELL: Yeah, yeah it'll print out the same iterable, 00:35:50.720 --> 00:35:52.342 align:middle line:84% I guess, if you do it again, yeah. 00:35:52.342 --> 00:35:54.800 align:middle line:84% STUDENT: If we're just doing like-- if we're just iterating 00:35:54.800 --> 00:35:58.770 align:middle line:84% over the dictionary, there can be chances where it comes up 00:35:58.770 --> 00:35:59.280 align:middle line:90% [INAUDIBLE]. 00:35:59.280 --> 00:36:01.920 align:middle line:84% ANA BELL: If you're iterating over the dictionary, 00:36:01.920 --> 00:36:04.690 align:middle line:84% not in the Python version we're using. 00:36:04.690 --> 00:36:08.520 align:middle line:84% But in a previous version, if you ran on your machine 00:36:08.520 --> 00:36:10.562 align:middle line:84% or if I ran the same code on my machine, 00:36:10.562 --> 00:36:12.270 align:middle line:84% it might have given me a different order. 00:36:12.270 --> 00:36:15.080 align:middle line:90% 00:36:15.080 --> 00:36:18.410 align:middle line:84% But in the versions we're using from now on in Python, 00:36:18.410 --> 00:36:20.600 align:middle line:84% because you guys all probably downloaded the latest 00:36:20.600 --> 00:36:24.620 align:middle line:84% version of Anaconda and Spyder, it 00:36:24.620 --> 00:36:27.800 align:middle line:84% will guarantee the order that you inserted the elements in. 00:36:27.800 --> 00:36:30.620 align:middle line:84% But if somebody's using an older version of Python 00:36:30.620 --> 00:36:32.660 align:middle line:84% takes your code and runs it, they 00:36:32.660 --> 00:36:37.250 align:middle line:84% might actually get A, A, B, B or some other order 00:36:37.250 --> 00:36:40.547 align:middle line:90% for these functions here. 00:36:40.547 --> 00:36:41.380 align:middle line:90% STUDENT: [INAUDIBLE] 00:36:41.380 --> 00:36:43.570 align:middle line:90% ANA BELL: Yeah, you're welcome. 00:36:43.570 --> 00:36:46.360 align:middle line:84% So these being iterable just means that we 00:36:46.360 --> 00:36:48.940 align:middle line:90% can have something like 4 I-- 00:36:48.940 --> 00:36:52.930 align:middle line:84% so 4 k in grades.keys basically giving us 00:36:52.930 --> 00:36:56.680 align:middle line:84% a loop where k is going to be each element in this list. 00:36:56.680 --> 00:37:00.690 align:middle line:90% 00:37:00.690 --> 00:37:01.770 align:middle line:90% So that's fine. 00:37:01.770 --> 00:37:04.320 align:middle line:84% So we can iterate over the keys, or we can iterate over 00:37:04.320 --> 00:37:05.410 align:middle line:90% the values directly. 00:37:05.410 --> 00:37:08.220 align:middle line:84% But what I find personally most effective 00:37:08.220 --> 00:37:11.530 align:middle line:84% is to iterate over each entry in the dictionary. 00:37:11.530 --> 00:37:15.510 align:middle line:84% So not just over the keys or the values by themselves, 00:37:15.510 --> 00:37:18.880 align:middle line:84% to iterate over the keys and the values together. 00:37:18.880 --> 00:37:25.630 align:middle line:84% So to do that, we use this function called grades.items. 00:37:25.630 --> 00:37:28.030 align:middle line:84% And unsurprisingly, this will return also 00:37:28.030 --> 00:37:31.330 align:middle line:84% an iterable, where each element in my iterable 00:37:31.330 --> 00:37:33.310 align:middle line:84% is not just the key or the value. 00:37:33.310 --> 00:37:38.280 align:middle line:84% It's a tuple of the key comma the value, OK? 00:37:38.280 --> 00:37:40.290 align:middle line:84% And again, we can cast it to a list 00:37:40.290 --> 00:37:42.600 align:middle line:84% to give us something that's more recognizable. 00:37:42.600 --> 00:37:45.180 align:middle line:84% You can see now each element in the returned list 00:37:45.180 --> 00:37:48.150 align:middle line:84% is going to be the tuple where I have an entry, right? 00:37:48.150 --> 00:37:51.960 align:middle line:84% So my entry Ana comma B is this first element in my return 00:37:51.960 --> 00:37:54.310 align:middle line:84% list, and then Matt A and then John B and then Katy A. 00:37:54.310 --> 00:37:57.450 align:middle line:84% So I grab these entries together where 00:37:57.450 --> 00:37:59.700 align:middle line:84% I have access to both the key and the value 00:37:59.700 --> 00:38:02.443 align:middle line:90% for that entry, which means-- 00:38:02.443 --> 00:38:04.110 align:middle line:84% and this is the important part-- that we 00:38:04.110 --> 00:38:05.460 align:middle line:90% can do something like this. 00:38:05.460 --> 00:38:07.960 align:middle line:84% And we can do this for the previous slide as well. 00:38:07.960 --> 00:38:13.020 align:middle line:84% But for this particular grades.items iteration, 00:38:13.020 --> 00:38:17.910 align:middle line:84% if we're grabbing a key value pair out of items, 00:38:17.910 --> 00:38:19.980 align:middle line:84% that means we can do something like this. 00:38:19.980 --> 00:38:23.940 align:middle line:84% For k comma v in grades.items means 00:38:23.940 --> 00:38:28.140 align:middle line:84% that Python will map k to the key for that entry 00:38:28.140 --> 00:38:30.510 align:middle line:84% and v to the value for that entry 00:38:30.510 --> 00:38:34.080 align:middle line:84% as I'm iterating over each one of these pairs, right? 00:38:34.080 --> 00:38:36.240 align:middle line:84% So with each iteration, I have access 00:38:36.240 --> 00:38:39.310 align:middle line:84% to both the key and the value for that entry, 00:38:39.310 --> 00:38:41.540 align:middle line:90% which is pretty useful. 00:38:41.540 --> 00:38:43.930 align:middle line:84% So if I have this line of code here, 00:38:43.930 --> 00:38:48.310 align:middle line:84% if I print key k has value v, the k and the v 00:38:48.310 --> 00:38:50.760 align:middle line:84% will change with each entry, right? 00:38:50.760 --> 00:38:53.260 align:middle line:84% I'm just grabbing both the key and the value for that entry. 00:38:53.260 --> 00:38:54.310 align:middle line:90% Yeah, question. 00:38:54.310 --> 00:38:56.200 align:middle line:84% STUDENT: To use [INAUDIBLE] that item, 00:38:56.200 --> 00:38:58.110 align:middle line:84% is it actually like-- is it actually tuple, 00:38:58.110 --> 00:38:59.230 align:middle line:90% or just the actual object? 00:38:59.230 --> 00:39:00.440 align:middle line:90% ANA BELL: It's not a tuple. 00:39:00.440 --> 00:39:04.910 align:middle line:84% So the actual object type is this thing dict_items. 00:39:04.910 --> 00:39:07.400 align:middle line:84% So again, not a type that we've worked with before. 00:39:07.400 --> 00:39:09.520 align:middle line:90% But that's just the type, right? 00:39:09.520 --> 00:39:12.130 align:middle line:84% We've seen lists, tuples, dictionaries. 00:39:12.130 --> 00:39:15.580 align:middle line:84% Dict_items is another data type, yeah. 00:39:15.580 --> 00:39:17.620 align:middle line:84% But the cool thing is that it's an iterable. 00:39:17.620 --> 00:39:19.240 align:middle line:84% So it's a sequence of values, which 00:39:19.240 --> 00:39:22.090 align:middle line:84% means that you can cast it to a list, which 00:39:22.090 --> 00:39:23.410 align:middle line:90% is also a sequence of values. 00:39:23.410 --> 00:39:25.090 align:middle line:84% And it knows how to do that casting. 00:39:25.090 --> 00:39:26.860 align:middle line:84% And you get the more recognizable list 00:39:26.860 --> 00:39:27.777 align:middle line:90% that we've been using. 00:39:27.777 --> 00:39:30.330 align:middle line:90% 00:39:30.330 --> 00:39:33.680 align:middle line:90% Other questions. 00:39:33.680 --> 00:39:35.870 align:middle line:84% OK, so I really like using grade.items 00:39:35.870 --> 00:39:37.260 align:middle line:90% to iterate over entries. 00:39:37.260 --> 00:39:39.270 align:middle line:84% So let's have you try this exercise. 00:39:39.270 --> 00:39:42.170 align:middle line:84% So it's a function called count matches. 00:39:42.170 --> 00:39:46.410 align:middle line:90% It takes in one dictionary d. 00:39:46.410 --> 00:39:50.380 align:middle line:84% I didn't say what the elements are, but you can mix and match. 00:39:50.380 --> 00:39:54.000 align:middle line:84% So here, I have a dictionary with just int mapped to ints. 00:39:54.000 --> 00:39:57.390 align:middle line:84% And here I've got a dictionary where it maps ints and strings 00:39:57.390 --> 00:39:58.600 align:middle line:90% and things like that. 00:39:58.600 --> 00:40:00.840 align:middle line:84% And what I want this function to do 00:40:00.840 --> 00:40:04.050 align:middle line:84% is tell me how many entries in this input dictionaries 00:40:04.050 --> 00:40:06.670 align:middle line:90% have the key match its value. 00:40:06.670 --> 00:40:10.710 align:middle line:84% So here, in this first example, the key here is 1. 00:40:10.710 --> 00:40:11.460 align:middle line:90% The value is 2. 00:40:11.460 --> 00:40:12.450 align:middle line:90% So they don't match. 00:40:12.450 --> 00:40:14.280 align:middle line:84% These don't match, and these don't match. 00:40:14.280 --> 00:40:15.810 align:middle line:90% So the count should be 0. 00:40:15.810 --> 00:40:19.410 align:middle line:84% But down here in this example, the 1 doesn't match 2, 00:40:19.410 --> 00:40:20.200 align:middle line:90% so that's fine. 00:40:20.200 --> 00:40:24.210 align:middle line:84% But the key A matches its value, 1 count. 00:40:24.210 --> 00:40:27.010 align:middle line:84% Key 5 matches its value, 2 counts. 00:40:27.010 --> 00:40:28.530 align:middle line:90% So this should return count 2. 00:40:28.530 --> 00:40:31.270 align:middle line:90% 00:40:31.270 --> 00:40:36.250 align:middle line:84% All right, let's have you work on that down by line 137. 00:40:36.250 --> 00:40:37.690 align:middle line:84% And then we'll write it together. 00:40:37.690 --> 00:40:40.942 align:middle line:90% 00:40:40.942 --> 00:40:42.275 align:middle line:90% All right, how can I start this? 00:40:42.275 --> 00:40:44.950 align:middle line:90% 00:40:44.950 --> 00:40:45.550 align:middle line:90% Yes. 00:40:45.550 --> 00:40:47.080 align:middle line:90% STUDENT: A count. 