WEBVTT 00:00:00.000 --> 00:00:04.940 align:middle line:84% [SQUEAKING] [RUSTLING] [CLICKING] 00:00:04.940 --> 00:00:16.302 align:middle line:90% 00:00:16.302 --> 00:00:21.440 align:middle line:84% ANA BELL: All right, so welcome to the first lecture of 6.100L. 00:00:21.440 --> 00:00:23.150 align:middle line:90% That's our new number. 00:00:23.150 --> 00:00:24.560 align:middle line:90% My name is Ana Bell. 00:00:24.560 --> 00:00:26.870 align:middle line:84% That's two separate names, first name Ana, 00:00:26.870 --> 00:00:28.890 align:middle line:84% last name Bell-- super confusing. 00:00:28.890 --> 00:00:31.310 align:middle line:84% But I've been a lecturer here in the EECS Department 00:00:31.310 --> 00:00:33.800 align:middle line:84% for probably almost 10 years now. 00:00:33.800 --> 00:00:36.170 align:middle line:84% And I've been doing the intro course for a while. 00:00:36.170 --> 00:00:39.230 align:middle line:84% I'm really happy to be teaching this full semester version 00:00:39.230 --> 00:00:41.750 align:middle line:90% of 6.100A. 00:00:41.750 --> 00:00:45.530 align:middle line:84% So today what we're going to do is go over 00:00:45.530 --> 00:00:48.560 align:middle line:84% a little bit of course administrative information, 00:00:48.560 --> 00:00:50.840 align:middle line:84% and then we'll dive right into some thoughts 00:00:50.840 --> 00:00:53.420 align:middle line:84% about computers, high level how they work. 00:00:53.420 --> 00:00:56.700 align:middle line:84% And then we'll start going into some Python basics. 00:00:56.700 --> 00:00:59.000 align:middle line:84% So we're going to get coding right away. 00:00:59.000 --> 00:01:01.880 align:middle line:84% So I highly encourage you, since you're in this class, 00:01:01.880 --> 00:01:04.519 align:middle line:84% to download the lecture slides beforehand, 00:01:04.519 --> 00:01:08.690 align:middle line:84% to take notes, and run code when I do. 00:01:08.690 --> 00:01:10.730 align:middle line:84% Some of the lectures are interactive. 00:01:10.730 --> 00:01:12.260 align:middle line:84% And we'll have breaks, so there'll 00:01:12.260 --> 00:01:14.480 align:middle line:84% be a place where you can take a break 00:01:14.480 --> 00:01:16.440 align:middle line:90% to actually do some coding. 00:01:16.440 --> 00:01:18.137 align:middle line:90% And that's important-- 00:01:18.137 --> 00:01:19.470 align:middle line:90% I call them "you try it" breaks. 00:01:19.470 --> 00:01:21.553 align:middle line:84% That's important to make sure that you're actually 00:01:21.553 --> 00:01:24.780 align:middle line:84% practicing what we are learning right at this time. 00:01:24.780 --> 00:01:28.290 align:middle line:84% The main idea for lectures is, yes, I will do some teaching, 00:01:28.290 --> 00:01:29.790 align:middle line:84% but there will also be opportunities 00:01:29.790 --> 00:01:33.990 align:middle line:84% for questions and for you guys to try some programming 00:01:33.990 --> 00:01:35.010 align:middle line:90% right on the spot. 00:01:35.010 --> 00:01:36.780 align:middle line:84% Even if you don't finish writing a program 00:01:36.780 --> 00:01:39.390 align:middle line:84% that we start talking about, I will finish it, 00:01:39.390 --> 00:01:41.430 align:middle line:84% and we can all talk about it together. 00:01:41.430 --> 00:01:46.270 align:middle line:84% And I'll show you some pitfalls and things like that. 00:01:46.270 --> 00:01:47.760 align:middle line:84% There will be lots of opportunities 00:01:47.760 --> 00:01:52.320 align:middle line:84% to practice in this class at various degrees of granularity. 00:01:52.320 --> 00:01:54.510 align:middle line:84% And then there's also lots of opportunities 00:01:54.510 --> 00:01:58.050 align:middle line:84% that I have in the handouts to do extra practice 00:01:58.050 --> 00:02:02.330 align:middle line:84% at home and through a bunch of different resources as well. 00:02:02.330 --> 00:02:05.590 align:middle line:84% The reason why I stress participation and practice is 00:02:05.590 --> 00:02:08.410 align:middle line:84% because part of the reason you're here is you 00:02:08.410 --> 00:02:10.180 align:middle line:90% want to learn how to program. 00:02:10.180 --> 00:02:12.160 align:middle line:84% You don't know how to program yet. 00:02:12.160 --> 00:02:14.890 align:middle line:84% And programming is actually a skill. 00:02:14.890 --> 00:02:16.090 align:middle line:90% it's like math or reading. 00:02:16.090 --> 00:02:18.100 align:middle line:84% It's something that you have to practice. 00:02:18.100 --> 00:02:21.290 align:middle line:84% You can't just watch me type in a bunch of lines of code 00:02:21.290 --> 00:02:23.800 align:middle line:84% And then when it comes time to do the quiz, 00:02:23.800 --> 00:02:25.600 align:middle line:84% you automatically know how to do it. 00:02:25.600 --> 00:02:29.530 align:middle line:84% You need to do it often, more and more so that it 00:02:29.530 --> 00:02:32.110 align:middle line:90% becomes sort of second nature. 00:02:32.110 --> 00:02:35.680 align:middle line:84% So the three big things you'll get out of this class are 00:02:35.680 --> 00:02:37.300 align:middle line:84% knowledge of concepts, obviously-- 00:02:37.300 --> 00:02:39.640 align:middle line:84% we're going to learn some computer science ideas-- 00:02:39.640 --> 00:02:43.840 align:middle line:84% programming skill, and problem solving-- 00:02:43.840 --> 00:02:45.190 align:middle line:90% problem solving skills. 