WEBVTT 00:00:00.000 --> 00:00:01.984 align:middle line:90% [SQUEAKING] 00:00:01.984 --> 00:00:03.968 align:middle line:90% [RUSTLING] 00:00:03.968 --> 00:00:06.448 align:middle line:90% [CLICKING] 00:00:06.448 --> 00:00:16.870 align:middle line:90% 00:00:16.870 --> 00:00:20.020 align:middle line:84% ANA BELL: So let's start today's lecture. 00:00:20.020 --> 00:00:23.440 align:middle line:84% Today, we're going to be talking about the idea of iteration. 00:00:23.440 --> 00:00:25.090 align:middle line:84% And iteration is another way we're 00:00:25.090 --> 00:00:28.700 align:middle line:84% going to add control flow to our programs. 00:00:28.700 --> 00:00:34.990 align:middle line:84% But before we do that, let's do a little bit of a recap on-- 00:00:34.990 --> 00:00:37.360 align:middle line:84% sorry, let's do a little bit of a recap of what we've 00:00:37.360 --> 00:00:39.710 align:middle line:84% done so far last lecture because last lecture, 00:00:39.710 --> 00:00:41.830 align:middle line:84% we actually introduced a different mechanism 00:00:41.830 --> 00:00:44.320 align:middle line:90% for control flow, branching. 00:00:44.320 --> 00:00:46.720 align:middle line:84% And the control flow is basically a way for us 00:00:46.720 --> 00:00:49.300 align:middle line:84% to tell Python not to go systematically 00:00:49.300 --> 00:00:50.620 align:middle line:90% through the code. 00:00:50.620 --> 00:00:53.620 align:middle line:84% Branching was a way for us to tell Python, hey, based 00:00:53.620 --> 00:00:55.720 align:middle line:84% on some condition being true or false, 00:00:55.720 --> 00:00:59.920 align:middle line:84% either evaluate some set of code or another set of code, 00:00:59.920 --> 00:01:01.283 align:middle line:90% which was not going linearly. 00:01:01.283 --> 00:01:02.950 align:middle line:84% We were actually kind of skipping around 00:01:02.950 --> 00:01:04.830 align:middle line:90% through the code. 00:01:04.830 --> 00:01:07.190 align:middle line:84% So that's what we learned at the end of the lecture. 00:01:07.190 --> 00:01:10.200 align:middle line:84% But we also learned about input and output-- 00:01:10.200 --> 00:01:12.830 align:middle line:84% so a way for us to write interactive programs. 00:01:12.830 --> 00:01:15.930 align:middle line:84% And we learned about a new data type, the string. 00:01:15.930 --> 00:01:18.200 align:middle line:84% So the string was a sequence of characters. 00:01:18.200 --> 00:01:21.740 align:middle line:84% Hopefully, you got a chance to do a little bit of exercises 00:01:21.740 --> 00:01:25.640 align:middle line:84% on MITx as practice for today's quizzes 00:01:25.640 --> 00:01:28.970 align:middle line:90% with strings and branching. 00:01:28.970 --> 00:01:29.470 align:middle line:90% OK. 00:01:29.470 --> 00:01:31.670 align:middle line:84% So in branching, what did we learn? 00:01:31.670 --> 00:01:33.880 align:middle line:84% We talked about how to actually add 00:01:33.880 --> 00:01:35.540 align:middle line:84% a branching point in our program. 00:01:35.540 --> 00:01:38.555 align:middle line:84% So we did that using these particular keywords. 00:01:38.555 --> 00:01:40.930 align:middle line:84% So when you type them in your program in the file editor, 00:01:40.930 --> 00:01:42.880 align:middle line:84% you'll see that they turn a different color. 00:01:42.880 --> 00:01:45.670 align:middle line:84% That tells you it's a special word in Python. 00:01:45.670 --> 00:01:48.310 align:middle line:84% And these keywords are how we told Python 00:01:48.310 --> 00:01:51.070 align:middle line:90% to put a branching point. 00:01:51.070 --> 00:01:57.100 align:middle line:84% And the colon of ended the branching conditional. 00:01:57.100 --> 00:01:59.650 align:middle line:84% And then anything that was indented 00:01:59.650 --> 00:02:03.340 align:middle line:84% as part of that conditional was code that would be executed 00:02:03.340 --> 00:02:04.930 align:middle line:90% when that condition was true. 00:02:04.930 --> 00:02:06.430 align:middle line:84% So I'm just going to quickly go over 00:02:06.430 --> 00:02:08.330 align:middle line:90% these, each one of these boxes. 00:02:08.330 --> 00:02:11.140 align:middle line:84% So the first one up here was the simplest way 00:02:11.140 --> 00:02:13.240 align:middle line:84% that we could add a conditional to our program. 00:02:13.240 --> 00:02:15.640 align:middle line:84% It basically said, hey, go through the program, when 00:02:15.640 --> 00:02:18.520 align:middle line:84% you reach this if condition, Python would 00:02:18.520 --> 00:02:21.880 align:middle line:84% check the condition and say, if that condition is true, 00:02:21.880 --> 00:02:26.510 align:middle line:84% execute the code that's indented as part of that block. 00:02:26.510 --> 00:02:28.850 align:middle line:84% If the condition was not true, don't do anything, 00:02:28.850 --> 00:02:31.880 align:middle line:84% just carry on with the remaining program. 00:02:31.880 --> 00:02:34.790 align:middle line:84% If we wanted to do something else-- so if the condition was 00:02:34.790 --> 00:02:36.650 align:middle line:84% not true, if we wanted to do something else, 00:02:36.650 --> 00:02:39.350 align:middle line:90% we added this else clause here. 00:02:39.350 --> 00:02:43.250 align:middle line:84% And the else also has some sort of code indented 00:02:43.250 --> 00:02:44.630 align:middle line:90% as part of its code block. 00:02:44.630 --> 00:02:50.820 align:middle line:84% And that code would be executed when that condition was false. 00:02:50.820 --> 00:02:51.390 align:middle line:90% OK. 00:02:51.390 --> 00:02:54.810 align:middle line:84% So that was a really simple if or if/else code structure. 00:02:54.810 --> 00:02:56.490 align:middle line:84% But sometimes, we want to have code 00:02:56.490 --> 00:02:59.190 align:middle line:84% that checks for many conditions, not just one. 00:02:59.190 --> 00:03:01.980 align:middle line:84% That's where the elif structure came in. 00:03:01.980 --> 00:03:06.670 align:middle line:84% So we would have an if condition that starts our code block. 00:03:06.670 --> 00:03:08.800 align:middle line:84% If that condition was true, as usual, 00:03:08.800 --> 00:03:11.740 align:middle line:84% we execute the code that's part of that block. 00:03:11.740 --> 00:03:15.280 align:middle line:84% Else if, so elif, we could insert another condition. 00:03:15.280 --> 00:03:17.787 align:middle line:84% And Python would say, OK, well if that one wasn't true, 00:03:17.787 --> 00:03:19.370 align:middle line:84% let me check if this next one is true. 00:03:19.370 --> 00:03:20.787 align:middle line:84% And then we would execute the code 00:03:20.787 --> 00:03:23.050 align:middle line:90% that's part of that code block. 00:03:23.050 --> 00:03:26.740 align:middle line:84% We can chain as many of these elifs as we want together. 00:03:26.740 --> 00:03:30.430 align:middle line:84% And Python will evaluate the very first one 00:03:30.430 --> 00:03:33.730 align:middle line:84% that it finds true that's part of this chain, 00:03:33.730 --> 00:03:36.250 align:middle line:90% even if more than one is true. 00:03:36.250 --> 00:03:38.660 align:middle line:84% It is possible none of those conditions were true, 00:03:38.660 --> 00:03:41.590 align:middle line:84% in which case Python would basically skip over all of them 00:03:41.590 --> 00:03:45.190 align:middle line:84% and do nothing, enter none of those code blocks. 00:03:45.190 --> 00:03:47.740 align:middle line:84% If you wanted to have a structure where if none 00:03:47.740 --> 00:03:50.740 align:middle line:84% of those conditions were true you wanted to do something, 00:03:50.740 --> 00:03:54.880 align:middle line:84% you could put an else at the end of a whole chain of if, elif, 00:03:54.880 --> 00:03:56.240 align:middle line:90% elif, elif's. 00:03:56.240 --> 00:03:59.290 align:middle line:84% And the else would be executed when none of those conditions 00:03:59.290 --> 00:04:00.410 align:middle line:90% are true. 00:04:00.410 --> 00:04:03.830 align:middle line:84% So hopefully, this is just recap. 00:04:03.830 --> 00:04:08.720 align:middle line:84% One tricky thing to remember is the if starts a code block. 00:04:08.720 --> 00:04:10.850 align:middle line:84% So the if can have an else associated 00:04:10.850 --> 00:04:13.650 align:middle line:84% with it or it can have an elif, elif, elif, 00:04:13.650 --> 00:04:16.220 align:middle line:90% and an else associated with it. 00:04:16.220 --> 00:04:19.730 align:middle line:84% But if condition and then followed by another if 00:04:19.730 --> 00:04:21.860 align:middle line:84% condition, both of those code blocks 00:04:21.860 --> 00:04:23.660 align:middle line:84% could potentially be executed because the 00:04:23.660 --> 00:04:24.710 align:middle line:90% if's are independent. 00:04:24.710 --> 00:04:26.180 align:middle line:90% It's not an else situation. 00:04:26.180 --> 00:04:30.695 align:middle line:84% They're just another if code block that gets started. 00:04:30.695 --> 00:04:33.490 align:middle line:90% 00:04:33.490 --> 00:04:34.870 align:middle line:84% Again, just to reiterate, the way 00:04:34.870 --> 00:04:37.328 align:middle line:84% you told Python which code to execute when the condition is 00:04:37.328 --> 00:04:39.160 align:middle line:90% true is by indentation. 00:04:39.160 --> 00:04:41.950 align:middle line:84% And indentation is something you have to do. 00:04:41.950 --> 00:04:45.340 align:middle line:90% It's not optional in Python. 00:04:45.340 --> 00:04:45.840 align:middle line:90% OK. 00:04:45.840 --> 00:04:47.510 align:middle line:84% So let's take what we've learned so far 00:04:47.510 --> 00:04:49.430 align:middle line:84% and code up a really simple game. 00:04:49.430 --> 00:04:51.710 align:middle line:84% So this is a very simple variation 00:04:51.710 --> 00:04:54.410 align:middle line:84% of the Lost Woods in Zelda, my version of it. 00:04:54.410 --> 00:04:58.340 align:middle line:84% Let's say it's a trick level where you have your character 00:04:58.340 --> 00:05:00.050 align:middle line:90% and they enter the Lost Woods. 00:05:00.050 --> 00:05:01.940 align:middle line:84% They're presented with this screen. 00:05:01.940 --> 00:05:05.030 align:middle line:84% And the trick here is you ask the user if they 00:05:05.030 --> 00:05:06.710 align:middle line:90% want to go left or right. 00:05:06.710 --> 00:05:08.810 align:middle line:84% If they say right, you're basically 00:05:08.810 --> 00:05:10.850 align:middle line:84% going to present them with the exact same screen 00:05:10.850 --> 00:05:11.790 align:middle line:90% all over again. 00:05:11.790 --> 00:05:14.900 align:middle line:84% So it's kind of representing that they're lost in the woods. 00:05:14.900 --> 00:05:17.025 align:middle line:84% And as long as they say I want to keep going right, 00:05:17.025 --> 00:05:19.317 align:middle line:84% I want to keep going right, I want to keep going right, 00:05:19.317 --> 00:05:21.960 align:middle line:84% they're basically going to see the same screen over and over 00:05:21.960 --> 00:05:22.460 align:middle line:90% again. 00:05:22.460 --> 00:05:24.500 align:middle line:84% And the trick to getting out of the woods 00:05:24.500 --> 00:05:28.420 align:middle line:90% is to say I want to go left. 00:05:28.420 --> 00:05:30.970 align:middle line:84% So no matter how far, how many times they've 00:05:30.970 --> 00:05:34.180 align:middle line:84% said right in a row, as long as they type in left, 00:05:34.180 --> 00:05:36.340 align:middle line:90% they're out of the woods. 