00:40:47.080 --> 00:40:48.190 align:middle line:90% ANA BELL: A count, yes. 00:40:48.190 --> 00:40:49.430 align:middle line:90% STUDENT: [INAUDIBLE] 00:40:49.430 --> 00:40:50.732 align:middle line:90% ANA BELL: 0, yep. 00:40:50.732 --> 00:40:52.210 align:middle line:90% STUDENT: A for loop? 00:40:52.210 --> 00:40:53.737 align:middle line:90% ANA BELL: A for loop, yep. 00:40:53.737 --> 00:41:00.360 align:middle line:84% STUDENT: [INAUDIBLE] v, k in d.items [INAUDIBLE].. 00:41:00.360 --> 00:41:03.028 align:middle line:84% ANA BELL: Yep, as a function, yep. 00:41:03.028 --> 00:41:05.350 align:middle line:90% STUDENT: If v equal to k? 00:41:05.350 --> 00:41:06.640 align:middle line:90% ANA BELL: v equal to k. 00:41:06.640 --> 00:41:09.370 align:middle line:84% Yep, so this is where my value equals my key 00:41:09.370 --> 00:41:11.615 align:middle line:90% for that particular entry. 00:41:11.615 --> 00:41:13.840 align:middle line:84% STUDENT: Count equals count plus 1 00:41:13.840 --> 00:41:18.150 align:middle line:84% ANA BELL: Count equals count plus 1, perfect. 00:41:18.150 --> 00:41:21.230 align:middle line:90% Yep, return count. 00:41:21.230 --> 00:41:22.970 align:middle line:84% Did anybody do it a different way? 00:41:22.970 --> 00:41:26.130 align:middle line:90% 00:41:26.130 --> 00:41:28.050 align:middle line:90% OK, awesome. 00:41:28.050 --> 00:41:29.350 align:middle line:90% Yeah. 00:41:29.350 --> 00:41:32.950 align:middle line:84% STUDENT: Why do you have to call it like [INAUDIBLE]?? 00:41:32.950 --> 00:41:35.277 align:middle line:84% I had to do it a different way [INAUDIBLE].. 00:41:35.277 --> 00:41:36.610 align:middle line:90% ANA BELL: We can write it, yeah. 00:41:36.610 --> 00:41:39.220 align:middle line:84% STUDENT: Yeah, I did like the count equals 0, 00:41:39.220 --> 00:41:42.400 align:middle line:84% and then I did like for x in like [INAUDIBLE] 00:41:42.400 --> 00:41:44.560 align:middle line:84% that it was just going to call the value. 00:41:44.560 --> 00:41:49.540 align:middle line:84% And [INAUDIBLE] like at x equals [INAUDIBLE].. 00:41:49.540 --> 00:41:51.790 align:middle line:84% I thought that it was just going to call [INAUDIBLE].. 00:41:51.790 --> 00:41:55.600 align:middle line:84% ANA BELL: Yeah, so we can say for x and d.keys or something 00:41:55.600 --> 00:41:57.610 align:middle line:90% like that, right? 00:41:57.610 --> 00:41:59.050 align:middle line:90% Something like that, or no? 00:41:59.050 --> 00:42:01.600 align:middle line:84% We can also say for x and d, I think. 00:42:01.600 --> 00:42:04.820 align:middle line:84% That might work too because it'll grab the key for us. 00:42:04.820 --> 00:42:07.610 align:middle line:90% But just to be safe, keys. 00:42:07.610 --> 00:42:09.420 align:middle line:84% And now we need to grab the value. 00:42:09.420 --> 00:42:12.740 align:middle line:84% So how do you grab the value associated with key x? 00:42:12.740 --> 00:42:15.680 align:middle line:90% STUDENT: Like key brackets. 00:42:15.680 --> 00:42:16.717 align:middle line:90% No, brackets are-- 00:42:16.717 --> 00:42:18.050 align:middle line:90% ANA BELL: Yeah, square brackets. 00:42:18.050 --> 00:42:19.133 align:middle line:90% It's just indexing, right? 00:42:19.133 --> 00:42:21.320 align:middle line:90% So d square brackets x. 00:42:21.320 --> 00:42:25.670 align:middle line:84% Oops, if d square brackets x equals-- 00:42:25.670 --> 00:42:29.900 align:middle line:84% so that's the value equals the key, right? 00:42:29.900 --> 00:42:33.780 align:middle line:90% Then, again, we count plus 1. 00:42:33.780 --> 00:42:36.540 align:middle line:90% So this is our other way. 00:42:36.540 --> 00:42:39.530 align:middle line:90% 00:42:39.530 --> 00:42:40.340 align:middle line:90% Yep. 00:42:40.340 --> 00:42:43.910 align:middle line:84% So we don't have to use items, but items makes things easier 00:42:43.910 --> 00:42:47.480 align:middle line:84% because we have in hand a variable that's 00:42:47.480 --> 00:42:49.700 align:middle line:84% the value and the variable that's the key. 00:42:49.700 --> 00:42:57.940 align:middle line:84% And doing things like indexing starts to get confusing if-- 00:42:57.940 --> 00:42:59.900 align:middle line:90% it can be confusing. 00:42:59.900 --> 00:43:01.820 align:middle line:84% But yeah, both ways are very valid. 00:43:01.820 --> 00:43:06.240 align:middle line:84% So let's run it, and it should work. 00:43:06.240 --> 00:43:08.240 align:middle line:84% So the first count is 0, as we expected. 00:43:08.240 --> 00:43:11.510 align:middle line:90% And the second count is 2. 00:43:11.510 --> 00:43:13.040 align:middle line:90% Any questions about this code? 00:43:13.040 --> 00:43:14.700 align:middle line:90% Does it make sense? 00:43:14.700 --> 00:43:17.120 align:middle line:84% Is there another way that somebody tried it? 00:43:17.120 --> 00:43:19.920 align:middle line:90% 00:43:19.920 --> 00:43:20.420 align:middle line:90% Nope? 00:43:20.420 --> 00:43:21.470 align:middle line:90% OK, good. 00:43:21.470 --> 00:43:26.940 align:middle line:90% 00:43:26.940 --> 00:43:31.800 align:middle line:84% OK, so dictionaries are mutable objects, right? 00:43:31.800 --> 00:43:35.110 align:middle line:84% So all the aliasing and cloning rules apply. 00:43:35.110 --> 00:43:37.650 align:middle line:84% Remember when we talked about lists 00:43:37.650 --> 00:43:41.190 align:middle line:84% and using the equal sign between a list 00:43:41.190 --> 00:43:43.410 align:middle line:90% and another variable name? 00:43:43.410 --> 00:43:45.450 align:middle line:84% Just a plain old equal sign means 00:43:45.450 --> 00:43:48.300 align:middle line:84% that you are making an alias for that list. 00:43:48.300 --> 00:43:50.440 align:middle line:84% Same thing applies to dictionaries. 00:43:50.440 --> 00:43:54.185 align:middle line:84% So saying D1 equals D2, where D2 is a dictionary, 00:43:54.185 --> 00:43:55.560 align:middle line:84% it means that you've just created 00:43:55.560 --> 00:43:57.400 align:middle line:90% an alias for that dictionary. 00:43:57.400 --> 00:44:00.270 align:middle line:84% So if you change the dictionary for either of those variables, 00:44:00.270 --> 00:44:02.910 align:middle line:84% you're changing the object itself. 00:44:02.910 --> 00:44:04.530 align:middle line:84% If you want to actually make a copy, 00:44:04.530 --> 00:44:07.830 align:middle line:84% you use d.copy where d is the name of the dictionary you'd 00:44:07.830 --> 00:44:10.680 align:middle line:84% like a copy, and that gets you a copy of that dictionary. 00:44:10.680 --> 00:44:15.030 align:middle line:84% And then you can change it without changing the original. 00:44:15.030 --> 00:44:17.210 align:middle line:84% So let's talk a little bit about the values 00:44:17.210 --> 00:44:19.880 align:middle line:84% for a dictionary and the keys because there 00:44:19.880 --> 00:44:23.450 align:middle line:84% are some restrictions on the keys for the dictionary. 00:44:23.450 --> 00:44:25.350 align:middle line:90% No restrictions on the values. 00:44:25.350 --> 00:44:28.310 align:middle line:84% So dictionary values can be any type, right? 00:44:28.310 --> 00:44:30.440 align:middle line:84% You can have a dictionary value that's 00:44:30.440 --> 00:44:32.517 align:middle line:90% a float, int, string, tuple. 00:44:32.517 --> 00:44:34.100 align:middle line:84% You can have a dictionary value that's 00:44:34.100 --> 00:44:36.140 align:middle line:84% a list, which is immutable object. 00:44:36.140 --> 00:44:39.230 align:middle line:84% You can have a dictionary value that's another dictionary. 00:44:39.230 --> 00:44:43.020 align:middle line:84% All are OK, whatever you'd like for the values to be. 00:44:43.020 --> 00:44:45.380 align:middle line:84% You can have dictionary values that are duplicates. 00:44:45.380 --> 00:44:49.160 align:middle line:84% So you can have one key that maps to value 5, another key 00:44:49.160 --> 00:44:51.020 align:middle line:90% that maps to value 5. 00:44:51.020 --> 00:44:52.760 align:middle line:90% All good. 00:44:52.760 --> 00:44:54.900 align:middle line:90% OK? 00:44:54.900 --> 00:44:57.020 align:middle line:84% So the values don't need to be unique. 00:44:57.020 --> 00:45:00.380 align:middle line:84% We do have restrictions on the keys though, OK? 00:45:00.380 --> 00:45:02.330 align:middle line:84% The first restriction on the keys 00:45:02.330 --> 00:45:05.030 align:middle line:84% is that it has to be unique, right? 00:45:05.030 --> 00:45:11.990 align:middle line:84% So if you're mapping a key 1 to value 5, 00:45:11.990 --> 00:45:18.580 align:middle line:84% you cannot map a key 1 to value 6 because if you go and look up 00:45:18.580 --> 00:45:23.050 align:middle line:84% the value associated with 1, how does Python know which value 00:45:23.050 --> 00:45:26.080 align:middle line:84% you'd like, the 5 or the 6, right? 00:45:26.080 --> 00:45:30.050 align:middle line:84% So the keys have to be unique, first of all. 00:45:30.050 --> 00:45:33.950 align:middle line:84% Second, the keys have to be immutable, technically 00:45:33.950 --> 00:45:34.790 align:middle line:90% hashable. 00:45:34.790 --> 00:45:37.230 align:middle line:84% But for the purposes of this class, 00:45:37.230 --> 00:45:39.420 align:middle line:84% just think of them as having to be immutable. 00:45:39.420 --> 00:45:42.380 align:middle line:84% So a key can only be one of these types that we've 00:45:42.380 --> 00:45:45.110 align:middle line:84% seen so far, int, float, string, tuple, or bool. 00:45:45.110 --> 00:45:46.760 align:middle line:84% You cannot have a key that's a list. 00:45:46.760 --> 00:45:48.590 align:middle line:84% You cannot have a key that's a dictionary 00:45:48.590 --> 00:45:51.620 align:middle line:90% because they're mutable objects. 00:45:51.620 --> 00:45:56.180 align:middle line:84% So let's look at that a little bit further in detail. 00:45:56.180 --> 00:46:01.990 align:middle line:84% So the reason why we can't have a key that is mutable 00:46:01.990 --> 00:46:05.620 align:middle line:84% is because of the way keys are stored in Python, sorry, 00:46:05.620 --> 00:46:08.280 align:middle line:84% the way the dictionaries are stored in Python. 