00:02:45.190 --> 00:02:48.167 align:middle line:84% Lectures and exams basically help you 00:02:48.167 --> 00:02:50.500 align:middle line:84% with your knowledge of-- test your knowledge of concepts 00:02:50.500 --> 00:02:52.690 align:middle line:84% and help you get knowledge of concepts. 00:02:52.690 --> 00:02:56.650 align:middle line:84% Finger exercises give you the programming skills. 00:02:56.650 --> 00:02:59.950 align:middle line:84% And the problem sets help you with problem solving. 00:02:59.950 --> 00:03:04.070 align:middle line:84% Basically, if you're given an English version of-- a problem 00:03:04.070 --> 00:03:07.520 align:middle line:84% in English, how do you go from that 00:03:07.520 --> 00:03:10.730 align:middle line:84% to thinking about what computer science concepts can I apply? 00:03:10.730 --> 00:03:14.300 align:middle line:84% And then after that, how do I take those computer science 00:03:14.300 --> 00:03:17.340 align:middle line:84% concepts and actually do the programming? 00:03:17.340 --> 00:03:20.760 align:middle line:84% So what are some topics we'll be covering? 00:03:20.760 --> 00:03:23.480 align:middle line:84% We will be, at the core of it, learning 00:03:23.480 --> 00:03:24.800 align:middle line:90% computational thinking. 00:03:24.800 --> 00:03:28.880 align:middle line:84% So in the future, when you encounter a problem, 00:03:28.880 --> 00:03:31.910 align:middle line:84% your first thought shouldn't be, How do I mathematically 00:03:31.910 --> 00:03:34.100 align:middle line:84% solve this? or, How do I brute force 00:03:34.100 --> 00:03:35.700 align:middle line:90% or manually solve this problem? 00:03:35.700 --> 00:03:39.248 align:middle line:84% How can I apply computation to help me solve this problem? 00:03:39.248 --> 00:03:40.790 align:middle line:84% And throughout these lectures, you're 00:03:40.790 --> 00:03:44.330 align:middle line:84% going to see some examples of us applying computation 00:03:44.330 --> 00:03:46.490 align:middle line:84% to a problem you might have already seen and maybe 00:03:46.490 --> 00:03:51.060 align:middle line:84% solved mathematically, which is pretty cool. 00:03:51.060 --> 00:03:52.910 align:middle line:84% Obviously, to get that, we're going 00:03:52.910 --> 00:03:54.710 align:middle line:84% to learn the Python programming language. 00:03:54.710 --> 00:03:56.360 align:middle line:84% Once we get the basics, we're going 00:03:56.360 --> 00:03:59.600 align:middle line:84% to see how we can start to structure our code 00:03:59.600 --> 00:04:01.580 align:middle line:84% to look a little bit better so we don't just 00:04:01.580 --> 00:04:04.670 align:middle line:84% have a bunch of code dumped in a file. 00:04:04.670 --> 00:04:06.530 align:middle line:84% We're going to start to organize our code 00:04:06.530 --> 00:04:11.360 align:middle line:84% and see how we can make it neat, readable, and modular. 00:04:11.360 --> 00:04:14.750 align:middle line:90% And then towards the-- 00:04:14.750 --> 00:04:16.970 align:middle line:84% not in this lecture but in a couple of lectures 00:04:16.970 --> 00:04:19.260 align:middle line:84% and as a theme throughout this class, 00:04:19.260 --> 00:04:20.959 align:middle line:84% we're going to look at some algorithms. 00:04:20.959 --> 00:04:23.300 align:middle line:84% They're not super complicated, but they're 00:04:23.300 --> 00:04:26.150 align:middle line:84% kind of the base algorithms for a bunch of algorithms 00:04:26.150 --> 00:04:28.190 align:middle line:84% you might see in the future if you 00:04:28.190 --> 00:04:30.708 align:middle line:90% decide to take more CS classes. 00:04:30.708 --> 00:04:32.250 align:middle line:84% Lastly, towards the end of the class, 00:04:32.250 --> 00:04:34.940 align:middle line:84% we're going to see algorithmic complexity, which basically 00:04:34.940 --> 00:04:37.670 align:middle line:84% means we're going to start asking or trying to answer 00:04:37.670 --> 00:04:40.160 align:middle line:84% the question, how do we know the programs 00:04:40.160 --> 00:04:42.070 align:middle line:90% we write are efficient? 00:04:42.070 --> 00:04:44.570 align:middle line:84% We can write programs, but how do we know that they're fast, 00:04:44.570 --> 00:04:47.328 align:middle line:84% and how do we know that they don't take up 00:04:47.328 --> 00:04:48.620 align:middle line:90% all the memory in the computer? 00:04:48.620 --> 00:04:51.710 align:middle line:84% So things like that, comparing different algorithms 00:04:51.710 --> 00:04:55.670 align:middle line:84% that do the same thing against each other. 00:04:55.670 --> 00:04:58.330 align:middle line:90% So if there's no questions-- 00:04:58.330 --> 00:05:00.340 align:middle line:84% again, as I said, a bunch of this information 00:05:00.340 --> 00:05:02.470 align:middle line:84% is already in the handout plus more-- 00:05:02.470 --> 00:05:05.840 align:middle line:90% we can begin. 00:05:05.840 --> 00:05:11.240 align:middle line:84% OK, so let's start by talking about knowledge. 00:05:11.240 --> 00:05:14.210 align:middle line:84% Declarative knowledge is a statement of fact, 00:05:14.210 --> 00:05:17.420 align:middle line:84% and a lot of us probably in math and in the past 00:05:17.420 --> 00:05:19.250 align:middle line:84% have worked with declarative knowledge. 