00:05:36.340 --> 00:05:38.755 align:middle line:84% So let's try to code that up with what we know, 00:05:38.755 --> 00:05:40.370 align:middle line:90% just conditionals. 00:05:40.370 --> 00:05:44.740 align:middle line:90% We have an if/else. 00:05:44.740 --> 00:05:47.320 align:middle line:84% The if says, if the user exits right, 00:05:47.320 --> 00:05:48.760 align:middle line:90% we're going to do something. 00:05:48.760 --> 00:05:52.660 align:middle line:84% And otherwise, we're going to say that the user said left, 00:05:52.660 --> 00:05:54.040 align:middle line:90% or something else, or exit. 00:05:54.040 --> 00:05:55.957 align:middle line:84% And then we're going to tell them that they've 00:05:55.957 --> 00:05:57.290 align:middle line:90% exited successfully. 00:05:57.290 --> 00:05:57.790 align:middle line:90% All right. 00:05:57.790 --> 00:05:59.623 align:middle line:84% Now, if they said exit right, what do we do? 00:05:59.623 --> 00:06:02.040 align:middle line:84% Well, we're going to show them the exact same thing again. 00:06:02.040 --> 00:06:04.810 align:middle line:84% So we're setting the background to the same woods background. 00:06:04.810 --> 00:06:07.837 align:middle line:84% And then they're presented with the choice all over again. 00:06:07.837 --> 00:06:08.920 align:middle line:90% Do you want to exit right? 00:06:08.920 --> 00:06:10.087 align:middle line:90% Or do you want to exit left? 00:06:10.087 --> 00:06:13.770 align:middle line:84% So if they say exit right, we would do something. 00:06:13.770 --> 00:06:18.600 align:middle line:84% And otherwise, we would tell them they successfully exited. 00:06:18.600 --> 00:06:20.370 align:middle line:90% Well, what if they exited right? 00:06:20.370 --> 00:06:23.280 align:middle line:84% If they exited right, then we would do something again, 00:06:23.280 --> 00:06:26.530 align:middle line:84% basically present them with the same situation. 00:06:26.530 --> 00:06:29.400 align:middle line:84% So we would set the woods back around again. 00:06:29.400 --> 00:06:31.950 align:middle line:84% And we would ask them if they want to go right or left again. 00:06:31.950 --> 00:06:35.180 align:middle line:84% And otherwise, if they said left, they exit. 00:06:35.180 --> 00:06:37.700 align:middle line:84% So we already see a problem, right? 00:06:37.700 --> 00:06:41.240 align:middle line:84% How deep do we make this nested loop situation? 00:06:41.240 --> 00:06:43.740 align:middle line:84% Here, we already have three, in the case the user said, 00:06:43.740 --> 00:06:45.830 align:middle line:84% I want to go right three times in a row. 00:06:45.830 --> 00:06:48.380 align:middle line:84% But we don't know how persistent the user will be. 00:06:48.380 --> 00:06:50.660 align:middle line:84% So how do we know, when we're writing our code, 00:06:50.660 --> 00:06:53.880 align:middle line:84% how deep to make this nested loop? 00:06:53.880 --> 00:06:55.140 align:middle line:90% We don't. 00:06:55.140 --> 00:06:57.870 align:middle line:84% We won't be able to really code this up very well 00:06:57.870 --> 00:06:59.500 align:middle line:90% with what we know so far. 00:06:59.500 --> 00:07:02.610 align:middle line:84% And so that's the motivation for introducing iteration 00:07:02.610 --> 00:07:05.010 align:middle line:84% because the situation on the previous slide 00:07:05.010 --> 00:07:07.080 align:middle line:84% fits really well with some task we 00:07:07.080 --> 00:07:09.870 align:middle line:84% want to repeat multiple times, as long 00:07:09.870 --> 00:07:11.280 align:middle line:90% as some condition is true. 00:07:11.280 --> 00:07:14.410 align:middle line:84% In our case, the condition is the user says, 00:07:14.410 --> 00:07:15.330 align:middle line:90% I want to exit right. 00:07:15.330 --> 00:07:18.420 align:middle line:84% So while the user keeps saying exit right, 00:07:18.420 --> 00:07:20.880 align:middle line:84% show them the woods background and ask them again 00:07:20.880 --> 00:07:22.830 align:middle line:90% which way do you want to go. 00:07:22.830 --> 00:07:26.720 align:middle line:84% And so while that's true, just repeat this set of things-- 00:07:26.720 --> 00:07:29.930 align:middle line:84% check that they said exit right, show them the woods background, 00:07:29.930 --> 00:07:32.120 align:middle line:84% ask them again, check that they said exit right, 00:07:32.120 --> 00:07:34.260 align:middle line:84% show them the background, ask them again. 00:07:34.260 --> 00:07:37.550 align:middle line:84% And if at any point they'd say I don't want to exit right, 00:07:37.550 --> 00:07:42.830 align:middle line:84% we break out of this loop and we rejoin the rest of the program. 00:07:42.830 --> 00:07:45.440 align:middle line:84% That's the terminology we use with if statements. 00:07:45.440 --> 00:07:47.330 align:middle line:84% We set the background to the exit background 00:07:47.330 --> 00:07:50.010 align:middle line:90% and they're out of the woods. 00:07:50.010 --> 00:07:52.560 align:middle line:84% So this sets the scene for a while loops. 00:07:52.560 --> 00:07:55.350 align:middle line:84% Here's another example of while loops in the context 00:07:55.350 --> 00:07:56.800 align:middle line:90% of watching a show. 00:07:56.800 --> 00:07:59.400 align:middle line:84% So if we want to start a new show on Netflix 00:07:59.400 --> 00:08:02.100 align:middle line:84% and we want to watch all episodes of the show in one 00:08:02.100 --> 00:08:04.110 align:middle line:84% shot, we're going to tell Netflix 00:08:04.110 --> 00:08:05.340 align:middle line:90% we're starting a new show. 00:08:05.340 --> 00:08:09.470 align:middle line:84% And while there are more episodes to watch in this show, 00:08:09.470 --> 00:08:12.260 align:middle line:84% we're going to keep watching the next episode. 00:08:12.260 --> 00:08:15.845 align:middle line:84% So if there are no more episodes to watch, then we're done. 00:08:15.845 --> 00:08:17.900 align:middle line:84% Python-- not Python, Netflix will suggest 00:08:17.900 --> 00:08:19.650 align:middle line:90% three more shows like this one. 00:08:19.650 --> 00:08:22.370 align:middle line:84% And while there are more episodes to watch-- so yes, 00:08:22.370 --> 00:08:24.110 align:middle line:84% there are more episodes to watch-- 00:08:24.110 --> 00:08:28.450 align:middle line:84% we're going to play the next episode in sequence. 00:08:28.450 --> 00:08:29.950 align:middle line:84% So that's the idea that we're trying 00:08:29.950 --> 00:08:31.540 align:middle line:90% to get at with while loops. 00:08:31.540 --> 00:08:33.429 align:middle line:84% In Python, this is how we code them. 00:08:33.429 --> 00:08:36.830 align:middle line:84% So we start a while loop with the keyword while. 00:08:36.830 --> 00:08:38.740 align:middle line:84% So this, again, will turn blue in Python 00:08:38.740 --> 00:08:40.270 align:middle line:90% because it's a special word. 00:08:40.270 --> 00:08:43.429 align:middle line:90% Some condition is true. 00:08:43.429 --> 00:08:45.950 align:middle line:84% So this is, again, some expression or something 00:08:45.950 --> 00:08:49.460 align:middle line:84% that will evaluate to a Boolean, like we talked about 00:08:49.460 --> 00:08:52.190 align:middle line:90% in last lecture. 00:08:52.190 --> 00:08:54.560 align:middle line:84% Colon-- and colon tells Python we're 00:08:54.560 --> 00:08:56.720 align:middle line:84% starting a code block that's part 00:08:56.720 --> 00:08:59.120 align:middle line:90% of the while loop being true. 00:08:59.120 --> 00:09:01.040 align:middle line:84% And as usual, the code block means 00:09:01.040 --> 00:09:04.230 align:middle line:84% we're going to indent these lines of code. 00:09:04.230 --> 00:09:07.880 align:middle line:84% So whatever we want to execute when the condition is true 00:09:07.880 --> 00:09:11.080 align:middle line:90% will be indented. 00:09:11.080 --> 00:09:14.620 align:middle line:84% When the indented statements are finished executing, 00:09:14.620 --> 00:09:20.270 align:middle line:84% Python automatically goes back and re-checks the condition. 00:09:20.270 --> 00:09:23.360 align:middle line:84% So it re-checks whether the condition is true or not. 00:09:23.360 --> 00:09:25.450 align:middle line:84% And this is done behind the scenes. 00:09:25.450 --> 00:09:27.580 align:middle line:84% When you code up a while loop, when 00:09:27.580 --> 00:09:30.640 align:middle line:84% you type in the keyword while, Python will automatically 00:09:30.640 --> 00:09:31.810 align:middle line:90% do this behavior. 00:09:31.810 --> 00:09:33.220 align:middle line:90% It will check the condition. 00:09:33.220 --> 00:09:35.410 align:middle line:84% It will execute the lines of code indented. 00:09:35.410 --> 00:09:37.940 align:middle line:84% And then it will go back and check the condition again. 00:09:37.940 --> 00:09:39.910 align:middle line:84% If it's still true, it will execute the lines 00:09:39.910 --> 00:09:40.900 align:middle line:90% of code indented again. 00:09:40.900 --> 00:09:42.700 align:middle line:84% And then it will check the condition again. 00:09:42.700 --> 00:09:44.560 align:middle line:84% So it's not something you have to code up. 00:09:44.560 --> 00:09:46.390 align:middle line:84% You don't have to tell it to go back. 00:09:46.390 --> 00:09:48.190 align:middle line:84% As long as you're writing this while loop, 00:09:48.190 --> 00:09:54.350 align:middle line:84% Python will automatically do that sequence of steps for you. 00:09:54.350 --> 00:09:56.420 align:middle line:84% So when the condition becomes false, 00:09:56.420 --> 00:09:59.610 align:middle line:84% Python will no longer execute the stuff inside, 00:09:59.610 --> 00:10:01.610 align:middle line:84% the stuff that's indented inside the while loop. 00:10:01.610 --> 00:10:04.430 align:middle line:84% And it'll go rejoin the rest of the program 00:10:04.430 --> 00:10:09.320 align:middle line:84% at the same indentation level as the while loop. 00:10:09.320 --> 00:10:13.610 align:middle line:84% So notice that the condition is kind of something 00:10:13.610 --> 00:10:15.980 align:middle line:90% that's dependent on-- 00:10:15.980 --> 00:10:20.420 align:middle line:84% or sorry, the fact that we're doing this code over and over 00:10:20.420 --> 00:10:23.510 align:middle line:84% again depends on this condition being true. 00:10:23.510 --> 00:10:27.700 align:middle line:84% So if the code inside is not ever 00:10:27.700 --> 00:10:30.490 align:middle line:84% changing anything related to our condition, 00:10:30.490 --> 00:10:33.370 align:middle line:90% then it's very likely-- 00:10:33.370 --> 00:10:36.790 align:middle line:84% it's actually for sure that this loop will execute 00:10:36.790 --> 00:10:38.210 align:middle line:90% infinite number of times. 00:10:38.210 --> 00:10:40.450 align:middle line:84% So this is the pitfall of while loops. 00:10:40.450 --> 00:10:42.950 align:middle line:84% It's possible that, if you're not careful, 00:10:42.950 --> 00:10:46.600 align:middle line:84% your code will execute an infinite number of times. 00:10:46.600 --> 00:10:49.235 align:middle line:90% And it'll just never terminate. 