00:46:08.280 --> 00:46:10.530 align:middle line:84% So I'm going to show you an example on the next slide. 00:46:10.530 --> 00:46:12.447 align:middle line:84% First I'm going to explain how they're stored, 00:46:12.447 --> 00:46:14.760 align:middle line:84% and then we'll go through an example showing you 00:46:14.760 --> 00:46:19.410 align:middle line:84% exactly why you can't have a mutable structure. 00:46:19.410 --> 00:46:23.120 align:middle line:84% So the way dictionaries are stored in Python 00:46:23.120 --> 00:46:26.940 align:middle line:84% is you first need a key to associate with a value. 00:46:26.940 --> 00:46:28.910 align:middle line:84% So everything starts off with the key you'd 00:46:28.910 --> 00:46:31.170 align:middle line:90% like to add to your dictionary. 00:46:31.170 --> 00:46:35.580 align:middle line:84% So Python basically runs a function called a hash function 00:46:35.580 --> 00:46:37.865 align:middle line:90% on the key. 00:46:37.865 --> 00:46:40.490 align:middle line:84% For simplicity's sake, let's say the key you're trying to store 00:46:40.490 --> 00:46:41.120 align:middle line:90% is a number. 00:46:41.120 --> 00:46:43.730 align:middle line:84% That hash function might return that same number. 00:46:43.730 --> 00:46:45.740 align:middle line:90% It might return something else. 00:46:45.740 --> 00:46:48.980 align:middle line:84% If you're trying to store a string as a key, 00:46:48.980 --> 00:46:51.680 align:middle line:84% Python again runs maybe a different hash function 00:46:51.680 --> 00:46:55.580 align:middle line:84% that takes in that string, which might be a bunch of characters, 00:46:55.580 --> 00:46:57.660 align:middle line:84% and it converts it to some number. 00:46:57.660 --> 00:47:00.560 align:middle line:84% So the hash function always takes in your key 00:47:00.560 --> 00:47:04.590 align:middle line:90% and converts it to a number, OK? 00:47:04.590 --> 00:47:09.240 align:middle line:84% That number, think of it like representing a memory location 00:47:09.240 --> 00:47:13.765 align:middle line:84% where you're going to store the value associated with that key. 00:47:13.765 --> 00:47:15.390 align:middle line:84% So you're always grabbing a number that 00:47:15.390 --> 00:47:16.650 align:middle line:90% represents a memory location. 00:47:16.650 --> 00:47:19.340 align:middle line:90% 00:47:19.340 --> 00:47:22.020 align:middle line:84% At that memory location, you'll store the value. 00:47:22.020 --> 00:47:25.100 align:middle line:84% So next time you want to look up the value associated 00:47:25.100 --> 00:47:27.980 align:middle line:84% with a key, you just run that same hash function. 00:47:27.980 --> 00:47:29.600 align:middle line:90% The function won't change. 00:47:29.600 --> 00:47:31.850 align:middle line:84% You run the same hash function on your object, 00:47:31.850 --> 00:47:34.400 align:middle line:84% and you'll be able to get that same integer back. 00:47:34.400 --> 00:47:37.250 align:middle line:84% You'll be able to grab that same value back. 00:47:37.250 --> 00:47:39.400 align:middle line:84% But if you're storing mutable objects, 00:47:39.400 --> 00:47:41.360 align:middle line:84% that means that object can change. 00:47:41.360 --> 00:47:44.380 align:middle line:84% So if you run the hash function, the thing that gives you 00:47:44.380 --> 00:47:47.770 align:middle line:84% a number on something that's changed, 00:47:47.770 --> 00:47:50.767 align:middle line:84% that number might not be the same 00:47:50.767 --> 00:47:52.600 align:middle line:84% because you've changed the thing that you're 00:47:52.600 --> 00:47:53.990 align:middle line:90% passing into the function. 00:47:53.990 --> 00:47:57.240 align:middle line:84% So why would it give you the same value back? 00:47:57.240 --> 00:47:58.860 align:middle line:90% So let's look at this example. 00:47:58.860 --> 00:48:00.410 align:middle line:90% So again, we're storing grades. 00:48:00.410 --> 00:48:03.170 align:middle line:84% And let's say we're trying to store a bunch of grades 00:48:03.170 --> 00:48:04.100 align:middle line:90% inside our memory. 00:48:04.100 --> 00:48:10.580 align:middle line:84% And let's say our memory is just 16 locations, so 0 through 15. 00:48:10.580 --> 00:48:12.830 align:middle line:84% So at these locations, I'm going to store grades 00:48:12.830 --> 00:48:14.740 align:middle line:90% associated with a person. 00:48:14.740 --> 00:48:17.890 align:middle line:84% The function I'm going to run on the student 00:48:17.890 --> 00:48:19.790 align:middle line:90% is using their name. 00:48:19.790 --> 00:48:23.290 align:middle line:84% So I'm going to store Ana's grade somewhere. 00:48:23.290 --> 00:48:26.830 align:middle line:84% But I need to run a function that takes in the string Ana 00:48:26.830 --> 00:48:29.530 align:middle line:90% and gets for me a number. 00:48:29.530 --> 00:48:32.360 align:middle line:84% That number is where I'm going to store my grade. 00:48:32.360 --> 00:48:35.370 align:middle line:84% So a simple hash function we might do is to say, 00:48:35.370 --> 00:48:39.560 align:middle line:84% well, I'm going to take A and map it to 1, B, map it to 2, 00:48:39.560 --> 00:48:41.480 align:middle line:90% C map it to 3, and so on. 00:48:41.480 --> 00:48:46.700 align:middle line:84% I can sum all of those numbers associated with my letters 00:48:46.700 --> 00:48:48.510 align:middle line:90% in my name, 16. 00:48:48.510 --> 00:48:52.310 align:middle line:84% And then I can mod it with 16, which is how many entries I 00:48:52.310 --> 00:48:53.880 align:middle line:90% have in my memory. 00:48:53.880 --> 00:48:56.510 align:middle line:84% So if I mod it with 16, that's going to give me 00:48:56.510 --> 00:48:59.400 align:middle line:90% a number 0 through 15, right? 00:48:59.400 --> 00:49:01.400 align:middle line:84% If you take the remainder when you divide by 16, 00:49:01.400 --> 00:49:04.450 align:middle line:84% you'll either get 0 all the way up through 15. 00:49:04.450 --> 00:49:06.660 align:middle line:84% So if I mod my name, that means I'm 00:49:06.660 --> 00:49:13.510 align:middle line:84% going to store my grade at memory location 0. 00:49:13.510 --> 00:49:15.840 align:middle line:90% So far, so good? 00:49:15.840 --> 00:49:18.240 align:middle line:84% So basically, I made up this hash function that 00:49:18.240 --> 00:49:21.160 align:middle line:90% tells me where to put my grade. 00:49:21.160 --> 00:49:23.170 align:middle line:90% Now I add another person. 00:49:23.170 --> 00:49:26.230 align:middle line:84% Again, I'm going to convert the letters in their name 00:49:26.230 --> 00:49:30.610 align:middle line:84% to numbers so that I can easily get a number out 00:49:30.610 --> 00:49:31.870 align:middle line:90% of their letters. 00:49:31.870 --> 00:49:36.700 align:middle line:84% So I'm basically hashing their letters to a number. 00:49:36.700 --> 00:49:39.130 align:middle line:84% Again, summing this for Eric is 35. 00:49:39.130 --> 00:49:40.810 align:middle line:84% I'm going to mod it with 16, which 00:49:40.810 --> 00:49:45.430 align:middle line:84% means I'm going to put Eric's grade at location 3. 00:49:45.430 --> 00:49:47.320 align:middle line:90% Next person, John, same thing. 00:49:47.320 --> 00:49:49.210 align:middle line:90% I add the numbers, mod 16. 00:49:49.210 --> 00:49:51.830 align:middle line:84% I'm going to put John at location 15. 00:49:51.830 --> 00:49:55.390 align:middle line:84% So this is my memory where I'm storing the values associated 00:49:55.390 --> 00:49:56.540 align:middle line:90% with these students. 00:49:56.540 --> 00:49:59.950 align:middle line:84% So if I want to grab back my grade, 00:49:59.950 --> 00:50:02.750 align:middle line:84% I run the exact same hash function. 00:50:02.750 --> 00:50:05.590 align:middle line:84% So I'm going to run the same hash function on my name. 00:50:05.590 --> 00:50:07.000 align:middle line:90% My name hasn't changed, right? 00:50:07.000 --> 00:50:08.140 align:middle line:90% It's still the string. 00:50:08.140 --> 00:50:10.460 align:middle line:84% I'm not allowed to change it because it's a string. 00:50:10.460 --> 00:50:13.390 align:middle line:84% And so I'm going to get the same value back, 0. 00:50:13.390 --> 00:50:16.420 align:middle line:84% So that means to grab the letter associated with my name, 00:50:16.420 --> 00:50:18.940 align:middle line:84% I just need to go straight into my memory location 00:50:18.940 --> 00:50:23.630 align:middle line:84% and look up the value at that memory location. 00:50:23.630 --> 00:50:26.150 align:middle line:90% So I know it's going to be a C. 00:50:26.150 --> 00:50:35.280 align:middle line:84% Now, let's say I'm storing a list, a student name as a list. 00:50:35.280 --> 00:50:38.660 align:middle line:84% So again, Ana, Eric, and John are immutable, right? 00:50:38.660 --> 00:50:39.960 align:middle line:90% They will not change. 00:50:39.960 --> 00:50:44.440 align:middle line:84% But if I store Kate as a list, her name might change. 00:50:44.440 --> 00:50:46.690 align:middle line:84% Again, I can run the same hash function on her name. 00:50:46.690 --> 00:50:48.370 align:middle line:84% That means her grade when I first 00:50:48.370 --> 00:50:51.340 align:middle line:84% store it is going to be at location 5. 00:50:51.340 --> 00:50:54.530 align:middle line:84% So I'm storing Kate at location 5. 00:50:54.530 --> 00:50:58.340 align:middle line:84% All these three strings I know I can get back because they 00:50:58.340 --> 00:51:00.000 align:middle line:90% cannot change. 00:51:00.000 --> 00:51:03.290 align:middle line:84% But let's say that Kate goes and changes her name 00:51:03.290 --> 00:51:06.970 align:middle line:84% from Kate with a K to Kate with a C. It's 00:51:06.970 --> 00:51:09.880 align:middle line:84% the same object, right, the same person. 00:51:09.880 --> 00:51:12.460 align:middle line:84% She earned her grade, B, originally. 