00:05:19.250 --> 00:05:21.300 align:middle line:84% But this is not how computer science, this is not 00:05:21.300 --> 00:05:23.570 align:middle line:90% how this class works. 00:05:23.570 --> 00:05:25.940 align:middle line:84% In computer science what we do is we 00:05:25.940 --> 00:05:28.790 align:middle line:84% work with imperative knowledge, which is basically a recipe, 00:05:28.790 --> 00:05:30.540 align:middle line:90% how to do something. 00:05:30.540 --> 00:05:33.050 align:middle line:84% And when we're programming, all we're doing 00:05:33.050 --> 00:05:35.870 align:middle line:84% is writing a recipe for the computer to do something. 00:05:35.870 --> 00:05:37.920 align:middle line:90% That's it. 00:05:37.920 --> 00:05:40.530 align:middle line:90% So here's a numerical example. 00:05:40.530 --> 00:05:44.210 align:middle line:84% The first statement is a declarative statement. 00:05:44.210 --> 00:05:47.060 align:middle line:84% The square root of a number x is y such that y times y 00:05:47.060 --> 00:05:47.930 align:middle line:90% is equal to x. 00:05:47.930 --> 00:05:50.150 align:middle line:84% There are many possible values for x and y 00:05:50.150 --> 00:05:53.600 align:middle line:84% that this statement can be true, right? 00:05:53.600 --> 00:05:55.880 align:middle line:84% But if we gave that statement to a computer, 00:05:55.880 --> 00:05:57.380 align:middle line:84% it wouldn't know what to do with it. 00:05:57.380 --> 00:06:00.770 align:middle line:84% What we need to do is tell the computer 00:06:00.770 --> 00:06:03.440 align:middle line:84% how to find the square root of a number 00:06:03.440 --> 00:06:06.060 align:middle line:84% and then tell us what the square root of that number is. 00:06:06.060 --> 00:06:09.380 align:middle line:84% And so the computer then needs a recipe. 00:06:09.380 --> 00:06:11.300 align:middle line:84% So the recipe, a really simple one 00:06:11.300 --> 00:06:13.010 align:middle line:84% for finding the square root of a number, 00:06:13.010 --> 00:06:15.500 align:middle line:90% is steps one, two, three. 00:06:15.500 --> 00:06:17.540 align:middle line:84% So what we do is-- let's say we want 00:06:17.540 --> 00:06:20.680 align:middle line:90% to find the square root of 16. 00:06:20.680 --> 00:06:23.950 align:middle line:84% We obviously know it's four, but the computer doesn't. 00:06:23.950 --> 00:06:25.530 align:middle line:84% And so we give it an initial guess. 00:06:25.530 --> 00:06:27.680 align:middle line:90% Let's say the guess is three. 00:06:27.680 --> 00:06:29.400 align:middle line:90% How do we go from there? 00:06:29.400 --> 00:06:30.920 align:middle line:90% So the steps we follow-- 00:06:30.920 --> 00:06:35.960 align:middle line:84% step one, if 3 times 3, 9, is close enough to 16, 00:06:35.960 --> 00:06:37.140 align:middle line:90% we can stop. 00:06:37.140 --> 00:06:39.320 align:middle line:84% It's not really close enough for me. 00:06:39.320 --> 00:06:41.910 align:middle line:90% So let's keep going. 00:06:41.910 --> 00:06:44.810 align:middle line:84% Step two-- otherwise, we're going 00:06:44.810 --> 00:06:46.940 align:middle line:84% to make a new guess by averaging g, 00:06:46.940 --> 00:06:50.180 align:middle line:84% which is our original guess, 3, and x 00:06:50.180 --> 00:06:52.280 align:middle line:90% over g, which is 16 over 3. 00:06:52.280 --> 00:06:54.420 align:middle line:84% 16 was the square root we wanted to find. 00:06:54.420 --> 00:06:58.210 align:middle line:90% So our next guess is 4.17. 00:06:58.210 --> 00:07:01.000 align:middle line:84% OK, using the new guess, repeat the process 00:07:01.000 --> 00:07:02.200 align:middle line:90% until we are close enough. 00:07:02.200 --> 00:07:03.910 align:middle line:90% So we go back to step one. 00:07:03.910 --> 00:07:05.800 align:middle line:84% That's the first part of the process. 00:07:05.800 --> 00:07:07.210 align:middle line:90% We find guess squared. 00:07:07.210 --> 00:07:10.390 align:middle line:90% 4.17 squared is 17.36. 00:07:10.390 --> 00:07:13.450 align:middle line:84% So now we say, is that close enough? 00:07:13.450 --> 00:07:14.290 align:middle line:90% Not really. 00:07:14.290 --> 00:07:14.830 align:middle line:90% It's not. 00:07:14.830 --> 00:07:15.650 align:middle line:90% It's 17. 00:07:15.650 --> 00:07:17.600 align:middle line:84% It's not really even close to 16. 00:07:17.600 --> 00:07:19.450 align:middle line:90% So let's do it again. 00:07:19.450 --> 00:07:26.140 align:middle line:84% We make a new guess by averaging 4.17 and 16 divided by 4.17. 00:07:26.140 --> 00:07:30.150 align:middle line:84% That gives us our new guess, 4.0035. 00:07:30.150 --> 00:07:34.210 align:middle line:84% OK, next step, using the new guess, we repeat the process. 00:07:34.210 --> 00:07:38.430 align:middle line:90% So 4.0035 squared is 16.277-- 00:07:38.430 --> 00:07:40.080 align:middle line:90% .0277. 00:07:40.080 --> 00:07:42.720 align:middle line:90% Is that close enough to x? 00:07:42.720 --> 00:07:44.490 align:middle line:84% Yeah, I could be happy with this. 00:07:44.490 --> 00:07:47.970 align:middle line:84% I could stop there because we're within sort of plus/minus 1. 