00:10:49.235 --> 00:10:50.860 align:middle line:84% And I'll show you how to deal with that 00:10:50.860 --> 00:10:53.390 align:middle line:90% in a couple of slides. 00:10:53.390 --> 00:10:58.810 align:middle line:84% So let's try to code up this Lost Woods program just 00:10:58.810 --> 00:11:00.050 align:middle line:90% with a while loop. 00:11:00.050 --> 00:11:06.310 align:middle line:84% So here, we've got our question that we ask the user. 00:11:06.310 --> 00:11:08.150 align:middle line:90% Do you want to go left or right? 00:11:08.150 --> 00:11:11.380 align:middle line:84% And we're going to grab the user input as a string 00:11:11.380 --> 00:11:14.440 align:middle line:84% and save it in a variable called where. 00:11:14.440 --> 00:11:16.090 align:middle line:84% So whatever the user types in, it 00:11:16.090 --> 00:11:17.920 align:middle line:84% will be saved in the variable called where. 00:11:17.920 --> 00:11:20.230 align:middle line:84% So in my computer memory, the way 00:11:20.230 --> 00:11:22.780 align:middle line:84% this looks like, if the user types in right, 00:11:22.780 --> 00:11:25.720 align:middle line:84% that particular sequence of characters, that will 00:11:25.720 --> 00:11:28.470 align:middle line:90% be saved as the variable where. 00:11:28.470 --> 00:11:30.750 align:middle line:84% So then we finish this first line of the code here. 00:11:30.750 --> 00:11:32.970 align:middle line:84% And then we check while the value 00:11:32.970 --> 00:11:37.030 align:middle line:84% of where is equivalent to right, what are we going to do? 00:11:37.030 --> 00:11:39.660 align:middle line:84% We're going to ask the user again, where do you want to go? 00:11:39.660 --> 00:11:41.260 align:middle line:90% Left or right? 00:11:41.260 --> 00:11:44.290 align:middle line:90% So I'm going to say right again. 00:11:44.290 --> 00:11:47.540 align:middle line:84% And then this memory is going to look exactly the same. 00:11:47.540 --> 00:11:49.570 align:middle line:84% If the user keeps typing in right, 00:11:49.570 --> 00:11:56.990 align:middle line:84% I keep reassigning the variable where to have the value right. 00:11:56.990 --> 00:12:00.870 align:middle line:84% At some point, the user might type in left, in which case, 00:12:00.870 --> 00:12:05.150 align:middle line:84% we're going to lose the binding from a variable, where, 00:12:05.150 --> 00:12:08.720 align:middle line:84% from the specific sequence of characters, right, 00:12:08.720 --> 00:12:11.510 align:middle line:84% we're going to bind it now to the characters left. 00:12:11.510 --> 00:12:14.270 align:middle line:84% So at some point, after repeating this many times, 00:12:14.270 --> 00:12:15.980 align:middle line:90% the user will type in left. 00:12:15.980 --> 00:12:19.650 align:middle line:84% And we're going to have where is equal to left. 00:12:19.650 --> 00:12:21.350 align:middle line:84% And at that point, when the condition 00:12:21.350 --> 00:12:24.080 align:middle line:84% is being checked again, Python will say, nope, 00:12:24.080 --> 00:12:25.590 align:middle line:90% this is not equivalent. 00:12:25.590 --> 00:12:27.980 align:middle line:84% So I'm not going to go inside this code block. 00:12:27.980 --> 00:12:30.380 align:middle line:84% I'm just going to go down here and print "You got out 00:12:30.380 --> 00:12:32.750 align:middle line:90% of the Lost Forest." 00:12:32.750 --> 00:12:36.110 align:middle line:84% So in code, the way this looks is-- 00:12:36.110 --> 00:12:37.400 align:middle line:90% it's this first one here. 00:12:37.400 --> 00:12:44.490 align:middle line:90% 00:12:44.490 --> 00:12:47.000 align:middle line:84% So you're in the lost forest, go left or right. 00:12:47.000 --> 00:12:50.360 align:middle line:84% So if I type in right, it just keeps 00:12:50.360 --> 00:12:52.400 align:middle line:90% asking me which way to go. 00:12:52.400 --> 00:12:55.300 align:middle line:84% And at some point, I can type in left and I'm out. 00:12:55.300 --> 00:12:58.555 align:middle line:90% 00:12:58.555 --> 00:12:59.680 align:middle line:90% So it's pretty cool, right? 00:12:59.680 --> 00:13:04.110 align:middle line:84% We just made our own level in this text adventure. 00:13:04.110 --> 00:13:05.930 align:middle line:84% Let's have you think about this question. 00:13:05.930 --> 00:13:10.850 align:middle line:84% What if the user types in capital RIGHT? 00:13:10.850 --> 00:13:12.510 align:middle line:90% What do you think will happen? 00:13:12.510 --> 00:13:15.350 align:middle line:84% Are we going to ask the user to go left or right again? 00:13:15.350 --> 00:13:17.990 align:middle line:84% Or are we going to tell them that they got out 00:13:17.990 --> 00:13:19.974 align:middle line:90% of the forest? 00:13:19.974 --> 00:13:20.970 align:middle line:90% STUDENT: They got out. 00:13:20.970 --> 00:13:22.178 align:middle line:90% ANA BELL: Yeah, they got out. 00:13:22.178 --> 00:13:23.220 align:middle line:90% Do you want to say why? 00:13:23.220 --> 00:13:24.720 align:middle line:84% STUDENT: Because it's not lowercase. 00:13:24.720 --> 00:13:27.150 align:middle line:84% ANA BELL: Yeah, exactly, because it's not lowercase. 00:13:27.150 --> 00:13:29.320 align:middle line:84% So remember, when we're doing comparison-- 00:13:29.320 --> 00:13:34.170 align:middle line:84% so the == on strings, it has to be the same case. 00:13:34.170 --> 00:13:35.500 align:middle line:90% It's case sensitive. 00:13:35.500 --> 00:13:38.340 align:middle line:84% And so capital RIGHT or even some combination, 00:13:38.340 --> 00:13:47.350 align:middle line:84% like just Right is also going to give us that we got out. 00:13:47.350 --> 00:13:50.580 align:middle line:84% So this is counterintuitive to what 00:13:50.580 --> 00:13:56.220 align:middle line:84% we see as humans because we see right no matter what 00:13:56.220 --> 00:13:59.370 align:middle line:90% to mean right. 00:13:59.370 --> 00:14:01.290 align:middle line:84% So a workaround for this would be 00:14:01.290 --> 00:14:04.950 align:middle line:84% to use a command on the string to maybe convert everything 00:14:04.950 --> 00:14:07.350 align:middle line:84% to lowercase, just so it's more easily compared 00:14:07.350 --> 00:14:10.230 align:middle line:90% or something like that. 00:14:10.230 --> 00:14:10.730 align:middle line:90% OK. 00:14:10.730 --> 00:14:15.530 align:middle line:84% So another use of while loops is with numbers. 00:14:15.530 --> 00:14:17.250 align:middle line:90% Let's look at this example. 00:14:17.250 --> 00:14:19.678 align:middle line:84% I'm going to ask the user for an integer. 00:14:19.678 --> 00:14:21.720 align:middle line:84% And then I'm going to do something really simple. 00:14:21.720 --> 00:14:24.620 align:middle line:84% I'm going to print x to the screen however many times 00:14:24.620 --> 00:14:26.300 align:middle line:90% the user told me. 00:14:26.300 --> 00:14:27.800 align:middle line:84% So if the user gives me 4, I'm going 00:14:27.800 --> 00:14:31.130 align:middle line:84% to print x four times to the screen. 00:14:31.130 --> 00:14:34.550 align:middle line:84% So what is this code doing in memory? 00:14:34.550 --> 00:14:37.150 align:middle line:84% Well, the user gives me, let's say, 4. 00:14:37.150 --> 00:14:39.140 align:middle line:90% What happens step by step? 00:14:39.140 --> 00:14:40.820 align:middle line:90% First, we see our while loop. 00:14:40.820 --> 00:14:44.110 align:middle line:84% So I'm going to check whether 4, the current value of n, 00:14:44.110 --> 00:14:45.310 align:middle line:90% is greater than 0. 00:14:45.310 --> 00:14:46.720 align:middle line:90% Yes, that's true. 00:14:46.720 --> 00:14:49.010 align:middle line:84% I'm going to print x to the screen. 00:14:49.010 --> 00:14:51.730 align:middle line:84% And then I'm going to do the next line of code. 00:14:51.730 --> 00:14:53.350 align:middle line:84% That's part of this indented block, 00:14:53.350 --> 00:14:58.350 align:middle line:84% which is to take n and assign it to whatever n is minus 1. 00:14:58.350 --> 00:15:00.360 align:middle line:90% So I'm going to lose the 4. 00:15:00.360 --> 00:15:03.660 align:middle line:84% And I'm going to take 4 minus 1 to be 3, create a new object, 00:15:03.660 --> 00:15:06.410 align:middle line:90% and bind n to the 3. 00:15:06.410 --> 00:15:07.340 align:middle line:90% OK? 00:15:07.340 --> 00:15:09.722 align:middle line:84% Next, Python-- again, it's part of a while loop. 00:15:09.722 --> 00:15:12.180 align:middle line:84% So automatically, it looks at the condition again and says, 00:15:12.180 --> 00:15:14.870 align:middle line:84% well, now, the value of n is still greater than 0. 00:15:14.870 --> 00:15:16.880 align:middle line:90% Yeah, 3 is still greater than 0. 00:15:16.880 --> 00:15:18.950 align:middle line:84% So again, we're going to lose the binding-- 00:15:18.950 --> 00:15:20.810 align:middle line:84% sorry, we're printing x to the screen first. 00:15:20.810 --> 00:15:24.620 align:middle line:84% And then we lose the binding from the current value of n, 3, 00:15:24.620 --> 00:15:27.250 align:middle line:90% to 2. 00:15:27.250 --> 00:15:30.250 align:middle line:84% So we're decrementing n by 1 each time 00:15:30.250 --> 00:15:32.630 align:middle line:90% through this while loop. 00:15:32.630 --> 00:15:34.830 align:middle line:84% Then, again, Python checks the condition. 00:15:34.830 --> 00:15:36.830 align:middle line:84% It says 2 is still greater than 0. 00:15:36.830 --> 00:15:39.110 align:middle line:84% So again, we print another x to the screen. 00:15:39.110 --> 00:15:40.850 align:middle line:90% And then we decrement n by 1. 00:15:40.850 --> 00:15:42.860 align:middle line:90% So we're binding n to 1. 00:15:42.860 --> 00:15:44.480 align:middle line:84% Again, Python checks the condition. 00:15:44.480 --> 00:15:45.860 align:middle line:90% Is 1 still greater than 0? 00:15:45.860 --> 00:15:46.520 align:middle line:90% Yes. 00:15:46.520 --> 00:15:48.300 align:middle line:84% So we print another x to the screen. 00:15:48.300 --> 00:15:50.780 align:middle line:84% So we've printed four x's now to the screen. 00:15:50.780 --> 00:15:54.470 align:middle line:84% And then Python says now, I'm going to make n to be 0. 00:15:54.470 --> 00:15:57.660 align:middle line:84% And then at this point, Python will do another check. 00:15:57.660 --> 00:16:01.880 align:middle line:84% It's going to say, is 0 greater than 0? 00:16:01.880 --> 00:16:03.570 align:middle line:90% And that's going to be false. 00:16:03.570 --> 00:16:06.650 align:middle line:84% So it's not going to execute the code block anymore. 00:16:06.650 --> 00:16:08.330 align:middle line:90% And the program will be done. 00:16:08.330 --> 00:16:10.910 align:middle line:84% There's no code to rejoin anymore. 00:16:10.910 --> 00:16:12.900 align:middle line:84% There's just the end of the program. 00:16:12.900 --> 00:16:15.460 align:middle line:84% So we would have printed four x's to the screen. 00:16:15.460 --> 00:16:17.210 align:middle line:84% And this is in the Python file I gave you. 00:16:17.210 --> 00:16:21.990 align:middle line:84% You can feel free to run it to double check. 00:16:21.990 --> 00:16:23.880 align:middle line:90% My question is, what happens-- 00:16:23.880 --> 00:16:25.650 align:middle line:84% and this is a really common mistake. 