00:51:12.460 --> 00:51:14.830 align:middle line:84% So if I want to grab her grade back, 00:51:14.830 --> 00:51:16.640 align:middle line:84% even though her name has changed, 00:51:16.640 --> 00:51:18.670 align:middle line:84% I would still like to grab the B associated 00:51:18.670 --> 00:51:21.240 align:middle line:90% with her as a person. 00:51:21.240 --> 00:51:24.240 align:middle line:84% But Kate with a C, if I run that same hash function 00:51:24.240 --> 00:51:28.940 align:middle line:84% that I ran to put her grade in my table, 00:51:28.940 --> 00:51:32.930 align:middle line:84% tells me that I now need to look up her grade at memory location 00:51:32.930 --> 00:51:37.950 align:middle line:84% 13, no longer at memory location 5. 00:51:37.950 --> 00:51:41.050 align:middle line:90% She's not there. 00:51:41.050 --> 00:51:45.070 align:middle line:84% OK, so now it's like did the student disappear and all 00:51:45.070 --> 00:51:46.450 align:middle line:90% that stuff. 00:51:46.450 --> 00:51:47.140 align:middle line:90% So you see? 00:51:47.140 --> 00:51:52.990 align:middle line:84% Now, that's the reason why I cannot have a mutable object 00:51:52.990 --> 00:51:55.990 align:middle line:84% as a key to my list because if that object changes, 00:51:55.990 --> 00:51:59.170 align:middle line:84% running that hash function on that changed object might not 00:51:59.170 --> 00:52:01.660 align:middle line:84% give me the same memory location where I originally stored 00:52:01.660 --> 00:52:05.830 align:middle line:84% the value associated with that object. 00:52:05.830 --> 00:52:08.390 align:middle line:90% Does that make sense? 00:52:08.390 --> 00:52:12.160 align:middle line:84% OK, so let's revisit our original example, 00:52:12.160 --> 00:52:14.760 align:middle line:84% the one where we tried to store everything 00:52:14.760 --> 00:52:17.820 align:middle line:84% in a master list, all these grades in a master list. 00:52:17.820 --> 00:52:22.040 align:middle line:84% Now, let's store it in a master dictionary. 00:52:22.040 --> 00:52:23.690 align:middle line:90% So I've got my grades. 00:52:23.690 --> 00:52:26.990 align:middle line:84% Notice, curly bracket curly bracket is a dictionary. 00:52:26.990 --> 00:52:31.320 align:middle line:84% I've got two students in my class, Ana, right? 00:52:31.320 --> 00:52:33.120 align:middle line:84% So this is Ana's information and Bob. 00:52:33.120 --> 00:52:36.840 align:middle line:90% That's Bob's information. 00:52:36.840 --> 00:52:39.690 align:middle line:84% So just two students in my master dictionary. 00:52:39.690 --> 00:52:45.470 align:middle line:84% So the key Ana is going to be one entry key. 00:52:45.470 --> 00:52:47.600 align:middle line:90% Key Bob is the other entry key. 00:52:47.600 --> 00:52:50.780 align:middle line:84% And what's the information associated with these keys-- 00:52:50.780 --> 00:52:53.660 align:middle line:84% well, with Ana, I've got this dictionary 00:52:53.660 --> 00:52:55.400 align:middle line:90% associated with her name. 00:52:55.400 --> 00:52:57.360 align:middle line:90% So that's this big thing here. 00:52:57.360 --> 00:53:00.170 align:middle line:84% I'll explain it in a bit, and similarly with Bob. 00:53:00.170 --> 00:53:03.140 align:middle line:84% Bob has one thing associated with him, 00:53:03.140 --> 00:53:04.730 align:middle line:90% and it's another dictionary. 00:53:04.730 --> 00:53:07.560 align:middle line:90% 00:53:07.560 --> 00:53:10.830 align:middle line:84% So I'm mapping strings to dictionaries here. 00:53:10.830 --> 00:53:13.100 align:middle line:84% And that's fine because values in a dictionary 00:53:13.100 --> 00:53:15.920 align:middle line:90% can be other dictionaries. 00:53:15.920 --> 00:53:19.150 align:middle line:84% So what are these dictionaries about? 00:53:19.150 --> 00:53:22.360 align:middle line:84% Well, the number of items in the dictionary 00:53:22.360 --> 00:53:25.450 align:middle line:84% for a particular person, there's three elements, right? 00:53:25.450 --> 00:53:28.430 align:middle line:84% So comma, comma separates my three elements. 00:53:28.430 --> 00:53:32.230 align:middle line:84% The first one is going to be mapped with key mq. 00:53:32.230 --> 00:53:35.200 align:middle line:84% The second one mapped with key ps for problem set, 00:53:35.200 --> 00:53:39.210 align:middle line:84% and the last one mapped with the string fin for final grade. 00:53:39.210 --> 00:53:41.850 align:middle line:84% So each one of these students has this dictionary 00:53:41.850 --> 00:53:42.790 align:middle line:90% associated with them. 00:53:42.790 --> 00:53:45.060 align:middle line:84% And that dictionary then itself has 00:53:45.060 --> 00:53:47.640 align:middle line:84% three entries, one for the micro quiz, one for the ps, 00:53:47.640 --> 00:53:50.800 align:middle line:90% one for the final scores. 00:53:50.800 --> 00:53:54.310 align:middle line:84% So now, what's the values associated with those keys? 00:53:54.310 --> 00:53:56.530 align:middle line:84% Well, the micro quiz is going to be a list. 00:53:56.530 --> 00:53:58.280 align:middle line:84% The problem set is going to be a list. 00:53:58.280 --> 00:54:00.260 align:middle line:84% And the final is going to be a string. 00:54:00.260 --> 00:54:05.680 align:middle line:84% So a really nice representation of my class, right? 00:54:05.680 --> 00:54:07.940 align:middle line:90% And same for Bob. 00:54:07.940 --> 00:54:11.690 align:middle line:84% So now, what if we want to grab a student's exam 00:54:11.690 --> 00:54:15.770 align:middle line:84% grade or the student's list of exam grades? 00:54:15.770 --> 00:54:19.130 align:middle line:84% Remember that big function with the two nested 00:54:19.130 --> 00:54:20.810 align:middle line:90% for loops and the nested ifs? 00:54:20.810 --> 00:54:25.290 align:middle line:90% That becomes this line. 00:54:25.290 --> 00:54:25.995 align:middle line:90% Isn't that cool? 00:54:25.995 --> 00:54:28.683 align:middle line:90% 00:54:28.683 --> 00:54:29.600 align:middle line:90% Applause, I like that. 00:54:29.600 --> 00:54:30.770 align:middle line:90% Yeah, exactly. 00:54:30.770 --> 00:54:34.160 align:middle line:84% We should applaud this because look how easy it is now to-- 00:54:34.160 --> 00:54:35.660 align:middle line:90% yes, thank you, yes. 00:54:35.660 --> 00:54:37.700 align:middle line:90% [APPLAUSE] 00:54:37.700 --> 00:54:39.110 align:middle line:90% Dictionaries are awesome, guys. 00:54:39.110 --> 00:54:42.560 align:middle line:84% So yeah, so look, that line becomes this-- 00:54:42.560 --> 00:54:46.100 align:middle line:84% grabbing one quiz score becomes this single line of code 00:54:46.100 --> 00:54:47.480 align:middle line:90% right here. 00:54:47.480 --> 00:54:49.720 align:middle line:90% So let's break it down. 00:54:49.720 --> 00:54:51.970 align:middle line:84% Again, we do left to right whenever 00:54:51.970 --> 00:54:54.620 align:middle line:84% we've got this chain of stuff going on. 00:54:54.620 --> 00:54:57.130 align:middle line:84% So the first thing we say is, well, we're looking up 00:54:57.130 --> 00:54:59.140 align:middle line:90% grades at some index. 00:54:59.140 --> 00:55:02.110 align:middle line:84% So grades at some index gives me that dictionary, 00:55:02.110 --> 00:55:06.590 align:middle line:84% so something like this whole thing here, right. 00:55:06.590 --> 00:55:07.320 align:middle line:90% OK? 00:55:07.320 --> 00:55:07.820 align:middle line:90% Good. 00:55:07.820 --> 00:55:08.890 align:middle line:90% That's the first chain. 00:55:08.890 --> 00:55:12.580 align:middle line:84% Now, this box here gets replaced with that dictionary. 00:55:12.580 --> 00:55:15.100 align:middle line:84% And I'm doing another index into that dictionary. 00:55:15.100 --> 00:55:18.790 align:middle line:84% So that means I'm going to grab the mq associated 00:55:18.790 --> 00:55:23.460 align:middle line:90% with that dictionary. 00:55:23.460 --> 00:55:29.890 align:middle line:84% So the value associated with mq is going to be this list 544. 00:55:29.890 --> 00:55:33.570 align:middle line:84% So this box here gets replaced with the list 544. 00:55:33.570 --> 00:55:36.660 align:middle line:84% And then, if I want to grab just the first quiz value, 00:55:36.660 --> 00:55:40.110 align:middle line:84% I say now I'm going to index in the list 544 at index 0. 00:55:40.110 --> 00:55:41.880 align:middle line:90% So that grabs for me just the 5. 00:55:41.880 --> 00:55:46.630 align:middle line:84% So then the first quiz score for Ana was a 5. 00:55:46.630 --> 00:55:47.410 align:middle line:90% It's pretty bad. 00:55:47.410 --> 00:55:51.210 align:middle line:90% 00:55:51.210 --> 00:55:59.075 align:middle line:84% OK, so let's have you think about this. 00:55:59.075 --> 00:56:02.140 align:middle line:84% This is a function, nothing to code here. 00:56:02.140 --> 00:56:03.160 align:middle line:90% Just to think. 00:56:03.160 --> 00:56:08.850 align:middle line:84% This is a function that grabs the average 00:56:08.850 --> 00:56:17.250 align:middle line:84% of every single thing where that thing is what in the class. 00:56:17.250 --> 00:56:21.840 align:middle line:84% So if what is mq, as is down here in this example, 00:56:21.840 --> 00:56:24.860 align:middle line:84% if what is mq, this code is supposed 00:56:24.860 --> 00:56:27.440 align:middle line:84% to get the average of all of the micro quizzes 00:56:27.440 --> 00:56:29.340 align:middle line:84% for all the students in the class. 00:56:29.340 --> 00:56:32.420 align:middle line:84% So you basically want to grab the average of 10 00:56:32.420 --> 00:56:34.730 align:middle line:90% plus 8 plus 3. 00:56:34.730 --> 00:56:37.700 align:middle line:84% And if it's ps, I would like to grab 00:56:37.700 --> 00:56:39.507 align:middle line:84% the average of all the problem sets for all 00:56:39.507 --> 00:56:40.590 align:middle line:90% the students in the class. 00:56:40.590 --> 00:56:44.525 align:middle line:84% So the average of 10, 10, 7, 8, and 0. 