00:07:47.970 --> 00:07:49.300 align:middle line:90% So I'm OK with that. 00:07:49.300 --> 00:07:52.320 align:middle line:84% But if we want it to be within plus or minus 1 times 10 00:07:52.320 --> 00:07:55.170 align:middle line:84% to the negative 6 or 7 or something like that, 00:07:55.170 --> 00:07:58.760 align:middle line:84% then we would continue the process. 00:07:58.760 --> 00:08:01.840 align:middle line:84% So really what we had there is an algorithm. 00:08:01.840 --> 00:08:03.608 align:middle line:90% It's a sequence of steps-- 00:08:03.608 --> 00:08:04.900 align:middle line:90% step one, step two, step three. 00:08:04.900 --> 00:08:07.450 align:middle line:84% There's some sort of flow of control. 00:08:07.450 --> 00:08:12.070 align:middle line:84% We had a place where we said if the guess is close enough, 00:08:12.070 --> 00:08:13.640 align:middle line:90% then we can stop. 00:08:13.640 --> 00:08:15.550 align:middle line:90% Otherwise, we do something else. 00:08:15.550 --> 00:08:17.710 align:middle line:84% We had another flow of control where 00:08:17.710 --> 00:08:19.240 align:middle line:90% we said repeat this thing. 00:08:19.240 --> 00:08:21.100 align:middle line:84% So we're kind of not going linearly, 00:08:21.100 --> 00:08:23.160 align:middle line:90% but we're changing the flow. 00:08:23.160 --> 00:08:25.740 align:middle line:84% And then lastly is a way to stop. 00:08:25.740 --> 00:08:27.810 align:middle line:84% We don't want the algorithm to go on forever. 00:08:27.810 --> 00:08:29.370 align:middle line:84% We would like to stop at some point. 00:08:29.370 --> 00:08:32.429 align:middle line:84% And this stopping point, I was kind of vague about it. 00:08:32.429 --> 00:08:35.940 align:middle line:84% But it could be when we were within plus or minus 1 00:08:35.940 --> 00:08:39.669 align:middle line:90% of the actual answer. 00:08:39.669 --> 00:08:44.680 align:middle line:84% And so recipes are basically algorithms, right? 00:08:44.680 --> 00:08:47.520 align:middle line:84% My grandmother was basically teaching algorithms 00:08:47.520 --> 00:08:52.020 align:middle line:84% when she would teach me to bake a cake. 00:08:52.020 --> 00:08:55.380 align:middle line:84% She didn't call it that, but she was really. 00:08:55.380 --> 00:08:58.410 align:middle line:84% And so even recipes have that same structure. 00:08:58.410 --> 00:08:59.697 align:middle line:90% There's a sequence of steps. 00:08:59.697 --> 00:09:00.780 align:middle line:90% There's a flow of control. 00:09:00.780 --> 00:09:03.000 align:middle line:84% Like, if you don't have egg, use egg substitute. 00:09:03.000 --> 00:09:06.690 align:middle line:84% Or repeat sticking a toothpick to see 00:09:06.690 --> 00:09:09.520 align:middle line:84% if it comes out clean every minute or something like that. 00:09:09.520 --> 00:09:12.210 align:middle line:90% And then there's a way to stop. 00:09:12.210 --> 00:09:13.680 align:middle line:84% When the toothpick comes out clean, 00:09:13.680 --> 00:09:16.800 align:middle line:84% you take it out of the oven, and you eat it. 00:09:16.800 --> 00:09:19.700 align:middle line:84% And so computers are machines that execute these algorithms. 00:09:19.700 --> 00:09:22.190 align:middle line:90% They're actually dumb. 00:09:22.190 --> 00:09:24.240 align:middle line:90% Computers are not very smart. 00:09:24.240 --> 00:09:26.220 align:middle line:84% They don't make decisions on their own. 00:09:26.220 --> 00:09:28.490 align:middle line:84% They just follow these sequences of steps 00:09:28.490 --> 00:09:30.440 align:middle line:90% that we told them to do. 00:09:30.440 --> 00:09:35.000 align:middle line:84% Computers are good at storing lots and lots of data. 00:09:35.000 --> 00:09:36.680 align:middle line:84% We can't really do that, but computers 00:09:36.680 --> 00:09:40.520 align:middle line:84% can store gigabytes of storage, terabytes even. 00:09:40.520 --> 00:09:42.450 align:middle line:84% And computers can do operations really, 00:09:42.450 --> 00:09:44.720 align:middle line:84% really quickly, which is something we can't do. 00:09:44.720 --> 00:09:46.170 align:middle line:84% They're good at those two things, 00:09:46.170 --> 00:09:47.820 align:middle line:90% but they're not very smart. 00:09:47.820 --> 00:09:49.880 align:middle line:84% They can't make decisions unless they're 00:09:49.880 --> 00:09:54.180 align:middle line:90% told to make the decisions. 00:09:54.180 --> 00:09:59.240 align:middle line:84% So really, the computer only does what you tell it to do. 00:09:59.240 --> 00:10:02.830 align:middle line:84% And that's one of the big ideas that I want you to come away 00:10:02.830 --> 00:10:05.080 align:middle line:90% from this lecture with. 00:10:05.080 --> 00:10:07.420 align:middle line:84% Computer only does what you tell it to do. 00:10:07.420 --> 00:10:09.760 align:middle line:84% The sequences of steps that you tell it to do, 00:10:09.760 --> 00:10:12.520 align:middle line:84% that's the only thing it follows. 00:10:12.520 --> 00:10:15.810 align:middle line:84% So a little brief history just to make you appreciate 00:10:15.810 --> 00:10:19.320 align:middle line:84% programming, Python programming language before we actually get 00:10:19.320 --> 00:10:22.120 align:middle line:90% started with it is-- 00:10:22.120 --> 00:10:24.840 align:middle line:84% so before the 1940s, we had these things 00:10:24.840 --> 00:10:27.240 align:middle line:90% called fixed program computers. 