00:16:25.650 --> 00:16:28.590 align:middle line:84% What happens if we forget this last line? 00:16:28.590 --> 00:16:32.420 align:middle line:90% 00:16:32.420 --> 00:16:33.380 align:middle line:90% We can try it. 00:16:33.380 --> 00:16:36.784 align:middle line:90% I can try it in here. 00:16:36.784 --> 00:16:37.640 align:middle line:90% STUDENT: [INAUDIBLE] 00:16:37.640 --> 00:16:38.750 align:middle line:90% ANA BELL: Yeah, exactly. 00:16:38.750 --> 00:16:40.010 align:middle line:90% It's going to go on forever. 00:16:40.010 --> 00:16:42.540 align:middle line:84% I'll show you what that looks like. 00:16:42.540 --> 00:16:47.340 align:middle line:84% So this is the code when we just have it working as usual. 00:16:47.340 --> 00:16:50.510 align:middle line:84% So if I type in 3, it prints three of those x's. 00:16:50.510 --> 00:16:53.390 align:middle line:84% But if I happen to forget to write this last line-- 00:16:53.390 --> 00:16:55.080 align:middle line:84% I'm just going to comment it out. 00:16:55.080 --> 00:16:58.310 align:middle line:84% And if I run the program, I can enter any number. 00:16:58.310 --> 00:17:03.500 align:middle line:84% And it'll just keep printing stuff to the console. 00:17:03.500 --> 00:17:06.599 align:middle line:84% It's just printing a whole bunch of stuff. 00:17:06.599 --> 00:17:11.869 align:middle line:84% So you can see this is all the stuff it printed. 00:17:11.869 --> 00:17:14.569 align:middle line:84% So yeah, we don't have a program that 00:17:14.569 --> 00:17:20.569 align:middle line:84% terminates because the condition here is never actually being-- 00:17:20.569 --> 00:17:22.490 align:middle line:84% the variable that's part of this condition 00:17:22.490 --> 00:17:26.339 align:middle line:84% is never actually being changed inside my loop. 00:17:26.339 --> 00:17:28.099 align:middle line:90% And so that's a big problem. 00:17:28.099 --> 00:17:30.680 align:middle line:84% When that happens, what we can do-- 00:17:30.680 --> 00:17:31.940 align:middle line:90% and what I just did here is-- 00:17:31.940 --> 00:17:33.740 align:middle line:84% I'm going to-- you can click the shell 00:17:33.740 --> 00:17:36.320 align:middle line:84% and hit Control-C or Command-C on a Mac. 00:17:36.320 --> 00:17:39.200 align:middle line:84% And that will just end the program manually. 00:17:39.200 --> 00:17:43.260 align:middle line:84% Or you can just click the red X in the corner. 00:17:43.260 --> 00:17:45.560 align:middle line:84% So here's another example of it going infinite. 00:17:45.560 --> 00:17:46.715 align:middle line:90% And there's this little-- 00:17:46.715 --> 00:17:48.860 align:middle line:90% sorry, red box in the corner. 00:17:48.860 --> 00:17:49.730 align:middle line:90% You can click that. 00:17:49.730 --> 00:17:52.250 align:middle line:84% Or you can click the three lines to interrupt kernel. 00:17:52.250 --> 00:17:55.140 align:middle line:90% All that will stop the program. 00:17:55.140 --> 00:17:56.840 align:middle line:84% So in this class, we're not actually 00:17:56.840 --> 00:17:59.280 align:middle line:84% going to write programs that take seconds to run. 00:17:59.280 --> 00:18:02.180 align:middle line:84% So if you find yourself waiting for your program for more 00:18:02.180 --> 00:18:04.310 align:middle line:84% than one or two seconds, then likely you've 00:18:04.310 --> 00:18:05.640 align:middle line:90% entered an infinite loop. 00:18:05.640 --> 00:18:08.030 align:middle line:84% So you'll want to stop it and try to see 00:18:08.030 --> 00:18:09.320 align:middle line:90% where your program went wrong. 00:18:09.320 --> 00:18:13.930 align:middle line:90% 00:18:13.930 --> 00:18:14.560 align:middle line:90% OK. 00:18:14.560 --> 00:18:19.160 align:middle line:84% So give this a try, if you want, just 00:18:19.160 --> 00:18:21.500 align:middle line:84% so you get the hang of stopping an infinite program 00:18:21.500 --> 00:18:26.060 align:middle line:84% because you'll very likely write a program that 00:18:26.060 --> 00:18:26.960 align:middle line:90% doesn't terminate. 00:18:26.960 --> 00:18:31.205 align:middle line:84% So while true-- what's the condition here? 00:18:31.205 --> 00:18:33.750 align:middle line:90% 00:18:33.750 --> 00:18:34.860 align:middle line:90% It's just true, right? 00:18:34.860 --> 00:18:37.140 align:middle line:84% So there's no condition that's being checked. 00:18:37.140 --> 00:18:44.020 align:middle line:84% This program will run always an infinite times no matter what. 00:18:44.020 --> 00:18:47.890 align:middle line:84% So that's just this little You Try It down here on line 44. 00:18:47.890 --> 00:18:49.270 align:middle line:90% Just run it as soon. 00:18:49.270 --> 00:18:52.150 align:middle line:84% As you run it, it's just going to print that to the screen 00:18:52.150 --> 00:18:53.320 align:middle line:90% over and over again. 00:18:53.320 --> 00:18:56.110 align:middle line:84% Be sure to click the shell to put the focus on there 00:18:56.110 --> 00:19:02.440 align:middle line:84% and hit Control-C or hit the red X. 00:19:02.440 --> 00:19:07.030 align:middle line:84% All right, so the big idea with while loops 00:19:07.030 --> 00:19:10.640 align:middle line:84% is that they can repeat the code inside them indefinitely. 00:19:10.640 --> 00:19:12.880 align:middle line:84% So we have to be a little bit careful 00:19:12.880 --> 00:19:17.290 align:middle line:84% with what our conditions are and whether we're actually 00:19:17.290 --> 00:19:20.470 align:middle line:84% making progress towards having that condition become 00:19:20.470 --> 00:19:22.460 align:middle line:90% false at some point. 00:19:22.460 --> 00:19:24.760 align:middle line:84% And when that happens, when they run indefinitely, 00:19:24.760 --> 00:19:30.730 align:middle line:84% you'll have to manually intervene to close the program. 00:19:30.730 --> 00:19:31.230 align:middle line:90% OK. 00:19:31.230 --> 00:19:38.370 align:middle line:84% So now that we've seen a loop with a little bit of numerical 00:19:38.370 --> 00:19:41.700 align:middle line:84% computation inside it-- so we were changing the value of n 00:19:41.700 --> 00:19:42.720 align:middle line:90% inside our loop-- 00:19:42.720 --> 00:19:45.010 align:middle line:84% let's have you work on this little program. 00:19:45.010 --> 00:19:48.390 align:middle line:84% It's an extension of the Lost Woods. 00:19:48.390 --> 00:19:50.790 align:middle line:84% This is exactly the same program that I just 00:19:50.790 --> 00:19:52.420 align:middle line:90% ran a few slides ago. 00:19:52.420 --> 00:19:56.430 align:middle line:84% But what I want you to add is an extra printout. 00:19:56.430 --> 00:20:03.750 align:middle line:84% So when the user says right more than two times, 00:20:03.750 --> 00:20:06.780 align:middle line:84% the next time you ask them whether they go left or right, 00:20:06.780 --> 00:20:09.840 align:middle line:84% I'd like you to print a sad face right before you 00:20:09.840 --> 00:20:11.000 align:middle line:90% ask them that question. 00:20:11.000 --> 00:20:12.250 align:middle line:90% It can be on a different line. 00:20:12.250 --> 00:20:15.140 align:middle line:84% It doesn't have to be on the same line. 00:20:15.140 --> 00:20:20.420 align:middle line:84% And the way to do that is to try to create a new variable that's 00:20:20.420 --> 00:20:22.580 align:middle line:84% going to be like your counter, that keeps track 00:20:22.580 --> 00:20:26.390 align:middle line:84% of how many times the user has-- how many times 00:20:26.390 --> 00:20:30.160 align:middle line:90% this while loop has repeated. 00:20:30.160 --> 00:20:32.160 align:middle line:84% So I'll give you a couple of moments to do that. 00:20:32.160 --> 00:20:35.670 align:middle line:84% And then we'll write it together. 00:20:35.670 --> 00:20:38.490 align:middle line:84% As usual, the You Try It is in here. 00:20:38.490 --> 00:20:42.260 align:middle line:84% So you can just uncomment the code with-- 00:20:42.260 --> 00:20:46.880 align:middle line:84% in Spyder, its Control-1 or Command-1 to batch uncomment. 00:20:46.880 --> 00:20:51.790 align:middle line:84% And then you can work off of this code to try to improve it. 00:20:51.790 --> 00:20:54.380 align:middle line:90% 00:20:54.380 --> 00:20:54.880 align:middle line:90% OK. 00:20:54.880 --> 00:20:57.100 align:middle line:84% So does anyone have a start for me? 00:20:57.100 --> 00:20:58.540 align:middle line:90% How can we do this? 00:20:58.540 --> 00:21:01.090 align:middle line:84% You don't have to give it to me in full. 00:21:01.090 --> 00:21:04.000 align:middle line:84% We can work our way up to the final program. 00:21:04.000 --> 00:21:06.070 align:middle line:84% But what's your first thought here? 00:21:06.070 --> 00:21:18.388 align:middle line:90% 00:21:18.388 --> 00:21:18.888 align:middle line:90% Yes? 00:21:18.888 --> 00:21:19.704 align:middle line:90% Sorry. 00:21:19.704 --> 00:21:22.740 align:middle line:84% STUDENT: [INAUDIBLE] variable like n or something 00:21:22.740 --> 00:21:26.158 align:middle line:84% incremented every time the while loop goes through. 00:21:26.158 --> 00:21:26.700 align:middle line:90% ANA BELL: OK. 00:21:26.700 --> 00:21:28.650 align:middle line:84% So we can create a variable, n, at the beginning 00:21:28.650 --> 00:21:29.310 align:middle line:90% of our program. 00:21:29.310 --> 00:21:30.850 align:middle line:90% What do you want to make it? 00:21:30.850 --> 00:21:31.350 align:middle line:90% STUDENT: 0. 00:21:31.350 --> 00:21:34.170 align:middle line:90% ANA BELL: 0, OK, good. 00:21:34.170 --> 00:21:37.020 align:middle line:84% 0 will keep track of-- or n will keep track of how many times 00:21:37.020 --> 00:21:38.530 align:middle line:90% we've gone through the loop. 00:21:38.530 --> 00:21:45.690 align:middle line:84% So inside our program, when do we want to change n? 00:21:45.690 --> 00:21:46.560 align:middle line:90% Sorry. 00:21:46.560 --> 00:21:48.840 align:middle line:84% Every time we go through the loop, right? 00:21:48.840 --> 00:21:51.400 align:middle line:84% So every time we want to go through the loop, 00:21:51.400 --> 00:21:54.280 align:middle line:84% we want to change n to be a new value. 00:21:54.280 --> 00:21:56.790 align:middle line:84% So maybe we want to increase it by 1. 00:21:56.790 --> 00:22:01.140 align:middle line:90% So n is equal to n plus 1. 00:22:01.140 --> 00:22:04.203 align:middle line:84% So now, this will keep track of how many times 00:22:04.203 --> 00:22:05.370 align:middle line:90% we've gone through the loop. 00:22:05.370 --> 00:22:09.570 align:middle line:84% And we can actually double check this by printing n. 00:22:09.570 --> 00:22:16.360 align:middle line:84% So if we run it and we say right, we've gone once. 00:22:16.360 --> 00:22:18.040 align:middle line:90% Right, we've got twice. 00:22:18.040 --> 00:22:19.720 align:middle line:90% Right, we've gone three times. 00:22:19.720 --> 00:22:24.680 align:middle line:84% So this means we're incrementing it correctly. 00:22:24.680 --> 00:22:27.800 align:middle line:84% Now, what can I do with this variable n now that I have it 00:22:27.800 --> 00:22:31.280 align:middle line:84% and I know it's incrementing correctly? 00:22:31.280 --> 00:22:32.060 align:middle line:90% Yeah? 00:22:32.060 --> 00:22:34.005 align:middle line:84% STUDENT: Set up an if statement [INAUDIBLE].. 00:22:34.005 --> 00:22:35.880 align:middle line:84% ANA BELL: Yep, we can set up an if statement. 