00:56:44.525 --> 00:56:47.240 align:middle line:90% 00:56:47.240 --> 00:56:48.590 align:middle line:90% So we've got a loop. 00:56:48.590 --> 00:56:51.380 align:middle line:84% That goes through every student in the keys. 00:56:51.380 --> 00:56:56.540 align:middle line:84% So the student, stud, stud here is going 00:56:56.540 --> 00:56:59.270 align:middle line:90% to be this dictionary, right? 00:56:59.270 --> 00:57:02.240 align:middle line:84% So given this dictionary, what line 00:57:02.240 --> 00:57:06.770 align:middle line:84% should you insert here such that you're creating a list, just 00:57:06.770 --> 00:57:10.470 align:middle line:84% a single top-level list of all of the values in there? 00:57:10.470 --> 00:57:12.410 align:middle line:84% So the thing you actually want to end up with, 00:57:12.410 --> 00:57:14.208 align:middle line:84% and if we're looking at the ps scores 00:57:14.208 --> 00:57:16.250 align:middle line:84% just because it's a little easier to think about, 00:57:16.250 --> 00:57:22.970 align:middle line:84% is going to be 10, 10, 7, 8, and 0. 00:57:22.970 --> 00:57:25.400 align:middle line:84% So in the end, what I would like to get 00:57:25.400 --> 00:57:28.700 align:middle line:84% in my all data, this list that I'm maintaining here, 00:57:28.700 --> 00:57:33.510 align:middle line:84% is something like this for the ps. 00:57:33.510 --> 00:57:35.850 align:middle line:84% So think about which one of these lines 00:57:35.850 --> 00:57:37.170 align:middle line:90% will accomplish that. 00:57:37.170 --> 00:57:41.250 align:middle line:90% 00:57:41.250 --> 00:57:44.430 align:middle line:84% And just to help you out we can say 00:57:44.430 --> 00:57:48.870 align:middle line:90% student is my dictionary of-- 00:57:48.870 --> 00:57:50.385 align:middle line:90% who thinks it's the first one? 00:57:50.385 --> 00:57:53.030 align:middle line:90% 00:57:53.030 --> 00:57:55.130 align:middle line:90% The second one? 00:57:55.130 --> 00:57:57.270 align:middle line:90% Third one? 00:57:57.270 --> 00:57:58.630 align:middle line:90% Fourth one? 00:57:58.630 --> 00:58:02.550 align:middle line:90% 00:58:02.550 --> 00:58:04.440 align:middle line:84% Nobody thought it's the first one? 00:58:04.440 --> 00:58:06.900 align:middle line:90% Are you guys sure? 00:58:06.900 --> 00:58:09.540 align:middle line:84% OK, why do you think it's the second one? 00:58:09.540 --> 00:58:11.700 align:middle line:90% Is it because of the append? 00:58:11.700 --> 00:58:12.750 align:middle line:90% Yeah? 00:58:12.750 --> 00:58:14.020 align:middle line:90% Let's think about it. 00:58:14.020 --> 00:58:18.240 align:middle line:84% So all data is a list, and what are we appending? 00:58:18.240 --> 00:58:23.630 align:middle line:84% So what is data at stud at what going to give us? 00:58:23.630 --> 00:58:29.070 align:middle line:84% Data at student is the dictionary, this dictionary 00:58:29.070 --> 00:58:32.810 align:middle line:90% here, right, this value here. 00:58:32.810 --> 00:58:36.910 align:middle line:84% And if we take this value and index into the what, 00:58:36.910 --> 00:58:39.280 align:middle line:84% will it be an integer or will it be a list? 00:58:39.280 --> 00:58:42.120 align:middle line:90% 00:58:42.120 --> 00:58:43.120 align:middle line:90% A list. 00:58:43.120 --> 00:58:46.470 align:middle line:84% So when we append a list to another list, 00:58:46.470 --> 00:58:47.940 align:middle line:90% what is that going to give us? 00:58:47.940 --> 00:58:51.300 align:middle line:90% 00:58:51.300 --> 00:58:55.040 align:middle line:84% So if we have a list already with a, 00:58:55.040 --> 00:58:57.710 align:middle line:84% b, c, if I append another list to this, 00:58:57.710 --> 00:59:00.807 align:middle line:84% will it put the element within that list or the list itself? 00:59:00.807 --> 00:59:01.640 align:middle line:90% STUDENT: [INAUDIBLE] 00:59:01.640 --> 00:59:02.900 align:middle line:90% ANA BELL: Yeah, exactly. 00:59:02.900 --> 00:59:05.850 align:middle line:90% 00:59:05.850 --> 00:59:08.450 align:middle line:84% So that's not going to work for us. 00:59:08.450 --> 00:59:11.510 align:middle line:90% Clearly, D is not right either. 00:59:11.510 --> 00:59:14.750 align:middle line:84% And definitely, indexing into data at student at what 00:59:14.750 --> 00:59:16.790 align:middle line:90% is not going to be right, OK? 00:59:16.790 --> 00:59:21.060 align:middle line:90% So that leaves one other choice. 00:59:21.060 --> 00:59:23.650 align:middle line:90% The first one. 00:59:23.650 --> 00:59:26.860 align:middle line:84% So let's see why the first one works. 00:59:26.860 --> 00:59:28.330 align:middle line:90% We're concatenating, right? 00:59:28.330 --> 00:59:30.280 align:middle line:90% So the plus concatenates. 00:59:30.280 --> 00:59:34.390 align:middle line:84% So let's say I already have a list, a, b, c. 00:59:34.390 --> 00:59:35.980 align:middle line:84% I'm going to concatenate something 00:59:35.980 --> 00:59:38.920 align:middle line:84% I already have with data at student 00:59:38.920 --> 00:59:40.870 align:middle line:90% at what, which we said is what? 00:59:40.870 --> 00:59:42.707 align:middle line:84% Is it a single element or another list? 00:59:42.707 --> 00:59:43.540 align:middle line:90% STUDENT: [INAUDIBLE] 00:59:43.540 --> 00:59:45.220 align:middle line:84% ANA BELL: Exactly, so we concatenate 00:59:45.220 --> 00:59:49.280 align:middle line:84% with something like 10, 10, or something like that. 00:59:49.280 --> 00:59:54.610 align:middle line:84% So that will return for us a, b, c, 10, 10, 00:59:54.610 --> 00:59:57.280 align:middle line:84% which will allow us to do something like getting 00:59:57.280 --> 01:00:00.925 align:middle line:90% the sum of all these elements. 01:00:00.925 --> 01:00:01.800 align:middle line:90% Questions about that? 01:00:01.800 --> 01:00:02.520 align:middle line:90% Does that make sense? 01:00:02.520 --> 01:00:03.330 align:middle line:90% Is that all right? 01:00:03.330 --> 01:00:04.110 align:middle line:90% Yes. 01:00:04.110 --> 01:00:06.858 align:middle line:90% STUDENT: [INAUDIBLE] 01:00:06.858 --> 01:00:08.000 align:middle line:90% 01:00:08.000 --> 01:00:08.625 align:middle line:90% ANA BELL: Yeah. 01:00:08.625 --> 01:00:11.180 align:middle line:90% 01:00:11.180 --> 01:00:14.046 align:middle line:84% STUDENT: Why would it [INAUDIBLE]?? 01:00:14.046 --> 01:00:18.540 align:middle line:90% 01:00:18.540 --> 01:00:24.138 align:middle line:84% Like, how come it returns 10 [INAUDIBLE]?? 01:00:24.138 --> 01:00:26.790 align:middle line:84% ANA BELL: Because we're indexing into mq. 01:00:26.790 --> 01:00:30.030 align:middle line:84% So if you index into mq, mq is your key. 01:00:30.030 --> 01:00:32.890 align:middle line:84% So you grab the value associated with that key. 01:00:32.890 --> 01:00:35.340 align:middle line:84% So that would be this list here, the 10. 01:00:35.340 --> 01:00:38.770 align:middle line:84% Or for ps, it would be the list 10, 10. 01:00:38.770 --> 01:00:42.880 align:middle line:84% So quick recap on lists and dictionaries before we do one 01:00:42.880 --> 01:00:44.480 align:middle line:90% final longer example. 01:00:44.480 --> 01:00:47.680 align:middle line:84% So again, lists are ordered sequences of elements, right? 01:00:47.680 --> 01:00:49.820 align:middle line:84% There is some element at index 0. 01:00:49.820 --> 01:00:52.540 align:middle line:84% There's some element at index 1, some element at index 2. 01:00:52.540 --> 01:00:56.590 align:middle line:84% So we do have these quote unquote "indices," right? 01:00:56.590 --> 01:00:59.290 align:middle line:84% But there's an order to these indices, 01:00:59.290 --> 01:01:02.440 align:middle line:84% and there has to be an element index 0 and further 01:01:02.440 --> 01:01:04.600 align:middle line:90% up from there, right? 01:01:04.600 --> 01:01:07.090 align:middle line:84% Dictionaries also have these quote unquote 01:01:07.090 --> 01:01:08.830 align:middle line:90% "indices," which we call keys. 01:01:08.830 --> 01:01:09.970 align:middle line:90% But these are custom. 01:01:09.970 --> 01:01:13.360 align:middle line:90% So you can basically rearrange-- 01:01:13.360 --> 01:01:16.450 align:middle line:84% you can think of it as being allowed to rearrange indices 01:01:16.450 --> 01:01:17.590 align:middle line:90% however you'd like, right? 01:01:17.590 --> 01:01:22.470 align:middle line:84% There's no order to the indices in a dictionary. 01:01:22.470 --> 01:01:25.410 align:middle line:84% There are some restrictions on the keys or these indices, 01:01:25.410 --> 01:01:28.920 align:middle line:84% so they can't be immutable or hashable. 01:01:28.920 --> 01:01:32.100 align:middle line:84% But other than that, the things that you 01:01:32.100 --> 01:01:35.460 align:middle line:84% store related to that key can be any type, 01:01:35.460 --> 01:01:39.920 align:middle line:84% just like you can store any type in a list. 01:01:39.920 --> 01:01:43.963 align:middle line:84% So the last thing I'd like to go through is a larger example. 01:01:43.963 --> 01:01:45.880 align:middle line:84% And this will showcase a whole bunch of things 01:01:45.880 --> 01:01:47.880 align:middle line:84% that we've been talking about so far. 01:01:47.880 --> 01:01:51.880 align:middle line:84% It'll showcase how to, first of all, 01:01:51.880 --> 01:01:54.040 align:middle line:84% create dictionaries, which is what we did today. 01:01:54.040 --> 01:01:57.080 align:middle line:84% It'll showcase how to reuse functions, 01:01:57.080 --> 01:02:00.140 align:middle line:84% how to write functions and reuse functions in other places. 01:02:00.140 --> 01:02:03.760 align:middle line:84% It will showcase a little bit of mutability as well. 01:02:03.760 --> 01:02:06.740 align:middle line:84% But this is all in a larger example. 01:02:06.740 --> 01:02:09.850 align:middle line:84% And if I go a little bit fast through this, 01:02:09.850 --> 01:02:11.590 align:middle line:84% I've given you Python tutor links. 01:02:11.590 --> 01:02:15.770 align:middle line:84% And it's also in the file to run on your own. 01:02:15.770 --> 01:02:20.800 align:middle line:84% So the goal of this last example is 01:02:20.800 --> 01:02:23.380 align:middle line:84% to basically find the most common words 01:02:23.380 --> 01:02:24.700 align:middle line:90% in a song's lyrics. 