00:10:27.240 --> 00:10:29.280 align:middle line:84% A pocket calculator as an example of that. 00:10:29.280 --> 00:10:33.910 align:middle line:90% Every button was an operation. 00:10:33.910 --> 00:10:36.010 align:middle line:84% In the little screen, you could use parentheses 00:10:36.010 --> 00:10:38.110 align:middle line:84% to put a bunch of different operations together, 00:10:38.110 --> 00:10:41.500 align:middle line:84% but there was no way to store all these operations together 00:10:41.500 --> 00:10:46.227 align:middle line:84% to later put in different inputs for that same sequence 00:10:46.227 --> 00:10:46.810 align:middle line:90% of operations. 00:10:46.810 --> 00:10:49.270 align:middle line:84% You had to input it every single-- input those sequences 00:10:49.270 --> 00:10:52.980 align:middle line:90% of operations every single time. 00:10:52.980 --> 00:10:57.810 align:middle line:84% After the 1940s, stored programs computers came into play. 00:10:57.810 --> 00:11:03.600 align:middle line:84% And they were able to store instructions to do things 00:11:03.600 --> 00:11:06.390 align:middle line:90% as data. 00:11:06.390 --> 00:11:09.390 align:middle line:84% And there was a special program called an interpreter that 00:11:09.390 --> 00:11:10.860 align:middle line:90% executed these instructions. 00:11:10.860 --> 00:11:14.370 align:middle line:84% It knew how to follow simple sequences of steps. 00:11:14.370 --> 00:11:17.520 align:middle line:84% When the program told it to go to a different location, 00:11:17.520 --> 00:11:18.180 align:middle line:90% it did. 00:11:18.180 --> 00:11:23.780 align:middle line:84% So it was basically executing these instructions. 00:11:23.780 --> 00:11:28.730 align:middle line:84% And the instructions that it did were arithmetic and logical, 00:11:28.730 --> 00:11:30.770 align:middle line:84% so addition, subtraction, things like that; 00:11:30.770 --> 00:11:34.280 align:middle line:84% simple tests like checking for equality between two values; 00:11:34.280 --> 00:11:36.500 align:middle line:84% and moving data, so taking this value 00:11:36.500 --> 00:11:39.560 align:middle line:84% and putting it at a different memory location. 00:11:39.560 --> 00:11:41.860 align:middle line:84% So I just wanted to give you a really brief overview, 00:11:41.860 --> 00:11:44.410 align:middle line:84% and this is not super accurate, but it gives you 00:11:44.410 --> 00:11:47.410 align:middle line:84% a sense of how exactly things happen 00:11:47.410 --> 00:11:50.420 align:middle line:90% low level in the computer. 00:11:50.420 --> 00:11:52.660 align:middle line:84% So the computer basically has memory, 00:11:52.660 --> 00:11:54.040 align:middle line:90% where things are stored. 00:11:54.040 --> 00:11:57.730 align:middle line:84% It has an arithmetic logic unit that does operations. 00:11:57.730 --> 00:12:00.130 align:middle line:84% It knows how to add things, subtract things, multiply 00:12:00.130 --> 00:12:01.810 align:middle line:90% things, compare things. 00:12:01.810 --> 00:12:03.970 align:middle line:84% And then it has the control unit, where 00:12:03.970 --> 00:12:06.590 align:middle line:90% this program counter is set. 00:12:06.590 --> 00:12:08.610 align:middle line:84% And this is where you put a program in. 00:12:08.610 --> 00:12:10.570 align:middle line:90% So let's see if this works. 00:12:10.570 --> 00:12:12.280 align:middle line:90% This is a program. 00:12:12.280 --> 00:12:14.480 align:middle line:90% And up here is our memory. 00:12:14.480 --> 00:12:18.250 align:middle line:84% So we have a bunch of memory locations, 3456, 3457. 00:12:18.250 --> 00:12:20.500 align:middle line:84% And at each of these memory locations, 00:12:20.500 --> 00:12:24.750 align:middle line:84% we have some values stored, prefilled. 00:12:24.750 --> 00:12:28.170 align:middle line:84% So when we first run this program, what ends up happening 00:12:28.170 --> 00:12:31.260 align:middle line:84% is that the interpreter sees the first instruction, 00:12:31.260 --> 00:12:35.650 align:middle line:84% Add, the values at 3456 and 3457 together. 00:12:35.650 --> 00:12:38.190 align:middle line:84% So it goes to these memory locations here, 00:12:38.190 --> 00:12:40.710 align:middle line:84% grabs the 3 and the 4, and sends them 00:12:40.710 --> 00:12:42.480 align:middle line:90% to the Arithmetic Logic Unit. 00:12:42.480 --> 00:12:44.710 align:middle line:84% The ALU knows how to do the addition. 00:12:44.710 --> 00:12:49.560 align:middle line:84% So it adds 3 plus 4, 7, and sends the result back here. 00:12:49.560 --> 00:12:52.710 align:middle line:84% Now, we never told it to store that result anywhere. 00:12:52.710 --> 00:12:54.930 align:middle line:84% But the next instruction says Store 00:12:54.930 --> 00:12:57.270 align:middle line:84% the value you just got back from the ALU 00:12:57.270 --> 00:12:59.760 align:middle line:90% at this memory location, 3458. 00:12:59.760 --> 00:13:04.380 align:middle line:84% So the next step basically takes that 7 00:13:04.380 --> 00:13:08.320 align:middle line:84% and stores it at memory location 3458. 00:13:08.320 --> 00:13:12.650 align:middle line:84% Super tedious-- all we did was add 3 plus 4. 00:13:12.650 --> 00:13:14.060 align:middle line:90% We do that again. 