00:22:35.880 --> 00:22:41.660 align:middle line:84% So we can check if that value of n is greater than 2 00:22:41.660 --> 00:22:43.940 align:middle line:84% according to the specification here. 00:22:43.940 --> 00:22:47.960 align:middle line:84% What do you want to do when if is greater than 2? 00:22:47.960 --> 00:22:49.190 align:middle line:90% Print something, right? 00:22:49.190 --> 00:22:50.750 align:middle line:90% So we can print the sad face. 00:22:50.750 --> 00:22:54.320 align:middle line:90% 00:22:54.320 --> 00:22:56.640 align:middle line:90% And let's try to run it now. 00:22:56.640 --> 00:23:00.230 align:middle line:84% So if we immediately hit left, it still works. 00:23:00.230 --> 00:23:03.110 align:middle line:84% If we go right one time, nothing. 00:23:03.110 --> 00:23:04.610 align:middle line:90% Another time, nothing. 00:23:04.610 --> 00:23:06.800 align:middle line:90% Right a last time, sad. 00:23:06.800 --> 00:23:10.655 align:middle line:84% And from now on, it's going to keep showing me the sad face. 00:23:10.655 --> 00:23:15.660 align:middle line:90% 00:23:15.660 --> 00:23:16.620 align:middle line:90% OK. 00:23:16.620 --> 00:23:19.870 align:middle line:90% Questions about this code? 00:23:19.870 --> 00:23:20.980 align:middle line:90% Yeah? 00:23:20.980 --> 00:23:22.930 align:middle line:84% STUDENT: Is it possible to test for something 00:23:22.930 --> 00:23:24.580 align:middle line:90% that's not equivalent. 00:23:24.580 --> 00:23:26.950 align:middle line:84% Is there a sign that's different than the two equals 00:23:26.950 --> 00:23:28.345 align:middle line:90% or not equivalent? 00:23:28.345 --> 00:23:30.970 align:middle line:84% ANA BELL: Can we check for non-equivalency? 00:23:30.970 --> 00:23:31.660 align:middle line:90% Here? 00:23:31.660 --> 00:23:33.560 align:middle line:90% STUDENT: Yeah. 00:23:33.560 --> 00:23:36.260 align:middle line:84% ANA BELL: So this particular check checks 00:23:36.260 --> 00:23:37.820 align:middle line:90% for what the user typed in. 00:23:37.820 --> 00:23:41.140 align:middle line:90% 00:23:41.140 --> 00:23:44.740 align:middle line:84% It's possible we can add this if statement that checks 00:23:44.740 --> 00:23:48.590 align:middle line:84% for the n in here and something else. 00:23:48.590 --> 00:23:51.060 align:middle line:84% But then we would have to have maybe another-- 00:23:51.060 --> 00:23:54.060 align:middle line:90% 00:23:54.060 --> 00:23:55.230 align:middle line:90% I'd have to think about it. 00:23:55.230 --> 00:23:57.990 align:middle line:84% But it might be possible to try to combine them 00:23:57.990 --> 00:23:59.520 align:middle line:90% inside the while loop. 00:23:59.520 --> 00:24:01.888 align:middle line:84% STUDENT: So there's a symbol that allows you to do that? 00:24:01.888 --> 00:24:03.180 align:middle line:90% ANA BELL: Oh, to do not equals? 00:24:03.180 --> 00:24:07.000 align:middle line:84% That would be the not equal sign, yeah. 00:24:07.000 --> 00:24:11.050 align:middle line:84% So another thing we can do with while loops is to iterate 00:24:11.050 --> 00:24:12.940 align:middle line:90% through numbers in a sequence. 00:24:12.940 --> 00:24:15.550 align:middle line:84% If we do this, there's a really common pattern, 00:24:15.550 --> 00:24:19.548 align:middle line:84% which actually leads us to the next kind of loop we're 00:24:19.548 --> 00:24:20.840 align:middle line:90% going to see on the next slide. 00:24:20.840 --> 00:24:24.380 align:middle line:84% But the pattern, when you want to iterate through numbers 00:24:24.380 --> 00:24:28.370 align:middle line:84% in a sequence, is you first set a loop variable 00:24:28.370 --> 00:24:29.960 align:middle line:90% before the while loop. 00:24:29.960 --> 00:24:34.070 align:middle line:84% Inside the condition for the loop, you do some sort of check 00:24:34.070 --> 00:24:35.820 align:middle line:90% with that variable. 00:24:35.820 --> 00:24:38.360 align:middle line:84% So n was my loop variable outside the loop. 00:24:38.360 --> 00:24:40.860 align:middle line:84% And then I test it inside the while loop. 00:24:40.860 --> 00:24:43.130 align:middle line:90% So n is less than 5. 00:24:43.130 --> 00:24:45.110 align:middle line:84% And then within the while loop, you 00:24:45.110 --> 00:24:48.290 align:middle line:84% can do whatever commands you want to do with that n. 00:24:48.290 --> 00:24:52.160 align:middle line:84% But then you have to remember to change it in some way 00:24:52.160 --> 00:24:54.350 align:middle line:84% because if you don't change it in some way, 00:24:54.350 --> 00:24:58.230 align:middle line:84% this while loop condition will always be true. 00:24:58.230 --> 00:25:02.810 align:middle line:84% So here, I'm incrementing n by 1 because it starts from 0. 00:25:02.810 --> 00:25:07.100 align:middle line:84% I want n to get to something, something above 5, 00:25:07.100 --> 00:25:09.125 align:middle line:84% which will lead to my condition becoming false. 00:25:09.125 --> 00:25:11.910 align:middle line:90% 00:25:11.910 --> 00:25:15.770 align:middle line:84% So this pattern actually exists in a bunch 00:25:15.770 --> 00:25:16.800 align:middle line:90% of different programs. 00:25:16.800 --> 00:25:21.750 align:middle line:84% So here's a program that calculates factorial for me. 00:25:21.750 --> 00:25:24.090 align:middle line:84% And here, I'm calculating 4 factorial. 00:25:24.090 --> 00:25:26.190 align:middle line:84% I'm not excited about the number 4. 00:25:26.190 --> 00:25:27.690 align:middle line:90% That's 4 factorial. 00:25:27.690 --> 00:25:29.470 align:middle line:90% How do we do this? 00:25:29.470 --> 00:25:32.670 align:middle line:84% Well, there's a lot of things I'm initializing here. 00:25:32.670 --> 00:25:35.310 align:middle line:84% But the more you work with loops, 00:25:35.310 --> 00:25:40.990 align:middle line:84% you'll get used to seeing what is the loop variable. 00:25:40.990 --> 00:25:45.090 align:middle line:84% So i is actually going to be my loop variable. 00:25:45.090 --> 00:25:47.790 align:middle line:84% Here, it's being set to some value, 00:25:47.790 --> 00:25:50.300 align:middle line:90% initially outside the loop. 00:25:50.300 --> 00:25:52.640 align:middle line:84% Inside the conditional, I'm doing some sort 00:25:52.640 --> 00:25:56.410 align:middle line:90% of condition check with it. 00:25:56.410 --> 00:25:59.560 align:middle line:84% And then inside the body of that conditional, 00:25:59.560 --> 00:26:02.540 align:middle line:90% I'm changing it in some way. 00:26:02.540 --> 00:26:04.540 align:middle line:90% That gives me some sort of-- 00:26:04.540 --> 00:26:09.010 align:middle line:84% that takes me to the end of my conditional here. 00:26:09.010 --> 00:26:10.480 align:middle line:90% So I'm setting i to 0. 00:26:10.480 --> 00:26:13.210 align:middle line:84% I'm incrementing i by 1 each time through the loop. 00:26:13.210 --> 00:26:16.180 align:middle line:84% And I'm making forward progress towards making 00:26:16.180 --> 00:26:20.440 align:middle line:84% i greater than x, at which point my conditional will 00:26:20.440 --> 00:26:23.460 align:middle line:90% become false. 00:26:23.460 --> 00:26:25.860 align:middle line:84% The rest of the code-- x is equal to 4-- 00:26:25.860 --> 00:26:29.550 align:middle line:84% just sets the thing I want to get the factorial of. 00:26:29.550 --> 00:26:33.627 align:middle line:84% And this factorial variable is kind of my running product. 00:26:33.627 --> 00:26:36.210 align:middle line:84% So it's the thing that I'm going to keep multiplying to figure 00:26:36.210 --> 00:26:37.980 align:middle line:90% out what the factorial is. 00:26:37.980 --> 00:26:40.230 align:middle line:84% So here, I'm initializing it to 1. 00:26:40.230 --> 00:26:42.420 align:middle line:84% And inside the loop, I'm multiplying it 00:26:42.420 --> 00:26:44.910 align:middle line:90% by my loop variable every time. 00:26:44.910 --> 00:26:49.690 align:middle line:84% So I'm not going to do a memory diagram for this example, 00:26:49.690 --> 00:26:52.902 align:middle line:90% but I will do the Python Tutor. 00:26:52.902 --> 00:26:54.610 align:middle line:84% And I'm going to step through to show you 00:26:54.610 --> 00:26:55.735 align:middle line:90% exactly what this is doing. 00:26:55.735 --> 00:26:57.130 align:middle line:90% So x is 4. 00:26:57.130 --> 00:26:58.630 align:middle line:90% i is 1 originally. 00:26:58.630 --> 00:26:59.860 align:middle line:90% And factorial is 1. 00:26:59.860 --> 00:27:02.380 align:middle line:84% So x is the thing I want to get the factorial of. 00:27:02.380 --> 00:27:04.810 align:middle line:84% i is going to be my loop variable. 00:27:04.810 --> 00:27:08.870 align:middle line:84% And factorial is my running product. 00:27:08.870 --> 00:27:13.180 align:middle line:84% So next step, i, 1 is less than or equal to 4. 00:27:13.180 --> 00:27:15.410 align:middle line:90% So I'm going to enter the loop. 00:27:15.410 --> 00:27:17.720 align:middle line:84% Python will calculate the factorial 00:27:17.720 --> 00:27:20.930 align:middle line:84% as whatever it is right now multiplied by 1, i. 00:27:20.930 --> 00:27:22.550 align:middle line:90% So it's still 1. 00:27:22.550 --> 00:27:24.380 align:middle line:84% And then I'm going to increment i 00:27:24.380 --> 00:27:27.320 align:middle line:84% to whatever it is from whatever it is now to 1. 00:27:27.320 --> 00:27:34.460 align:middle line:84% So I just want to mention this i += 1 is equivalent to saying i 00:27:34.460 --> 00:27:36.890 align:middle line:90% equals i plus 1. 00:27:36.890 --> 00:27:40.430 align:middle line:84% And this is true no matter what variable you have here. 00:27:40.430 --> 00:27:47.885 align:middle line:84% Basically, if you have fact *= 2 or something like that, 00:27:47.885 --> 00:27:54.470 align:middle line:84% that basically means factorial equals fact times 2. 00:27:54.470 --> 00:27:56.480 align:middle line:90% So that's the pattern here. 00:27:56.480 --> 00:27:58.740 align:middle line:84% These are equivalent and these are equivalent. 00:27:58.740 --> 00:28:01.830 align:middle line:84% This is just shorthand notation in programming. 00:28:01.830 --> 00:28:03.330 align:middle line:84% So that's what this line here means. 00:28:03.330 --> 00:28:08.060 align:middle line:90% i += 1 means I equals I plus 1. 00:28:08.060 --> 00:28:10.850 align:middle line:84% So at this line here, I'm taking whatever i is 00:28:10.850 --> 00:28:13.060 align:middle line:90% and adding 1 to it, 2. 00:28:13.060 --> 00:28:14.710 align:middle line:90% And then I do the check again. 00:28:14.710 --> 00:28:16.900 align:middle line:84% And remember, Python does this automatically. 00:28:16.900 --> 00:28:18.880 align:middle line:84% Because we're using a while loop, 00:28:18.880 --> 00:28:21.160 align:middle line:84% it goes back to the condition and checks 00:28:21.160 --> 00:28:24.010 align:middle line:84% it again using these new values for the variables. 00:28:24.010 --> 00:28:26.140 align:middle line:84% 2 is still less than or equal to 4. 00:28:26.140 --> 00:28:28.510 align:middle line:84% So again, we go inside the loop body. 00:28:28.510 --> 00:28:30.970 align:middle line:84% Factorial is whatever it is right now, 1, 00:28:30.970 --> 00:28:34.860 align:middle line:90% multiplied by i, 2. 00:28:34.860 --> 00:28:38.220 align:middle line:90% i is going to be 2 plus 1, 3. 