01:02:24.700 --> 01:02:27.340 align:middle line:84% And dictionaries are going to be really useful for doing 01:02:27.340 --> 01:02:28.575 align:middle line:90% something like this. 01:02:28.575 --> 01:02:30.200 align:middle line:84% So I'm going to show you, first of all, 01:02:30.200 --> 01:02:31.520 align:middle line:90% what we want to end up with. 01:02:31.520 --> 01:02:33.910 align:middle line:84% And then we can talk about how to divide this larger 01:02:33.910 --> 01:02:37.980 align:middle line:90% problem into smaller pieces. 01:02:37.980 --> 01:02:41.550 align:middle line:84% So OK, so those are all the pieces. 01:02:41.550 --> 01:02:43.080 align:middle line:84% But basically what I want to end up 01:02:43.080 --> 01:02:51.640 align:middle line:84% with is I want to have a song be stored as a string, OK? 01:02:51.640 --> 01:02:53.908 align:middle line:84% You'll recognize these, but these are very old. 01:02:53.908 --> 01:02:56.200 align:middle line:84% I actually haven't updated these songs for a few years. 01:02:56.200 --> 01:02:58.060 align:middle line:90% But yeah, anyway, don't judge. 01:02:58.060 --> 01:03:02.150 align:middle line:84% So I've got a song stored as a string. 01:03:02.150 --> 01:03:04.910 align:middle line:84% And I'm going to run each individual function. 01:03:04.910 --> 01:03:07.430 align:middle line:84% But in the end, what I'd like to do 01:03:07.430 --> 01:03:10.560 align:middle line:84% is come up with something like this. 01:03:10.560 --> 01:03:15.350 align:middle line:84% So I want to present the user the top-most common words 01:03:15.350 --> 01:03:17.000 align:middle line:90% in the song. 01:03:17.000 --> 01:03:18.517 align:middle line:90% So here I have a list. 01:03:18.517 --> 01:03:20.600 align:middle line:84% So you can see open closed square bracket tells me 01:03:20.600 --> 01:03:21.230 align:middle line:90% it's a list. 01:03:21.230 --> 01:03:23.160 align:middle line:84% And I've got elements in my list. 01:03:23.160 --> 01:03:25.340 align:middle line:84% So here's the first element in my list, 01:03:25.340 --> 01:03:30.770 align:middle line:84% which tells the user that the word I occurs 18 times. 01:03:30.770 --> 01:03:33.170 align:middle line:84% The next element in my list tells the user 01:03:33.170 --> 01:03:35.810 align:middle line:84% that the word we occurs 17 times. 01:03:35.810 --> 01:03:38.750 align:middle line:84% The next element tells the user that the words 01:03:38.750 --> 01:03:43.400 align:middle line:84% ain't ever getting older occur all 16 times, and then so on. 01:03:43.400 --> 01:03:46.700 align:middle line:84% So we're decreasing in frequency with the most common word 01:03:46.700 --> 01:03:48.140 align:middle line:90% occurring 18 times. 01:03:48.140 --> 01:03:55.550 align:middle line:84% And then I'm showing the user the most common words down to 01:03:55.550 --> 01:03:56.540 align:middle line:90% and including 6. 01:03:56.540 --> 01:03:58.550 align:middle line:84% So I would choose some arbitrary value 01:03:58.550 --> 01:04:01.100 align:middle line:84% I want to find in the song the words that occur at least 01:04:01.100 --> 01:04:03.360 align:middle line:90% six times, for example. 01:04:03.360 --> 01:04:05.325 align:middle line:84% OK, so that's the goal of this program. 01:04:05.325 --> 01:04:07.910 align:middle line:90% 01:04:07.910 --> 01:04:09.190 align:middle line:90% So how will we achieve this? 01:04:09.190 --> 01:04:11.350 align:middle line:84% It's obviously a pretty big task. 01:04:11.350 --> 01:04:16.390 align:middle line:84% I wouldn't want to code the entire thing right off the bat. 01:04:16.390 --> 01:04:20.910 align:middle line:84% But we can actually divide it into three smaller pieces. 01:04:20.910 --> 01:04:23.990 align:middle line:84% The first piece, and we're going to write the code for this, 01:04:23.990 --> 01:04:26.510 align:middle line:84% is to create something called a frequency dictionary. 01:04:26.510 --> 01:04:28.880 align:middle line:84% So given a string of words, we're 01:04:28.880 --> 01:04:32.390 align:middle line:84% going to create a dictionary that maps each word to how 01:04:32.390 --> 01:04:34.020 align:middle line:90% often it occurs. 01:04:34.020 --> 01:04:36.350 align:middle line:84% So fancy word, frequency dictionary, 01:04:36.350 --> 01:04:37.560 align:middle line:90% but it's pretty simple. 01:04:37.560 --> 01:04:42.770 align:middle line:84% It just maps a word to its count inside my long string. 01:04:42.770 --> 01:04:46.780 align:middle line:84% So this presents the data which is this string of words 01:04:46.780 --> 01:04:48.400 align:middle line:90% in a much nicer format, right? 01:04:48.400 --> 01:04:52.900 align:middle line:84% It's a dictionary that tells me the frequency of each word. 01:04:52.900 --> 01:04:56.080 align:middle line:84% Once I have that in hand, things get a little bit easier. 01:04:56.080 --> 01:04:58.060 align:middle line:84% I can write another function that 01:04:58.060 --> 01:05:02.450 align:middle line:84% finds the word that occurs most often in that dictionary. 01:05:02.450 --> 01:05:04.540 align:middle line:84% So the way I'm going to do that is look up 01:05:04.540 --> 01:05:07.270 align:middle line:84% the frequencies in the values, find 01:05:07.270 --> 01:05:10.090 align:middle line:84% the maximum of those values, and then figure out 01:05:10.090 --> 01:05:14.990 align:middle line:84% which keys are associated with that maximum value. 01:05:14.990 --> 01:05:17.000 align:middle line:84% And this is all made possible because I've 01:05:17.000 --> 01:05:21.200 align:middle line:84% reimagined my data in this frequency dictionary format. 01:05:21.200 --> 01:05:24.580 align:middle line:84% The last step, once I figure out how 01:05:24.580 --> 01:05:27.760 align:middle line:84% to write a function that returns for me the words that 01:05:27.760 --> 01:05:30.550 align:middle line:84% occur the most times, is to find the words that occur 01:05:30.550 --> 01:05:33.690 align:middle line:90% at least some number of times. 01:05:33.690 --> 01:05:37.620 align:middle line:84% And I'll go through an example of this one in a few slides 01:05:37.620 --> 01:05:38.670 align:middle line:90% when we get to it. 01:05:38.670 --> 01:05:41.610 align:middle line:84% But this last function here, number three, you 01:05:41.610 --> 01:05:43.653 align:middle line:84% can actually rewrite it in a whole bunch of ways. 01:05:43.653 --> 01:05:45.570 align:middle line:84% I'm just going to show you one way to write it 01:05:45.570 --> 01:05:47.220 align:middle line:90% that will involve mutability. 01:05:47.220 --> 01:05:50.490 align:middle line:84% But you don't have to do it using mutability. 01:05:50.490 --> 01:05:53.310 align:middle line:84% You can definitely do it in a whole bunch of-- 01:05:53.310 --> 01:05:56.670 align:middle line:84% with a whole bunch of other implementations. 01:05:56.670 --> 01:06:01.520 align:middle line:84% So let's begin by first creating a dictionary that maps 01:06:01.520 --> 01:06:03.840 align:middle line:90% the word to their frequencies. 01:06:03.840 --> 01:06:08.540 align:middle line:84% So I've picked a song that is a real song. 01:06:08.540 --> 01:06:10.940 align:middle line:90% And it has some repetition. 01:06:10.940 --> 01:06:13.850 align:middle line:84% And it's short that it fits in one line. 01:06:13.850 --> 01:06:16.010 align:middle line:90% So I've got this song here. 01:06:16.010 --> 01:06:18.830 align:middle line:84% And I've got my function generate word dictionary. 01:06:18.830 --> 01:06:22.290 align:middle line:90% The song is a string. 01:06:22.290 --> 01:06:24.980 align:middle line:84% So it's basically the song a little bit cleaned up, 01:06:24.980 --> 01:06:29.450 align:middle line:84% not in terms of words, but in terms of removing punctuation, 01:06:29.450 --> 01:06:31.820 align:middle line:84% removing commas, maybe exclamation-- 01:06:31.820 --> 01:06:36.210 align:middle line:84% or I might have kept quotations or something like that. 01:06:36.210 --> 01:06:40.040 align:middle line:84% But basically, it's removing all of the punctuation and stuff 01:06:40.040 --> 01:06:43.700 align:middle line:84% because that will mess up my word counts. 01:06:43.700 --> 01:06:45.280 align:middle line:84% So what is this function going to do 01:06:45.280 --> 01:06:47.560 align:middle line:90% given a string for my song? 01:06:47.560 --> 01:06:51.320 align:middle line:84% Well, first I'm going to convert all my letters to lowercase. 01:06:51.320 --> 01:06:53.810 align:middle line:84% This means that capital T-H-E will 01:06:53.810 --> 01:06:58.760 align:middle line:84% be counted as the same word as lowercase t-h-e, which 01:06:58.760 --> 01:07:00.500 align:middle line:90% is the correct way to do it. 01:07:00.500 --> 01:07:02.870 align:middle line:84% So convert everything to lowercase. 01:07:02.870 --> 01:07:05.070 align:middle line:84% Then I'm going to use our friend, 01:07:05.070 --> 01:07:08.420 align:middle line:84% the split function, remember, which takes in my string 01:07:08.420 --> 01:07:09.990 align:middle line:90% and splits on a character. 01:07:09.990 --> 01:07:12.790 align:middle line:84% So by default, it will split on the space. 01:07:12.790 --> 01:07:19.600 align:middle line:84% This puts our string of words in a very manageable format, 01:07:19.600 --> 01:07:21.760 align:middle line:90% a list of words. 01:07:21.760 --> 01:07:25.830 align:middle line:84% Much nicer to work with lists than work with a string. 01:07:25.830 --> 01:07:27.830 align:middle line:84% Now that I have my word list, I'm 01:07:27.830 --> 01:07:29.600 align:middle line:84% going to create my empty dictionary 01:07:29.600 --> 01:07:31.700 align:middle line:90% and then populate it. 01:07:31.700 --> 01:07:35.170 align:middle line:84% So I'm iterating over my list of words. 01:07:35.170 --> 01:07:37.540 align:middle line:90% And then I have a choice. 