00:13:14.060 --> 00:13:18.020 align:middle line:84% We add the values at 7889 and 7890. 00:13:18.020 --> 00:13:19.310 align:middle line:90% So it goes in the memory. 00:13:19.310 --> 00:13:23.410 align:middle line:84% It grabs the 5 and the 2, sends it to the ALU. 00:13:23.410 --> 00:13:26.710 align:middle line:84% The ALU calculates it as 7, brings it back, 00:13:26.710 --> 00:13:32.270 align:middle line:84% and then we store that in location 7891. 00:13:32.270 --> 00:13:34.970 align:middle line:84% And then after that, all we've done is two additions. 00:13:34.970 --> 00:13:38.330 align:middle line:84% And then the next instruction says Compare the values 00:13:38.330 --> 00:13:41.780 align:middle line:84% at memory locations 3458 and 7891. 00:13:41.780 --> 00:13:45.670 align:middle line:84% So we're going to compare the 7 with the 7. 00:13:45.670 --> 00:13:49.430 align:middle line:84% The ALU again does this comparison and says, 00:13:49.430 --> 00:13:51.350 align:middle line:84% all right, well, 7 and 7 are equivalent. 00:13:51.350 --> 00:13:54.790 align:middle line:84% So this is true or whatever it wants to give back 00:13:54.790 --> 00:13:56.860 align:middle line:90% to the interpreter. 00:13:56.860 --> 00:13:59.140 align:middle line:84% And then the last instruction here we 00:13:59.140 --> 00:14:01.820 align:middle line:84% have is Print the result of that comparison. 00:14:01.820 --> 00:14:04.930 align:middle line:84% So we print True because they were equal. 00:14:04.930 --> 00:14:06.910 align:middle line:84% Again, super high level, but it kind of 00:14:06.910 --> 00:14:10.160 align:middle line:84% gives you an appreciation for programming languages 00:14:10.160 --> 00:14:10.750 align:middle line:90% these days. 00:14:10.750 --> 00:14:14.260 align:middle line:84% This is very tedious to write if we had to write programs 00:14:14.260 --> 00:14:16.470 align:middle line:90% in this manner. 00:14:16.470 --> 00:14:17.970 align:middle line:84% Alan Turing a long time ago showed 00:14:17.970 --> 00:14:20.400 align:middle line:84% that you can compute anything with actually 00:14:20.400 --> 00:14:22.170 align:middle line:84% an even more basic set of primitives, 00:14:22.170 --> 00:14:23.700 align:middle line:90% not addition, subtraction. 00:14:23.700 --> 00:14:26.640 align:middle line:84% But instead, with a tape, you would actually 00:14:26.640 --> 00:14:29.820 align:middle line:84% have six primitives-- move the tape left, move the tape right, 00:14:29.820 --> 00:14:33.780 align:middle line:84% read the value at the tape, put a value on the tape, 00:14:33.780 --> 00:14:38.130 align:middle line:84% erase the value from the tape, and no operation. 00:14:38.130 --> 00:14:43.790 align:middle line:84% And so since he showed this what the result of it actually was 00:14:43.790 --> 00:14:46.760 align:middle line:90% is down here. 00:14:46.760 --> 00:14:48.710 align:middle line:84% Anything computable in one language 00:14:48.710 --> 00:14:50.820 align:middle line:84% is computable in any other programming language. 00:14:50.820 --> 00:14:53.660 align:middle line:84% So if we had some program written in Java, 00:14:53.660 --> 00:14:57.530 align:middle line:84% that basically boils down to something super 00:14:57.530 --> 00:15:02.540 align:middle line:84% long but something that is made up of these six primitives. 00:15:02.540 --> 00:15:05.150 align:middle line:84% That means that if we boil down this program to these six 00:15:05.150 --> 00:15:08.450 align:middle line:84% primitives, we can build back up the same program 00:15:08.450 --> 00:15:10.370 align:middle line:84% in a completely different language. 00:15:10.370 --> 00:15:11.660 align:middle line:90% And that's really powerful. 00:15:11.660 --> 00:15:14.400 align:middle line:90% That's a really cool statement. 00:15:14.400 --> 00:15:17.970 align:middle line:84% Now, we're not going to be working with those primitives. 00:15:17.970 --> 00:15:21.630 align:middle line:84% We're going to be using the Python primitives, which 00:15:21.630 --> 00:15:23.760 align:middle line:84% are more convenient, and they allow 00:15:23.760 --> 00:15:26.490 align:middle line:84% us to do a lot more things in much less time. 00:15:26.490 --> 00:15:28.260 align:middle line:84% I'm going to do a little comparison 00:15:28.260 --> 00:15:32.520 align:middle line:84% as we talk about the primitives of Python with English. 00:15:32.520 --> 00:15:36.090 align:middle line:84% So in English, some of the primitives might be words 00:15:36.090 --> 00:15:38.430 align:middle line:84% or even we can do letters or characters. 00:15:38.430 --> 00:15:41.785 align:middle line:90% But we can say it's words. 00:15:41.785 --> 00:15:43.410 align:middle line:84% With characters, we can build up words. 00:15:43.410 --> 00:15:44.993 align:middle line:84% With words, we can build up sentences. 00:15:44.993 --> 00:15:46.710 align:middle line:84% With sentences, we can build up stories. 