00:28:38.220 --> 00:28:40.020 align:middle line:84% And then again, I'm checking that 3 00:28:40.020 --> 00:28:41.190 align:middle line:90% is less than or equal to 4. 00:28:41.190 --> 00:28:42.000 align:middle line:90% It still is. 00:28:42.000 --> 00:28:43.740 align:middle line:84% So then we're going to do factorial 00:28:43.740 --> 00:28:47.790 align:middle line:84% is whatever it is now, 2, multiplied by whatever i is, 3. 00:28:47.790 --> 00:28:49.740 align:middle line:90% So now, it's 6. 00:28:49.740 --> 00:28:52.680 align:middle line:84% i is going to be one more than what it is right now, 4. 00:28:52.680 --> 00:28:55.080 align:middle line:84% 4 is still less than or equal to 4. 00:28:55.080 --> 00:28:57.840 align:middle line:84% We're going to go inside the body. 00:28:57.840 --> 00:29:03.030 align:middle line:84% factorial is whatever it is right now, 6, multiplied by 4. 00:29:03.030 --> 00:29:04.380 align:middle line:90% And that's 24. 00:29:04.380 --> 00:29:06.570 align:middle line:84% And then i is going to be whatever 00:29:06.570 --> 00:29:08.670 align:middle line:90% it is right now plus 1, 5. 00:29:08.670 --> 00:29:11.970 align:middle line:84% At this point, Python says, is 5 less than or equal to 4? 00:29:11.970 --> 00:29:13.320 align:middle line:90% No. 00:29:13.320 --> 00:29:15.100 align:middle line:90% And then it breaks the loop. 00:29:15.100 --> 00:29:19.380 align:middle line:84% And it goes to print this statement, 4 factorial is-- 00:29:19.380 --> 00:29:22.260 align:middle line:84% and then it grabs whatever the value of the factorial is. 00:29:22.260 --> 00:29:25.140 align:middle line:84% So here, I'm using this fstring print 00:29:25.140 --> 00:29:29.190 align:middle line:84% that notation that we learned about last lecture. 00:29:29.190 --> 00:29:33.620 align:middle line:84% So I encourage you to go through it yourself, just step by step. 00:29:33.620 --> 00:29:37.540 align:middle line:84% That's what Python Tutor's really, really useful for. 00:29:37.540 --> 00:29:38.040 align:middle line:90% OK. 00:29:38.040 --> 00:29:41.580 align:middle line:84% So let's look at a different kind of loop called a for loop. 00:29:41.580 --> 00:29:43.770 align:middle line:84% And the for loop is going to allow 00:29:43.770 --> 00:29:49.200 align:middle line:84% us to rewrite that special kind of while loop 00:29:49.200 --> 00:29:51.180 align:middle line:84% that we saw where we initialize a variable, 00:29:51.180 --> 00:29:52.980 align:middle line:84% we test the variable, we do something 00:29:52.980 --> 00:29:55.920 align:middle line:84% to the variable within the code in a more efficient, more 00:29:55.920 --> 00:29:57.220 align:middle line:90% readable way. 00:29:57.220 --> 00:30:01.890 align:middle line:84% So in terms of our Netflix example, a for loop 00:30:01.890 --> 00:30:04.680 align:middle line:84% would be equivalent to something like Netflix, 00:30:04.680 --> 00:30:07.170 align:middle line:84% if you're not interacting, with it cuts you off after four 00:30:07.170 --> 00:30:09.240 align:middle line:90% episodes to save bandwidth. 00:30:09.240 --> 00:30:12.540 align:middle line:84% And so there's a sequence of four episodes 00:30:12.540 --> 00:30:15.330 align:middle line:84% it knows it's going to go through if you're not 00:30:15.330 --> 00:30:16.870 align:middle line:90% interacting with it. 00:30:16.870 --> 00:30:20.550 align:middle line:84% So if you've already gone through your sequence of four 00:30:20.550 --> 00:30:23.580 align:middle line:84% episodes you're allowed to watch without any interaction, 00:30:23.580 --> 00:30:24.720 align:middle line:90% it's done. 00:30:24.720 --> 00:30:27.300 align:middle line:84% It cuts you off it says, "Are you still watching?" 00:30:27.300 --> 00:30:31.290 align:middle line:84% But if there are still more episodes, if it only showed you 00:30:31.290 --> 00:30:33.120 align:middle line:84% two out of the four, then it's going 00:30:33.120 --> 00:30:35.850 align:middle line:84% to keep showing you more episodes 00:30:35.850 --> 00:30:39.770 align:middle line:90% until it's shown you the four. 00:30:39.770 --> 00:30:44.420 align:middle line:84% So this is the program we had with while loops 00:30:44.420 --> 00:30:47.270 align:middle line:90% a couple slides ago. 00:30:47.270 --> 00:30:49.490 align:middle line:84% And remember, we are initializing a variable. 00:30:49.490 --> 00:30:51.830 align:middle line:84% We were testing the variable with some condition here. 00:30:51.830 --> 00:30:53.960 align:middle line:84% And then we were incrementing the variable 00:30:53.960 --> 00:30:56.930 align:middle line:84% or doing something that gives us nice forward progress 00:30:56.930 --> 00:30:59.990 align:middle line:84% towards making this condition false. 00:30:59.990 --> 00:31:02.150 align:middle line:90% But it's really verbose. 00:31:02.150 --> 00:31:03.560 align:middle line:90% Certainly, it works. 00:31:03.560 --> 00:31:05.150 align:middle line:84% You can do it, but it's very easy 00:31:05.150 --> 00:31:08.840 align:middle line:84% to forget to do this, something like this, in which case 00:31:08.840 --> 00:31:10.590 align:middle line:90% you'll get an infinite loop. 00:31:10.590 --> 00:31:14.220 align:middle line:84% With a for loop, those four lines of code 00:31:14.220 --> 00:31:18.180 align:middle line:84% just look like this, these two lines of code. 00:31:18.180 --> 00:31:19.980 align:middle line:84% So if there's a sequence of values 00:31:19.980 --> 00:31:22.020 align:middle line:84% you ever want to iterate over, that's 00:31:22.020 --> 00:31:25.590 align:middle line:90% what for loops are useful for. 00:31:25.590 --> 00:31:28.020 align:middle line:84% So the syntax for a for loop looks a little bit 00:31:28.020 --> 00:31:30.180 align:middle line:90% different than a while loop. 00:31:30.180 --> 00:31:33.240 align:middle line:90% It starts with a for keyword. 00:31:33.240 --> 00:31:35.700 align:middle line:84% This is a variable that you get to name, 00:31:35.700 --> 00:31:37.110 align:middle line:90% whatever name you'd like. 00:31:37.110 --> 00:31:40.080 align:middle line:84% The keyword in tells Python I'm going 00:31:40.080 --> 00:31:44.685 align:middle line:84% to make this variable take on values in this sequence. 00:31:44.685 --> 00:31:47.250 align:middle line:90% 00:31:47.250 --> 00:31:51.360 align:middle line:84% And again, we have a colon that tells 00:31:51.360 --> 00:31:54.640 align:middle line:84% Python we're going to start a code indentation here. 00:31:54.640 --> 00:31:57.390 align:middle line:84% And whatever lines of code you have that are indented 00:31:57.390 --> 00:32:02.140 align:middle line:84% are going to be executed however many sequence of values 00:32:02.140 --> 00:32:03.770 align:middle line:90% you have. 00:32:03.770 --> 00:32:06.580 align:middle line:84% So the first time through the loop, Python 00:32:06.580 --> 00:32:09.460 align:middle line:84% will make this variable name take on the first value 00:32:09.460 --> 00:32:10.580 align:middle line:90% in the sequence. 00:32:10.580 --> 00:32:14.390 align:middle line:84% And then it's going to execute this code. 00:32:14.390 --> 00:32:19.370 align:middle line:84% Automatically, Python, after it finishes executing these codes, 00:32:19.370 --> 00:32:22.310 align:middle line:84% it will go back and set this variable 00:32:22.310 --> 00:32:24.680 align:middle line:84% have the next value in the sequence 00:32:24.680 --> 00:32:26.960 align:middle line:84% and execute the same lines of code. 00:32:26.960 --> 00:32:29.660 align:middle line:84% When it's done, it's going to make the variable here 00:32:29.660 --> 00:32:31.610 align:middle line:84% take on the next values in the sequence 00:32:31.610 --> 00:32:33.360 align:middle line:90% and execute those lines of code. 00:32:33.360 --> 00:32:35.450 align:middle line:84% And so these lines of code will effectively 00:32:35.450 --> 00:32:41.970 align:middle line:84% be executed however many values you have in your sequence. 00:32:41.970 --> 00:32:46.080 align:middle line:84% So more practically speaking, here, we have a variable-- 00:32:46.080 --> 00:32:47.780 align:middle line:90% so n, in this case-- 00:32:47.780 --> 00:32:51.260 align:middle line:90% in some sequence of values. 00:32:51.260 --> 00:32:53.570 align:middle line:84% In this case, I'm saying range 5. 00:32:53.570 --> 00:32:56.510 align:middle line:84% We're going to print the value of n. 00:32:56.510 --> 00:32:58.790 align:middle line:84% So I'm going to introduce range now. 00:32:58.790 --> 00:33:05.000 align:middle line:84% Range is a way for us to grab numerical sequence of values 00:33:05.000 --> 00:33:07.730 align:middle line:90% that have some sort of pattern. 00:33:07.730 --> 00:33:10.240 align:middle line:84% So if we just say range some number, 00:33:10.240 --> 00:33:13.720 align:middle line:84% the pattern is we start at 0 and we go up to but not including 00:33:13.720 --> 00:33:14.810 align:middle line:90% that number. 00:33:14.810 --> 00:33:19.030 align:middle line:84% So range 5 means the sequence of values Python will iterate over 00:33:19.030 --> 00:33:21.340 align:middle line:90% are 0, 1, 2, 3, and 4. 00:33:21.340 --> 00:33:24.760 align:middle line:84% Range 10 means 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. 00:33:24.760 --> 00:33:26.770 align:middle line:84% So we go up to, but not including the value 00:33:26.770 --> 00:33:30.040 align:middle line:90% in the range starting from 0. 00:33:30.040 --> 00:33:32.770 align:middle line:84% So each time through the loop, Python 00:33:32.770 --> 00:33:36.160 align:middle line:84% will change the value of n to be every one of those values 00:33:36.160 --> 00:33:38.410 align:middle line:90% automatically. 00:33:38.410 --> 00:33:42.190 align:middle line:84% So these two lines here, for n in range 5 print n, 00:33:42.190 --> 00:33:44.140 align:middle line:84% the way they look like behind the scenes-- 00:33:44.140 --> 00:33:45.730 align:middle line:90% and Python does this for you-- 00:33:45.730 --> 00:33:51.040 align:middle line:84% is the first time it encounters for loop, it sets n to be 0. 00:33:51.040 --> 00:33:53.860 align:middle line:84% That's the first value in my sequence. 00:33:53.860 --> 00:33:57.900 align:middle line:84% And then it prints the value of n, 0. 00:33:57.900 --> 00:33:59.760 align:middle line:84% Next time through the loop, Python 00:33:59.760 --> 00:34:02.040 align:middle line:84% will say, OK, I've done what you asked me to do 00:34:02.040 --> 00:34:04.920 align:middle line:90% inside the code loop, print n. 00:34:04.920 --> 00:34:08.190 align:middle line:84% I'm going to make n have the next value in my sequence. 00:34:08.190 --> 00:34:12.600 align:middle line:84% So it loses the binding from the 0 and makes it be 1. 00:34:12.600 --> 00:34:13.617 align:middle line:90% OK, I've made n 1. 00:34:13.617 --> 00:34:14.909 align:middle line:90% Now, what do you want me to do? 00:34:14.909 --> 00:34:18.870 align:middle line:84% Well, I'm going to execute whatever is indented, print n. 00:34:18.870 --> 00:34:20.909 align:middle line:90% So I'm going to print 1. 00:34:20.909 --> 00:34:22.290 align:middle line:90% So I've already printed 0. 00:34:22.290 --> 00:34:24.050 align:middle line:90% Then I've printed 1. 00:34:24.050 --> 00:34:26.580 align:middle line:84% I'm finished executing the code inside the loop. 00:34:26.580 --> 00:34:30.020 align:middle line:84% So now, n is going to get the next value in the sequence. 