01:07:37.540 --> 01:07:39.940 align:middle line:84% Either I've seen this word already 01:07:39.940 --> 01:07:42.250 align:middle line:84% and I want to update the frequency, right? 01:07:42.250 --> 01:07:44.020 align:middle line:84% So I want to increase the frequency by 1 01:07:44.020 --> 01:07:47.590 align:middle line:84% because I've already added this word to my dictionary. 01:07:47.590 --> 01:07:50.440 align:middle line:84% Or this is the first time I'm seeing this word 01:07:50.440 --> 01:07:52.000 align:middle line:84% and I want to add it to my dictionary 01:07:52.000 --> 01:07:54.050 align:middle line:90% with a frequency of 1. 01:07:54.050 --> 01:07:58.010 align:middle line:84% So the first case here, the if, will update the frequency 01:07:58.010 --> 01:08:00.300 align:middle line:84% because I've already seen the word in my dictionary. 01:08:00.300 --> 01:08:03.950 align:middle line:84% So here I'm using in keyword to check if the key, the word, 01:08:03.950 --> 01:08:05.540 align:middle line:90% is already in my dictionary. 01:08:05.540 --> 01:08:08.710 align:middle line:84% If so, I increase its frequency by 1. 01:08:08.710 --> 01:08:10.510 align:middle line:84% Otherwise, this is the first time 01:08:10.510 --> 01:08:12.320 align:middle line:84% I'm adding my word to my dictionary. 01:08:12.320 --> 01:08:15.700 align:middle line:90% So give it a frequency of 1. 01:08:15.700 --> 01:08:17.840 align:middle line:84% And then I return the word dictionary. 01:08:17.840 --> 01:08:22.279 align:middle line:84% So this will map strings to integers. 01:08:22.279 --> 01:08:24.534 align:middle line:84% Let's work through it in the Python tutor. 01:08:24.534 --> 01:08:30.040 align:middle line:90% 01:08:30.040 --> 01:08:36.220 align:middle line:84% So step, step, step, step, step, lowercased my input string, 01:08:36.220 --> 01:08:40.450 align:middle line:84% step, I've split it, so now I've got this list of all 01:08:40.450 --> 01:08:44.100 align:middle line:90% of my words, step. 01:08:44.100 --> 01:08:45.300 align:middle line:90% This is where we begin. 01:08:45.300 --> 01:08:48.270 align:middle line:84% So I've created my empty dictionary over here. 01:08:48.270 --> 01:08:49.910 align:middle line:90% Keep an eye on this area here. 01:08:49.910 --> 01:08:52.220 align:middle line:90% It will become populated soon. 01:08:52.220 --> 01:08:55.548 align:middle line:84% The first word, w, is rah, right? 01:08:55.548 --> 01:08:56.840 align:middle line:90% It's the first word in my list. 01:08:56.840 --> 01:08:59.970 align:middle line:84% It's obviously the first time I'm seeing it. 01:08:59.970 --> 01:09:01.970 align:middle line:84% I have nothing in my dictionary right now. 01:09:01.970 --> 01:09:04.040 align:middle line:84% So I'm going to pop in my else, and I'm 01:09:04.040 --> 01:09:08.439 align:middle line:84% going to add it to my dictionary with a frequency of 1. 01:09:08.439 --> 01:09:09.880 align:middle line:90% Yay, that worked. 01:09:09.880 --> 01:09:15.550 align:middle line:84% Next word in my dictionary, in my list is this one, same word 01:09:15.550 --> 01:09:17.029 align:middle line:90% I've already seen. 01:09:17.029 --> 01:09:19.210 align:middle line:84% So I'm going to go inside the if and increase 01:09:19.210 --> 01:09:21.910 align:middle line:90% the frequency to 2, all right? 01:09:21.910 --> 01:09:23.550 align:middle line:90% rah is now 2. 01:09:23.550 --> 01:09:26.880 align:middle line:90% Next is ah, right? 01:09:26.880 --> 01:09:27.899 align:middle line:90% So here's my word. 01:09:27.899 --> 01:09:29.970 align:middle line:84% I've got the next one in my list. 01:09:29.970 --> 01:09:31.560 align:middle line:84% It's the first time I'm seeing it. 01:09:31.560 --> 01:09:34.350 align:middle line:84% Add it to my dictionary with a frequency of 1. 01:09:34.350 --> 01:09:37.380 align:middle line:90% Next word I'm seeing is, again. 01:09:37.380 --> 01:09:39.420 align:middle line:90% Increase its frequency to 2. 01:09:39.420 --> 01:09:41.430 align:middle line:90% And I'm going to go faster now. 01:09:41.430 --> 01:09:44.100 align:middle line:84% This is increasing the frequency to 3 01:09:44.100 --> 01:09:45.689 align:middle line:84% because I've seen it three times now. 01:09:45.689 --> 01:09:49.920 align:middle line:84% And then I'm adding rom for the first time, 01:09:49.920 --> 01:09:55.610 align:middle line:84% mah for the first time, and ro for the first time. 01:09:55.610 --> 01:10:00.260 align:middle line:84% And lastly, I'm going to increase mah frequency 01:10:00.260 --> 01:10:03.510 align:middle line:84% two more times because it occurs two more times in my song. 01:10:03.510 --> 01:10:06.170 align:middle line:84% So it's increased to 2, and now it's increased to 3. 01:10:06.170 --> 01:10:07.320 align:middle line:90% And then we're done. 01:10:07.320 --> 01:10:10.220 align:middle line:84% So we return the word dictionary. 01:10:10.220 --> 01:10:14.000 align:middle line:84% Really nice way to represent my list-- 01:10:14.000 --> 01:10:15.230 align:middle line:90% my song, right? 01:10:15.230 --> 01:10:18.040 align:middle line:90% Very nice. 01:10:18.040 --> 01:10:20.620 align:middle line:84% OK, so now that I have this frequency dictionary 01:10:20.620 --> 01:10:24.100 align:middle line:84% and I've put it up here-- this is what we ended up with-- 01:10:24.100 --> 01:10:26.410 align:middle line:84% how can we write a function that returns for me 01:10:26.410 --> 01:10:29.480 align:middle line:90% the most frequent word? 01:10:29.480 --> 01:10:34.150 align:middle line:84% So one thing we can recognize is the most frequent word 01:10:34.150 --> 01:10:38.170 align:middle line:84% has the highest value, Python dictionary 01:10:38.170 --> 01:10:40.660 align:middle line:90% value in my dictionary, right? 01:10:40.660 --> 01:10:46.390 align:middle line:84% So as a human, I would look to see which one of these entries 01:10:46.390 --> 01:10:47.620 align:middle line:90% have the biggest value. 01:10:47.620 --> 01:10:50.470 align:middle line:84% As a computer, I can't really do that because I 01:10:50.470 --> 01:10:53.270 align:middle line:84% have to do it a little bit more systematically. 01:10:53.270 --> 01:10:56.920 align:middle line:84% So what we can say is, well, let's look at our values 01:10:56.920 --> 01:10:59.360 align:middle line:84% and grab the maximum of the values. 01:10:59.360 --> 01:11:04.000 align:middle line:84% So here, I'm using this .values function on my dictionary 01:11:04.000 --> 01:11:08.060 align:middle line:84% to grab for me all of the values in my dictionary. 01:11:08.060 --> 01:11:12.310 align:middle line:84% So this will be the list 2 comma 3 comma 1 comma 3 comma 1. 01:11:12.310 --> 01:11:15.260 align:middle line:84% And then I'm running the max function on that list. 01:11:15.260 --> 01:11:18.550 align:middle line:84% So max of this list of numbers gives me the maximum value 01:11:18.550 --> 01:11:20.580 align:middle line:90% in that list, the 3. 01:11:20.580 --> 01:11:24.600 align:middle line:84% So highest now has the value integer 3. 01:11:24.600 --> 01:11:29.130 align:middle line:84% And now, all I need to do is iterate over my entries 01:11:29.130 --> 01:11:30.760 align:middle line:90% in my dictionary. 01:11:30.760 --> 01:11:33.090 align:middle line:90% So this is kv in the items. 01:11:33.090 --> 01:11:36.180 align:middle line:84% And all I'm checking now inside this iteration 01:11:36.180 --> 01:11:39.670 align:middle line:84% is if the value is equal to the highest. 01:11:39.670 --> 01:11:41.370 align:middle line:84% So as I'm looking at each entry, is 01:11:41.370 --> 01:11:43.890 align:middle line:84% the value for that entry the same as the highest one 01:11:43.890 --> 01:11:44.820 align:middle line:90% I've seen? 01:11:44.820 --> 01:11:47.868 align:middle line:84% If it is, I'm going to maintain a list of all the words 01:11:47.868 --> 01:11:49.410 align:middle line:84% with that highest value because there 01:11:49.410 --> 01:11:52.920 align:middle line:84% might be more than one word that has the highest 01:11:52.920 --> 01:11:56.340 align:middle line:84% value, as we saw when we actually ran it here, right? 01:11:56.340 --> 01:11:59.220 align:middle line:84% Here I had a list of all of these words 01:11:59.220 --> 01:12:00.630 align:middle line:90% that occurred 16 times. 01:12:00.630 --> 01:12:06.180 align:middle line:84% So that's the output that I want to maintain, OK? 01:12:06.180 --> 01:12:07.933 align:middle line:84% So I'm appending to my words list. 01:12:07.933 --> 01:12:09.600 align:middle line:84% And at the end, I'm returning this tuple 01:12:09.600 --> 01:12:12.640 align:middle line:84% with the words comma that highest value. 01:12:12.640 --> 01:12:18.760 align:middle line:84% So Python tutor, like in the previous time. 01:12:18.760 --> 01:12:21.030 align:middle line:84% So let's create our original dictionary. 01:12:21.030 --> 01:12:22.720 align:middle line:84% This is what we ended up with last time. 01:12:22.720 --> 01:12:24.750 align:middle line:90% So the highest value is 3 here. 01:12:24.750 --> 01:12:27.790 align:middle line:84% And I'm going to loop through each entry in my dictionary. 01:12:27.790 --> 01:12:31.600 align:middle line:84% So you can say C kv is going to be each one of these in order. 01:12:31.600 --> 01:12:34.720 align:middle line:84% So first it's rah w, then it's ah 3, and so on. 01:12:34.720 --> 01:12:38.260 align:middle line:84% Obviously, the 2 is not equal to the 3, so we move on. 01:12:38.260 --> 01:12:41.980 align:middle line:84% The 3 equals the 3, so we take the ah, and boom, 01:12:41.980 --> 01:12:43.083 align:middle line:90% add it to my list here. 01:12:43.083 --> 01:12:45.250 align:middle line:84% So this is the list I'm maintaining of all the words 01:12:45.250 --> 01:12:48.640 align:middle line:90% that occur with frequency 3. 