00:15:46.710 --> 00:15:50.700 align:middle line:84% With stories, we can build up books and things like that. 00:15:50.700 --> 00:15:52.740 align:middle line:84% In programming languages, the primitives 00:15:52.740 --> 00:15:58.860 align:middle line:84% are numbers, sequences of characters, operators 00:15:58.860 --> 00:16:04.500 align:middle line:84% like addition, multiplication, division, 00:16:04.500 --> 00:16:07.020 align:middle line:84% checking for equality, checking that something is greater 00:16:07.020 --> 00:16:09.160 align:middle line:90% than, things like that. 00:16:09.160 --> 00:16:12.210 align:middle line:84% So once we have these primitives in a language, 00:16:12.210 --> 00:16:15.700 align:middle line:84% we can start to build up the syntax of the language. 00:16:15.700 --> 00:16:20.130 align:middle line:84% So in English, having something like noun and noun and noun 00:16:20.130 --> 00:16:22.050 align:middle line:90% doesn't make any sense. 00:16:22.050 --> 00:16:23.980 align:middle line:84% Cat dog boy doesn't make much sense. 00:16:23.980 --> 00:16:26.050 align:middle line:90% It's not syntactically valid. 00:16:26.050 --> 00:16:31.100 align:middle line:84% But noun verb noun is syntactically valid. 00:16:31.100 --> 00:16:33.620 align:middle line:84% Similarly, in programming languages, 00:16:33.620 --> 00:16:37.890 align:middle line:84% we can have two objects kind of side by side. 00:16:37.890 --> 00:16:41.090 align:middle line:84% So here, this is a sequence of characters h and i. 00:16:41.090 --> 00:16:43.670 align:middle line:84% And this is the number 5 right beside that sequence 00:16:43.670 --> 00:16:45.270 align:middle line:90% of characters. 00:16:45.270 --> 00:16:47.372 align:middle line:84% But that doesn't make any sense, right? 00:16:47.372 --> 00:16:49.580 align:middle line:84% What does it mean to have this sequence of characters 00:16:49.580 --> 00:16:50.913 align:middle line:90% and that number right beside it? 00:16:50.913 --> 00:16:52.670 align:middle line:90% It has no meaning in Python. 00:16:52.670 --> 00:16:54.980 align:middle line:84% Instead, what we have to do is we 00:16:54.980 --> 00:16:57.480 align:middle line:84% have to add an operator in between these two objects. 00:16:57.480 --> 00:16:59.360 align:middle line:84% So here we add a little star operator 00:16:59.360 --> 00:17:01.550 align:middle line:84% in between the sequence of characters "hi" 00:17:01.550 --> 00:17:02.990 align:middle line:90% and the number 5. 00:17:02.990 --> 00:17:05.119 align:middle line:84% And in Python, the meaning to this 00:17:05.119 --> 00:17:08.300 align:middle line:84% is I want to repeat the sequence of characters "hi," 00:17:08.300 --> 00:17:09.770 align:middle line:90% h-i, five times. 00:17:09.770 --> 00:17:13.410 align:middle line:84% So this would basically give me hi, hi, hi, hi, hi. 00:17:13.410 --> 00:17:17.280 align:middle line:84% So once we have sentences in English 00:17:17.280 --> 00:17:21.060 align:middle line:84% and expressions that are syntactically valid, 00:17:21.060 --> 00:17:24.280 align:middle line:84% we can now talk about the static semantics of the language. 00:17:24.280 --> 00:17:27.150 align:middle line:84% So in English, saying something like "I are hungry" 00:17:27.150 --> 00:17:32.010 align:middle line:84% is syntactically correct, but it's not static-- 00:17:32.010 --> 00:17:37.590 align:middle line:84% it's not-- sorry, it doesn't have good static semantics. 00:17:37.590 --> 00:17:41.640 align:middle line:84% There's no meaning-- there is no meaning to that because the 00:17:41.640 --> 00:17:44.850 align:middle line:90% "are" is for you or plural. 00:17:44.850 --> 00:17:46.620 align:middle line:84% Similarly, in programming languages, 00:17:46.620 --> 00:17:49.410 align:middle line:84% and this will differ depending on what programming language 00:17:49.410 --> 00:17:51.000 align:middle line:90% you use-- 00:17:51.000 --> 00:17:53.070 align:middle line:84% here, in the previous slide, we saw 00:17:53.070 --> 00:17:56.610 align:middle line:84% that you can use the star operator between the sequence 00:17:56.610 --> 00:17:58.020 align:middle line:90% of characters and the number. 00:17:58.020 --> 00:18:01.690 align:middle line:84% And that meant repeat that sequence many times. 00:18:01.690 --> 00:18:04.860 align:middle line:84% But if we use a plus operator in between the sequence 00:18:04.860 --> 00:18:07.380 align:middle line:84% of characters and a number, that doesn't 00:18:07.380 --> 00:18:10.410 align:middle line:90% have any meaning in Python. 00:18:10.410 --> 00:18:12.330 align:middle line:84% So it has a static semantic error, 00:18:12.330 --> 00:18:14.700 align:middle line:84% even though it's syntactically valid, right? 00:18:14.700 --> 00:18:21.430 align:middle line:84% We have operator-- sorry, object operator object. 00:18:21.430 --> 00:18:24.490 align:middle line:84% So, so far, we've been able to find really nice parallels 00:18:24.490 --> 00:18:28.010 align:middle line:84% with English, the English language and the programming 00:18:28.010 --> 00:18:28.510 align:middle line:90% languages. 00:18:28.510 --> 00:18:30.730 align:middle line:84% But this is kind of where things break down, 00:18:30.730 --> 00:18:33.830 align:middle line:84% when we talk about the semantics of a language. 00:18:33.830 --> 00:18:37.450 align:middle line:84% So in English, you can have many different meanings. 00:18:37.450 --> 00:18:41.680 align:middle line:84% The chicken is ready to eat means let's eat this chicken. 00:18:41.680 --> 00:18:44.320 align:middle line:84% Or the chicken is ready to eat means the chicken wants 00:18:44.320 --> 00:18:46.060 align:middle line:90% to eat something. 