00:34:30.020 --> 00:34:34.239 align:middle line:84% Lose the binding from 1 and you get 2 and so on and so on. 00:34:34.239 --> 00:34:36.360 align:middle line:84% And by the end, this program will 00:34:36.360 --> 00:34:42.000 align:middle line:84% have printed 0, 1, 2, 3, and 4, every single value in my range. 00:34:42.000 --> 00:34:44.750 align:middle line:90% 00:34:44.750 --> 00:34:46.420 align:middle line:84% So it turns out that we can actually 00:34:46.420 --> 00:34:51.219 align:middle line:84% make range have three values inside the parentheses, not 00:34:51.219 --> 00:34:51.909 align:middle line:90% just one. 00:34:51.909 --> 00:34:55.570 align:middle line:84% One is shorthand notation if you ever want to start from 0 00:34:55.570 --> 00:34:58.570 align:middle line:84% and want to go and go up to and including-- 00:34:58.570 --> 00:35:01.990 align:middle line:84% sorry, up to but not including the value in the parentheses. 00:35:01.990 --> 00:35:04.210 align:middle line:84% But you can actually give it three values-- 00:35:04.210 --> 00:35:06.280 align:middle line:90% a start, a stop, and a step. 00:35:06.280 --> 00:35:10.120 align:middle line:84% And Python will automatically generate a sequence of values 00:35:10.120 --> 00:35:12.650 align:middle line:90% that matches this pattern. 00:35:12.650 --> 00:35:15.190 align:middle line:84% So this should seem familiar to you 00:35:15.190 --> 00:35:17.350 align:middle line:84% because we've seen something like this 00:35:17.350 --> 00:35:20.920 align:middle line:84% when we were doing strings, except that we 00:35:20.920 --> 00:35:24.190 align:middle line:84% weren't doing parentheses, we were doing square brackets. 00:35:24.190 --> 00:35:26.650 align:middle line:84% And we weren't doing commas, we were doing colons. 00:35:26.650 --> 00:35:28.630 align:middle line:90% But it's the exact same idea. 00:35:28.630 --> 00:35:32.410 align:middle line:84% Here, we're generating numbers, actual integers 00:35:32.410 --> 00:35:36.010 align:middle line:84% that we want a loop variable to take on. 00:35:36.010 --> 00:35:39.810 align:middle line:84% So if we omit start and step-- start by default is 0 00:35:39.810 --> 00:35:41.160 align:middle line:90% and step by default is 1. 00:35:41.160 --> 00:35:44.670 align:middle line:84% If we omit step, by default, it will be 1. 00:35:44.670 --> 00:35:48.510 align:middle line:84% So here, i in range 4, the variable i 00:35:48.510 --> 00:35:51.800 align:middle line:84% will take on the values 0, 1, 2, and 3. 00:35:51.800 --> 00:35:57.400 align:middle line:84% i in range 3, 5, i will take on the values 3 and 4. 00:35:57.400 --> 00:36:01.470 align:middle line:84% So we go up to, but not including the 5. 00:36:01.470 --> 00:36:03.660 align:middle line:84% Think about these three questions. 00:36:03.660 --> 00:36:07.950 align:middle line:84% So what are the range of values in the first one? 00:36:07.950 --> 00:36:10.360 align:middle line:90% And what are we going to print? 00:36:10.360 --> 00:36:12.690 align:middle line:84% So in 1, 4, 1, what range of values 00:36:12.690 --> 00:36:14.220 align:middle line:90% are we going to have i be? 00:36:14.220 --> 00:36:15.810 align:middle line:90% So i is going to be what? 00:36:15.810 --> 00:36:19.800 align:middle line:90% 00:36:19.800 --> 00:36:24.547 align:middle line:90% 1, 2, 3? 00:36:24.547 --> 00:36:25.380 align:middle line:90% STUDENT: [INAUDIBLE] 00:36:25.380 --> 00:36:26.505 align:middle line:90% ANA BELL: Yes, and we stop. 00:36:26.505 --> 00:36:29.580 align:middle line:84% We go up to, but not including the stop, which is a 4. 00:36:29.580 --> 00:36:31.547 align:middle line:90% And what are we printing? 00:36:31.547 --> 00:36:32.860 align:middle line:90% STUDENT: [INAUDIBLE] 00:36:32.860 --> 00:36:36.300 align:middle line:90% ANA BELL: Yeah, 1, 2, 3. 00:36:36.300 --> 00:36:37.890 align:middle line:90% How about the next one, j? 00:36:37.890 --> 00:36:39.427 align:middle line:90% What will the values of j be? 00:36:39.427 --> 00:36:40.260 align:middle line:90% STUDENT: [INAUDIBLE] 00:36:40.260 --> 00:36:44.270 align:middle line:90% ANA BELL: 1, 3. 00:36:44.270 --> 00:36:48.350 align:middle line:84% And that's it, yep, because we're going every other value. 00:36:48.350 --> 00:36:50.000 align:middle line:90% And what are we printing here? 00:36:50.000 --> 00:36:51.500 align:middle line:90% STUDENT: [INAUDIBLE] 00:36:51.500 --> 00:36:52.590 align:middle line:90% ANA BELL: Yeah, exactly. 00:36:52.590 --> 00:36:55.310 align:middle line:84% So we're doing an operation with each one of these j's. 00:36:55.310 --> 00:36:57.890 align:middle line:84% So we're going to print 2, and then 6. 00:36:57.890 --> 00:37:00.391 align:middle line:90% And how about the last one? 00:37:00.391 --> 00:37:03.620 align:middle line:90% STUDENT: [INAUDIBLE] 00:37:03.620 --> 00:37:05.360 align:middle line:84% ANA BELL: We're stepping backward, right? 00:37:05.360 --> 00:37:06.590 align:middle line:90% The negative 1. 00:37:06.590 --> 00:37:07.940 align:middle line:90% So what's our start? 00:37:07.940 --> 00:37:08.780 align:middle line:90% STUDENT: [INAUDIBLE] 00:37:08.780 --> 00:37:09.680 align:middle line:90% ANA BELL: 4. 00:37:09.680 --> 00:37:10.260 align:middle line:90% And then? 00:37:10.260 --> 00:37:10.760 align:middle line:90% STUDENT: 3 00:37:10.760 --> 00:37:15.220 align:middle line:90% ANA BELL: 3, 2, 1. 00:37:15.220 --> 00:37:15.820 align:middle line:90% And that's it. 00:37:15.820 --> 00:37:18.490 align:middle line:84% We're going down to, but not including the end, right? 00:37:18.490 --> 00:37:20.170 align:middle line:84% So we're not going to include the 0. 00:37:20.170 --> 00:37:22.475 align:middle line:90% And what are we printing here? 00:37:22.475 --> 00:37:24.850 align:middle line:90% STUDENT: [INAUDIBLE] 00:37:24.850 --> 00:37:29.380 align:middle line:84% ANA BELL: Yes, four dollar signs for the first time, and then 00:37:29.380 --> 00:37:33.760 align:middle line:84% three dollar signs, and then two dollar signs, and then 00:37:33.760 --> 00:37:36.980 align:middle line:90% one dollar sign, exactly. 00:37:36.980 --> 00:37:40.760 align:middle line:84% So the body obviously can do operations 00:37:40.760 --> 00:37:43.020 align:middle line:84% and can manipulate that loop variable. 00:37:43.020 --> 00:37:48.600 align:middle line:84% So each time that variable goes through, it changes. 00:37:48.600 --> 00:37:52.830 align:middle line:84% And then you can use that to your advantage. 00:37:52.830 --> 00:37:56.400 align:middle line:84% So here's another example of something useful. 00:37:56.400 --> 00:38:01.730 align:middle line:84% We can use for loops to keep track of how many times 00:38:01.730 --> 00:38:03.020 align:middle line:90% we're going through a loop. 00:38:03.020 --> 00:38:04.940 align:middle line:84% And in this particular case, we're 00:38:04.940 --> 00:38:07.850 align:middle line:84% writing a program that sums all the values 00:38:07.850 --> 00:38:13.260 align:middle line:84% from 0 all the way up to but not including whatever is in here. 00:38:13.260 --> 00:38:15.060 align:middle line:90% So how are we doing this? 00:38:15.060 --> 00:38:18.160 align:middle line:90% Let's do the memory diagram. 00:38:18.160 --> 00:38:22.640 align:middle line:84% We've got mysum is equal to 0 as the first line. 00:38:22.640 --> 00:38:28.600 align:middle line:84% So this will be 0 in memory, bound to the name mysum. 00:38:28.600 --> 00:38:32.410 align:middle line:84% And then for loop will generate for me the values 0 00:38:32.410 --> 00:38:36.370 align:middle line:90% through 9, including. 00:38:36.370 --> 00:38:40.210 align:middle line:84% So i, the first time through the loop, will have a value of 0. 00:38:40.210 --> 00:38:44.110 align:middle line:84% So we're going to do the operations or the code we're 00:38:44.110 --> 00:38:45.760 align:middle line:90% asked to do when i is 0. 00:38:45.760 --> 00:38:48.850 align:middle line:84% So mysum will be whatever it is right now plus whatever 00:38:48.850 --> 00:38:49.930 align:middle line:90% i is, 0. 00:38:49.930 --> 00:38:52.510 align:middle line:90% So it stays 0. 00:38:52.510 --> 00:38:54.430 align:middle line:84% Python's done with the code inside. 00:38:54.430 --> 00:38:57.280 align:middle line:84% So now, it's going to take i and change it 00:38:57.280 --> 00:39:01.150 align:middle line:84% to the next value in the sequence, 1. 00:39:01.150 --> 00:39:04.600 align:middle line:84% Now, we're going to do again the stuff inside the loop 00:39:04.600 --> 00:39:05.728 align:middle line:90% with i being 1. 00:39:05.728 --> 00:39:08.020 align:middle line:84% So we're going to take mysum, whatever it is right now, 00:39:08.020 --> 00:39:09.290 align:middle line:90% and add 1 to it. 00:39:09.290 --> 00:39:10.670 align:middle line:90% So it's 1. 00:39:10.670 --> 00:39:11.960 align:middle line:90% And then we're done there. 00:39:11.960 --> 00:39:15.470 align:middle line:84% So Python will take i to be the next value in the sequence, 2. 00:39:15.470 --> 00:39:17.570 align:middle line:84% And then we're going to do again mysum 00:39:17.570 --> 00:39:20.810 align:middle line:84% is whatever it is now, 1, plus whatever i is now, 2. 00:39:20.810 --> 00:39:22.720 align:middle line:90% So it's 3. 00:39:22.720 --> 00:39:25.617 align:middle line:84% Again, i will increment to 3 automatically. 00:39:25.617 --> 00:39:27.200 align:middle line:84% That's the next value in the sequence. 00:39:27.200 --> 00:39:29.470 align:middle line:90% So mysum will get a value of 6. 00:39:29.470 --> 00:39:33.100 align:middle line:84% And then i will change to 4, so on, and so on, 00:39:33.100 --> 00:39:36.010 align:middle line:90% and so on until i becomes 8. 00:39:36.010 --> 00:39:37.840 align:middle line:90% That's sort of towards the end. 00:39:37.840 --> 00:39:42.760 align:middle line:84% When i is 8, the value of mysum is 36, right? 00:39:42.760 --> 00:39:47.160 align:middle line:84% 0 plus 0 plus 1 plus 2 all the way up to 8 is 36. 00:39:47.160 --> 00:39:49.830 align:middle line:84% And then when i becomes 9, Python 00:39:49.830 --> 00:39:52.360 align:middle line:84% will take mysum, whatever it is right now, 36, 00:39:52.360 --> 00:39:54.060 align:middle line:90% add 9 to it to give us 45. 00:39:54.060 --> 00:39:57.933 align:middle line:90% 00:39:57.933 --> 00:39:59.850 align:middle line:84% And then that's the end of the program, right? 00:39:59.850 --> 00:40:03.910 align:middle line:84% There's nothing else to do, except to print mysum. 00:40:03.910 --> 00:40:07.710 align:middle line:84% So at the end of this loop, it's gone through 10 times 00:40:07.710 --> 00:40:09.900 align:middle line:90% adding 0 all the way up to 9. 00:40:09.900 --> 00:40:13.200 align:middle line:90% We're going to print 45. 00:40:13.200 --> 00:40:13.880 align:middle line:90% Yeah? 00:40:13.880 --> 00:40:15.350 align:middle line:90% STUDENT: So I tried running it. 00:40:15.350 --> 00:40:19.160 align:middle line:84% It printed 45 at the top, and then just went 00:40:19.160 --> 00:40:21.980 align:middle line:90% 0, 1, 2, 3, 4 [INAUDIBLE]. 00:40:21.980 --> 00:40:24.995 align:middle line:84% ANA BELL: Oh, maybe you have another print statement. 