01:12:48.640 --> 01:12:52.300 align:middle line:84% Next, no for rom, yes for mah, so I'm 01:12:52.300 --> 01:12:53.950 align:middle line:90% going to add it to my list. 01:12:53.950 --> 01:12:58.760 align:middle line:84% And then no for ro, and I'm done. 01:12:58.760 --> 01:13:02.350 align:middle line:84% So the return is going to be this list-- 01:13:02.350 --> 01:13:04.480 align:middle line:84% this tuple here with the list of the words that 01:13:04.480 --> 01:13:05.320 align:middle line:90% occur three times. 01:13:05.320 --> 01:13:09.180 align:middle line:90% 01:13:09.180 --> 01:13:12.040 align:middle line:90% OK, good. 01:13:12.040 --> 01:13:14.500 align:middle line:84% Last part, I'm not going to go through Python tutor. 01:13:14.500 --> 01:13:16.540 align:middle line:84% I did include a link to it because it becomes 01:13:16.540 --> 01:13:17.920 align:middle line:90% very messy with the arrows. 01:13:17.920 --> 01:13:21.010 align:middle line:84% But I do encourage you to try to follow it along 01:13:21.010 --> 01:13:22.280 align:middle line:90% on your own time. 01:13:22.280 --> 01:13:25.540 align:middle line:84% I will explain, however, the way that I 01:13:25.540 --> 01:13:27.020 align:middle line:90% chose to solve this problem. 01:13:27.020 --> 01:13:31.900 align:middle line:84% So I chose to solve this problem to include mutation and reusing 01:13:31.900 --> 01:13:34.930 align:middle line:84% the function that we just wrote that grabs for me 01:13:34.930 --> 01:13:38.380 align:middle line:84% the highest value and the words associated with that highest 01:13:38.380 --> 01:13:40.900 align:middle line:90% frequency value. 01:13:40.900 --> 01:13:43.960 align:middle line:90% So this is the idea. 01:13:43.960 --> 01:13:47.350 align:middle line:84% I have my original word dictionary, right? 01:13:47.350 --> 01:13:50.740 align:middle line:84% This is the frequency dictionary we created right off the bat. 01:13:50.740 --> 01:13:56.170 align:middle line:84% What I'm going to do is look to see which words occur 01:13:56.170 --> 01:13:58.460 align:middle line:90% with the highest frequency. 01:13:58.460 --> 01:14:01.390 align:middle line:84% So the highest frequency, my function from before, 01:14:01.390 --> 01:14:02.800 align:middle line:90% grabs for me-- 01:14:02.800 --> 01:14:04.570 align:middle line:90% it figures out that it's 3. 01:14:04.570 --> 01:14:06.640 align:middle line:84% And it figures out the words associated 01:14:06.640 --> 01:14:10.650 align:middle line:90% with that 3 are ah and mah. 01:14:10.650 --> 01:14:12.930 align:middle line:90% That's exactly what we just did. 01:14:12.930 --> 01:14:16.430 align:middle line:84% So I'm going to grab those words and those entries 01:14:16.430 --> 01:14:17.510 align:middle line:90% in the dictionary. 01:14:17.510 --> 01:14:20.270 align:middle line:84% And then, I'm going to mutate the dictionary 01:14:20.270 --> 01:14:23.060 align:middle line:84% to remove those words because I know those words occur 01:14:23.060 --> 01:14:26.780 align:middle line:90% with the highest frequency. 01:14:26.780 --> 01:14:28.990 align:middle line:84% So now, I've removed those words, 01:14:28.990 --> 01:14:31.660 align:middle line:84% and I've saved them because they were the result of the function 01:14:31.660 --> 01:14:33.880 align:middle line:90% that I had just ran, right? 01:14:33.880 --> 01:14:36.670 align:middle line:84% So I'm maintaining this frequency list, 01:14:36.670 --> 01:14:39.340 align:middle line:84% which will contain all the words that occur at least-- 01:14:39.340 --> 01:14:45.430 align:middle line:84% I guess I said greater than one time, so at least two times. 01:14:45.430 --> 01:14:48.320 align:middle line:84% So I'm going to grab the ones that occur 3 and 2 times. 01:14:48.320 --> 01:14:52.490 align:middle line:84% So right now, I had just grabbed the words that occur 3 times. 01:14:52.490 --> 01:14:54.930 align:middle line:84% I've removed them from my dictionary. 01:14:54.930 --> 01:14:56.570 align:middle line:84% So I've actually mutated my dictionary 01:14:56.570 --> 01:14:58.840 align:middle line:90% to remove those words. 01:14:58.840 --> 01:15:02.080 align:middle line:84% Now, if I run the exact same function 01:15:02.080 --> 01:15:04.870 align:middle line:84% that I just wrote on the previous slide 01:15:04.870 --> 01:15:10.180 align:middle line:84% on this mutated dictionary, which words will it give me? 01:15:10.180 --> 01:15:12.924 align:middle line:90% Which words occur the most now? 01:15:12.924 --> 01:15:13.840 align:middle line:90% STUDENT: [INAUDIBLE] 01:15:13.840 --> 01:15:15.100 align:middle line:90% ANA BELL: Exactly, right? 01:15:15.100 --> 01:15:17.080 align:middle line:84% Now the highest value in my dictionary, 01:15:17.080 --> 01:15:19.750 align:middle line:84% in this frequency dictionary, is 2 01:15:19.750 --> 01:15:22.480 align:middle line:84% because I mutated to remove what was previously the highest 01:15:22.480 --> 01:15:23.180 align:middle line:90% value. 01:15:23.180 --> 01:15:26.710 align:middle line:84% So I'm running the same function again on the mutated dictionary 01:15:26.710 --> 01:15:29.560 align:middle line:90% to give me just the rah, right? 01:15:29.560 --> 01:15:34.450 align:middle line:84% So I grab that, keep track of that in my frequency list, 01:15:34.450 --> 01:15:37.790 align:middle line:84% right, mutate the dictionary to remove that. 01:15:37.790 --> 01:15:40.990 align:middle line:84% And as I'm doing that, I'm also keeping track 01:15:40.990 --> 01:15:43.150 align:middle line:84% to make sure that the highest frequency I have 01:15:43.150 --> 01:15:46.030 align:middle line:84% in the remaining dictionary is at least whatever 01:15:46.030 --> 01:15:46.910 align:middle line:90% I was interested in. 01:15:46.910 --> 01:15:48.580 align:middle line:90% So here I want it at least 2. 01:15:48.580 --> 01:15:51.610 align:middle line:84% So this function, the one I will write, 01:15:51.610 --> 01:15:54.730 align:middle line:84% will no longer grab any other values from the dictionary 01:15:54.730 --> 01:15:57.800 align:middle line:84% because now frequency 1 I don't want to grab. 01:15:57.800 --> 01:16:01.007 align:middle line:90% So this is the resulting value. 01:16:01.007 --> 01:16:01.840 align:middle line:90% And that's the idea. 01:16:01.840 --> 01:16:03.465 align:middle line:84% We're using mutability and the function 01:16:03.465 --> 01:16:05.450 align:middle line:90% we just wrote to do this task. 01:16:05.450 --> 01:16:07.000 align:middle line:84% And this is the code that does that. 01:16:07.000 --> 01:16:10.120 align:middle line:84% So this runs the function we wrote previously. 01:16:10.120 --> 01:16:13.780 align:middle line:84% Step number 2 gives us that tuple 01:16:13.780 --> 01:16:15.490 align:middle line:90% with the list of all the words. 01:16:15.490 --> 01:16:18.310 align:middle line:84% This loop here makes sure I still 01:16:18.310 --> 01:16:22.000 align:middle line:84% have frequencies that are at least x in the dictionary. 01:16:22.000 --> 01:16:25.220 align:middle line:84% I grab the tuple that I just created, 01:16:25.220 --> 01:16:28.160 align:middle line:84% so something like this, and add it to my frequency list. 01:16:28.160 --> 01:16:30.710 align:middle line:84% So this is the resulting list that I'm keeping track of. 01:16:30.710 --> 01:16:33.980 align:middle line:84% And then this bit here removes the word from my dictionary. 01:16:33.980 --> 01:16:35.740 align:middle line:84% So I'm mutating the dictionary using 01:16:35.740 --> 01:16:39.980 align:middle line:84% this del keyword that we saw at the beginning of this lecture. 01:16:39.980 --> 01:16:42.580 align:middle line:90% Yeah, question. 01:16:42.580 --> 01:16:46.630 align:middle line:84% STUDENT: The other one like I know it says whilst basically 01:16:46.630 --> 01:16:50.410 align:middle line:90% while the 01:16:50.410 --> 01:16:52.156 align:middle line:90% ANA BELL: Yeah. 01:16:52.156 --> 01:16:55.330 align:middle line:84% STUDENT: I guess what I just [INAUDIBLE] 01:16:55.330 --> 01:17:01.410 align:middle line:84% why is still [INAUDIBLE] well the [INAUDIBLE]?? 01:17:01.410 --> 01:17:03.800 align:middle line:84% ANA BELL: So I think maybe it's because the function-- 01:17:03.800 --> 01:17:05.490 align:middle line:84% I forget what the specification said, 01:17:05.490 --> 01:17:09.680 align:middle line:84% but I don't know if it said at least 2 or greater than 2, 01:17:09.680 --> 01:17:11.900 align:middle line:90% or at least x or greater than x. 01:17:11.900 --> 01:17:13.550 align:middle line:84% It depends on which one I actually 01:17:13.550 --> 01:17:15.020 align:middle line:90% said in the specification. 01:17:15.020 --> 01:17:17.690 align:middle line:84% But you can imagine changing this to greater than or greater 01:17:17.690 --> 01:17:18.830 align:middle line:90% than or equal to. 01:17:18.830 --> 01:17:21.080 align:middle line:84% And then we're running this function again 01:17:21.080 --> 01:17:25.410 align:middle line:84% inside this while loop to grab the frequency [INAUDIBLE].. 01:17:25.410 --> 01:17:27.360 align:middle line:84% Yeah, so these are just the observations 01:17:27.360 --> 01:17:29.650 align:middle line:84% I actually stated at the beginning of this example, 01:17:29.650 --> 01:17:32.400 align:middle line:84% a bunch of the different things that we've learned that we're 01:17:32.400 --> 01:17:34.060 align:middle line:90% using within this example. 01:17:34.060 --> 01:17:38.040 align:middle line:84% So slicing or splitting, iterating over the list 01:17:38.040 --> 01:17:43.550 align:middle line:84% directly, mutability, using the items, things like that. 01:17:43.550 --> 01:17:45.130 align:middle line:90% OK, so that's it. 01:17:45.130 --> 01:17:47.080 align:middle line:90% That's all I have. 01:17:47.080 --> 01:17:48.300 align:middle line:90% I'll see you guys on Monday. 01:17:48.300 --> 01:17:49.710 align:middle line:90% Monday is Halloween. 01:17:49.710 --> 01:17:54.390 align:middle line:84% If you'd like to bring a costume, I love Halloween. 01:17:54.390 --> 01:17:59.510 align:middle line:84% I will wear something different than what I usually wear. 01:17:59.510 --> 01:18:05.000 align:middle line:90%