00:40:24.995 --> 00:40:26.870 align:middle line:84% Or it might be part of another program that's 00:40:26.870 --> 00:40:29.960 align:middle line:84% being run beforehand that you didn't comment out. 00:40:29.960 --> 00:40:31.820 align:middle line:90% Yeah, next question? 00:40:31.820 --> 00:40:34.550 align:middle line:84% STUDENT: [INAUDIBLE] the += syntax [INAUDIBLE]?? 00:40:34.550 --> 00:40:37.220 align:middle line:90% 00:40:37.220 --> 00:40:38.900 align:middle line:84% ANA BELL: The += equals what it means? 00:40:38.900 --> 00:40:49.740 align:middle line:84% Oh, it just means it would be like mysum equals mysum plus i. 00:40:49.740 --> 00:40:52.460 align:middle line:90% 00:40:52.460 --> 00:40:54.830 align:middle line:84% It's just shorthand notation because most 00:40:54.830 --> 00:40:57.170 align:middle line:84% of your variable names might be really long. 00:40:57.170 --> 00:40:59.340 align:middle line:84% And it's really annoying to retype them. 00:40:59.340 --> 00:41:02.840 align:middle line:84% And so that's generally why that shorthand notation exists, 00:41:02.840 --> 00:41:03.350 align:middle line:90% yeah. 00:41:03.350 --> 00:41:06.320 align:middle line:84% But it basically means take whatever mysum is 00:41:06.320 --> 00:41:09.960 align:middle line:90% and add i to it. 00:41:09.960 --> 00:41:13.650 align:middle line:84% And save it back into the variable mysum. 00:41:13.650 --> 00:41:14.150 align:middle line:90% OK. 00:41:14.150 --> 00:41:16.260 align:middle line:84% Let's have you try this code real quick. 00:41:16.260 --> 00:41:18.500 align:middle line:90% So here is code. 00:41:18.500 --> 00:41:21.840 align:middle line:84% It's already on the Python file to start out with. 00:41:21.840 --> 00:41:25.580 align:middle line:90% I want you to have this code-- 00:41:25.580 --> 00:41:28.070 align:middle line:90% it's pretty close to working. 00:41:28.070 --> 00:41:29.790 align:middle line:90% But there's one issue. 00:41:29.790 --> 00:41:32.120 align:middle line:84% So we have this for loop that starts at start 00:41:32.120 --> 00:41:33.260 align:middle line:90% and ends at end. 00:41:33.260 --> 00:41:35.168 align:middle line:90% And we're keeping a running sum. 00:41:35.168 --> 00:41:36.960 align:middle line:84% And then we're printing the sum at the end. 00:41:36.960 --> 00:41:39.380 align:middle line:84% So very similar to what we just saw, but what 00:41:39.380 --> 00:41:42.710 align:middle line:84% I want this code to do is I want it to go and sum up 00:41:42.710 --> 00:41:45.000 align:middle line:90% the start and the end. 00:41:45.000 --> 00:41:48.260 align:middle line:84% So if I have start as 3 and end is 5, 00:41:48.260 --> 00:41:52.820 align:middle line:84% I want it to add 3 plus 4 plus 5. 00:41:52.820 --> 00:41:56.000 align:middle line:84% And so this code is not doing quite that. 00:41:56.000 --> 00:42:01.770 align:middle line:84% And I would like you to fix it or to tell me how to fix it. 00:42:01.770 --> 00:42:04.760 align:middle line:84% So it's down here on line 140-ish. 00:42:04.760 --> 00:42:10.110 align:middle line:90% 00:42:10.110 --> 00:42:12.450 align:middle line:84% First thing you should do is run it and maybe 00:42:12.450 --> 00:42:14.940 align:middle line:84% see what answer it actually gives you. 00:42:14.940 --> 00:42:16.050 align:middle line:90% So I just ran it. 00:42:16.050 --> 00:42:16.920 align:middle line:90% It gave me a 7. 00:42:16.920 --> 00:42:20.480 align:middle line:90% 00:42:20.480 --> 00:42:22.730 align:middle line:84% When you're encountering an output that's 00:42:22.730 --> 00:42:25.930 align:middle line:84% not quite what you expect, one of the first things to do-- you 00:42:25.930 --> 00:42:27.410 align:middle line:84% can obviously use the Python Tutor. 00:42:27.410 --> 00:42:30.290 align:middle line:84% But another thing you can do is put print statements 00:42:30.290 --> 00:42:32.690 align:middle line:90% at various places. 00:42:32.690 --> 00:42:36.540 align:middle line:84% Useful places would be inside for loop. 00:42:36.540 --> 00:42:40.520 align:middle line:90% So here, we can print i. 00:42:40.520 --> 00:42:42.410 align:middle line:84% That's a reasonable thing to print out. 00:42:42.410 --> 00:42:44.870 align:middle line:84% And maybe we'll see exactly what values 00:42:44.870 --> 00:42:48.830 align:middle line:84% of i we are adding because we know the summing works. 00:42:48.830 --> 00:42:51.410 align:middle line:84% We just wrote the program on the previous line-- 00:42:51.410 --> 00:42:54.660 align:middle line:90% or previous slide. 00:42:54.660 --> 00:42:56.845 align:middle line:84% So we got 3, 4, 7, which is a little confusing. 00:42:56.845 --> 00:42:58.220 align:middle line:84% Let's make our print statement be 00:42:58.220 --> 00:43:01.010 align:middle line:84% a little bit better. i equals comma 00:43:01.010 --> 00:43:02.855 align:middle line:84% and then print the actual value of i. 00:43:02.855 --> 00:43:05.460 align:middle line:90% 00:43:05.460 --> 00:43:06.930 align:middle line:90% So what do you guys notice? 00:43:06.930 --> 00:43:08.370 align:middle line:90% i is 3. i is 4. 00:43:08.370 --> 00:43:09.690 align:middle line:90% And then it prints the sum 7. 00:43:09.690 --> 00:43:11.107 align:middle line:84% What's the problem with this code? 00:43:11.107 --> 00:43:14.320 align:middle line:90% 00:43:14.320 --> 00:43:15.950 align:middle line:90% Yeah? 00:43:15.950 --> 00:43:17.740 align:middle line:90% STUDENT: [INAUDIBLE] 00:43:17.740 --> 00:43:19.930 align:middle line:84% ANA BELL: Yeah, we're not adding 5, right? 00:43:19.930 --> 00:43:24.940 align:middle line:84% We're just adding 0, originally, plus 3 plus 4. 00:43:24.940 --> 00:43:27.570 align:middle line:90% And we've never added 5. 00:43:27.570 --> 00:43:29.340 align:middle line:90% So how can we fix that? 00:43:29.340 --> 00:43:30.720 align:middle line:90% Yeah? 00:43:30.720 --> 00:43:31.995 align:middle line:90% STUDENT: end plus 1. 00:43:31.995 --> 00:43:34.740 align:middle line:84% ANA BELL: Yeah, we can do n plus 1, exactly. 00:43:34.740 --> 00:43:37.680 align:middle line:84% So the range, remember, grabs the-- oh, plus 1. 00:43:37.680 --> 00:43:43.650 align:middle line:84% The range grabs these values as numbers that it's working with. 00:43:43.650 --> 00:43:45.360 align:middle line:90% So start is OK. 00:43:45.360 --> 00:43:46.470 align:middle line:90% end is OK. 00:43:46.470 --> 00:43:48.990 align:middle line:84% But we go up to but not including end. 00:43:48.990 --> 00:43:52.050 align:middle line:84% So if we go to end plus 1, we're going to go up to 00:43:52.050 --> 00:43:54.210 align:middle line:90% but not including end plus 1. 00:43:54.210 --> 00:43:59.190 align:middle line:84% So if we run it now, it looks much better. 00:43:59.190 --> 00:44:03.360 align:middle line:84% So we've got i is 3, i is 4, i is 5, and the sum is 12. 00:44:03.360 --> 00:44:03.860 align:middle line:90% Perfect. 00:44:03.860 --> 00:44:07.040 align:middle line:90% 00:44:07.040 --> 00:44:12.478 align:middle line:84% So print statements are very useful when debugging code. 00:44:12.478 --> 00:44:13.520 align:middle line:90% Questions about this one? 00:44:13.520 --> 00:44:15.275 align:middle line:90% Or does this make sense? 00:44:15.275 --> 00:44:18.320 align:middle line:90% 00:44:18.320 --> 00:44:20.240 align:middle line:90% OK. 00:44:20.240 --> 00:44:23.000 align:middle line:84% The last slide I want to do before we do a summary 00:44:23.000 --> 00:44:25.490 align:middle line:84% is just to show you that factorial code we 00:44:25.490 --> 00:44:28.890 align:middle line:84% saw using a while loop a few slides back. 00:44:28.890 --> 00:44:31.100 align:middle line:90% So it looks really verbose. 00:44:31.100 --> 00:44:33.050 align:middle line:84% We kind of have to think about it for a while 00:44:33.050 --> 00:44:36.320 align:middle line:84% before we realized what it's actually doing. 00:44:36.320 --> 00:44:38.270 align:middle line:84% But it was calculating the factorial. 00:44:38.270 --> 00:44:41.180 align:middle line:84% Obviously, good variable names helped us figure that out. 00:44:41.180 --> 00:44:45.620 align:middle line:84% That same code in a for loop looks 00:44:45.620 --> 00:44:47.610 align:middle line:84% like this-- with a for loop looks like this. 00:44:47.610 --> 00:44:50.480 align:middle line:84% So we still have the initialization of x to 4. 00:44:50.480 --> 00:44:52.730 align:middle line:84% We still initialize our factorial, 00:44:52.730 --> 00:44:56.420 align:middle line:90% our running product to a 1. 00:44:56.420 --> 00:45:00.530 align:middle line:84% But we're losing the four lines of code 00:45:00.530 --> 00:45:04.910 align:middle line:84% that make up that pattern of changing numbers 00:45:04.910 --> 00:45:06.440 align:middle line:90% with while loops into 2. 00:45:06.440 --> 00:45:09.440 align:middle line:84% So this line, i equals 1, this while loop 00:45:09.440 --> 00:45:13.280 align:middle line:84% with the conditional, and this incrementing of i become 00:45:13.280 --> 00:45:16.220 align:middle line:90% the for loop and that's it. 00:45:16.220 --> 00:45:19.310 align:middle line:84% The for loop takes care of all of that-- the initialization, 00:45:19.310 --> 00:45:23.630 align:middle line:84% the increment, and going up to but not including 00:45:23.630 --> 00:45:26.120 align:middle line:90% the last value, x plus 1. 00:45:26.120 --> 00:45:29.420 align:middle line:84% So we're going to multiply the factorial with 1, 00:45:29.420 --> 00:45:32.495 align:middle line:84% then 2, then 3, then 4 all the way up to and including x. 00:45:32.495 --> 00:45:36.800 align:middle line:90% 00:45:36.800 --> 00:45:39.500 align:middle line:84% So the big idea about for loops is 00:45:39.500 --> 00:45:42.170 align:middle line:84% they're going to repeat however long the sequence is. 00:45:42.170 --> 00:45:44.750 align:middle line:84% So you're able to repeat certain code 00:45:44.750 --> 00:45:46.520 align:middle line:84% a set number of times, which is really 00:45:46.520 --> 00:45:47.990 align:middle line:90% useful in some situations. 00:45:47.990 --> 00:45:49.610 align:middle line:84% While loops were useful in situations 00:45:49.610 --> 00:45:51.110 align:middle line:84% where you didn't know how many times 00:45:51.110 --> 00:45:53.450 align:middle line:84% you might want to repeat the code. 00:45:53.450 --> 00:45:54.750 align:middle line:90% So a quick summary-- 00:45:54.750 --> 00:45:56.630 align:middle line:84% we saw some looping mechanisms today. 00:45:56.630 --> 00:45:58.940 align:middle line:90% It was a lot of syntax, I know. 00:45:58.940 --> 00:46:02.030 align:middle line:84% But the finger exercises for today will certainly help. 00:46:02.030 --> 00:46:06.770 align:middle line:84% MITx also has extra help, extra exercises. 00:46:06.770 --> 00:46:09.110 align:middle line:84% It's really important to do them, 00:46:09.110 --> 00:46:12.860 align:middle line:84% just to get the mental model for how exactly these loops work, 00:46:12.860 --> 00:46:16.400 align:middle line:84% and how they assign variables, and how they do 00:46:16.400 --> 00:46:18.320 align:middle line:90% the checks behind the scenes. 00:46:18.320 --> 00:46:21.500 align:middle line:84% And it'll help you get faster at writing code 00:46:21.500 --> 00:46:24.380 align:middle line:90% and at doing quizzes as well. 00:46:24.380 --> 00:46:27.130 align:middle line:90% So that's it for today. 00:46:27.130 --> 00:46:41.000 align:middle line:90%