00:18:46.060 --> 00:18:49.690 align:middle line:84% Programming languages, there is no multiple meanings 00:18:49.690 --> 00:18:50.950 align:middle line:90% to a program that you write. 00:18:50.950 --> 00:18:53.680 align:middle line:84% Because the computer, the machine, 00:18:53.680 --> 00:18:58.000 align:middle line:84% the language follows the set of instructions to a T, 00:18:58.000 --> 00:19:01.870 align:middle line:84% there is no ambiguity about what it needs to do. 00:19:01.870 --> 00:19:03.610 align:middle line:84% It just follows the instructions and does 00:19:03.610 --> 00:19:08.350 align:middle line:84% what it needs to do to the end, till it reaches the-- 00:19:08.350 --> 00:19:10.060 align:middle line:90% it terminates the program. 00:19:10.060 --> 00:19:12.100 align:middle line:84% And so programs only have one meaning, 00:19:12.100 --> 00:19:14.080 align:middle line:84% but the problem is it might not be the meaning 00:19:14.080 --> 00:19:17.290 align:middle line:90% that you intended it to have. 00:19:17.290 --> 00:19:19.750 align:middle line:84% And that's when things start to go wrong. 00:19:19.750 --> 00:19:24.000 align:middle line:84% We can have syntactic errors in our program, spelling errors 00:19:24.000 --> 00:19:25.840 align:middle line:84% and indentation errors, things like that. 00:19:25.840 --> 00:19:27.270 align:middle line:90% And those are easy to catch. 00:19:27.270 --> 00:19:31.830 align:middle line:84% Static semantic errors are 90% probably easy to catch. 00:19:31.830 --> 00:19:35.790 align:middle line:84% But the problem comes in with the semantics. 00:19:35.790 --> 00:19:39.000 align:middle line:84% The meaning that you intended this program to have 00:19:39.000 --> 00:19:41.770 align:middle line:84% might not be what it's actually doing. 00:19:41.770 --> 00:19:44.800 align:middle line:84% And that's where most of my errors happen. 00:19:44.800 --> 00:19:47.130 align:middle line:84% And that's where I get super frustrated when I program. 00:19:47.130 --> 00:19:49.630 align:middle line:84% And that's probably where you guys will get super frustrated 00:19:49.630 --> 00:19:51.600 align:middle line:84% too because you write a program that you think 00:19:51.600 --> 00:19:56.040 align:middle line:84% is doing one thing, but instead, either it crashes right away, 00:19:56.040 --> 00:19:59.520 align:middle line:84% or it runs forever and doesn't really stop, or it terminates, 00:19:59.520 --> 00:20:01.110 align:middle line:84% but it gives you an incorrect answer. 00:20:01.110 --> 00:20:02.990 align:middle line:84% It's not what you were expecting. 00:20:02.990 --> 00:20:08.550 align:middle line:84% And we'll talk about this in a few lectures. 00:20:08.550 --> 00:20:10.400 align:middle line:84% So when we write programs, we're basically 00:20:10.400 --> 00:20:14.120 align:middle line:84% writing sequences of definitions and commands. 00:20:14.120 --> 00:20:17.360 align:middle line:84% And we're going to write these either in a file editor 00:20:17.360 --> 00:20:18.770 align:middle line:90% or in a shell. 00:20:18.770 --> 00:20:22.970 align:middle line:84% The first, today at least, we're writing in the shell directly. 00:20:22.970 --> 00:20:26.360 align:middle line:84% And half of tomorrow, we'll write in the shell 00:20:26.360 --> 00:20:30.830 align:middle line:84% because we're not really writing any-- 00:20:30.830 --> 00:20:33.790 align:middle line:84% we're not really writing many lines of code. 00:20:33.790 --> 00:20:34.790 align:middle line:90% We're just going to be-- 00:20:34.790 --> 00:20:39.980 align:middle line:84% I'm just going to be showing you some really quick things 00:20:39.980 --> 00:20:42.450 align:middle line:84% that we can do with the Python programming language. 00:20:42.450 --> 00:20:45.740 align:middle line:84% So hopefully you all have installed the programming 00:20:45.740 --> 00:20:47.480 align:middle line:90% environment. 00:20:47.480 --> 00:20:49.040 align:middle line:90% This is the Code Editor. 00:20:49.040 --> 00:20:51.750 align:middle line:84% So tomorrow, we'll start working in here. 00:20:51.750 --> 00:20:54.830 align:middle line:84% But for today, we're really just going to work in the shell. 00:20:54.830 --> 00:20:57.815 align:middle line:84% And even in the future, you can still 00:20:57.815 --> 00:20:58.940 align:middle line:90% type commands in the shell. 00:20:58.940 --> 00:21:01.730 align:middle line:84% I find the shell very useful if there's just something really 00:21:01.730 --> 00:21:04.230 align:middle line:84% quick that I want to check, that I 00:21:04.230 --> 00:21:06.817 align:middle line:84% don't want to write a program for and then run. 00:21:06.817 --> 00:21:08.400 align:middle line:84% It's just like a simple command that I 00:21:08.400 --> 00:21:09.942 align:middle line:84% want to check to make sure it's doing 00:21:09.942 --> 00:21:14.100 align:middle line:84% what I think it's doing before I insert it in my code editor. 00:21:14.100 --> 00:21:19.650 align:middle line:90% So here we have this. 00:21:19.650 --> 00:21:20.940 align:middle line:90% So mine is-- 00:21:20.940 --> 00:21:23.160 align:middle line:84% I guess I'm using the white theme just because I find 00:21:23.160 --> 00:21:25.290 align:middle line:90% it easier for you guys to see. 00:21:25.290 --> 00:21:26.940 align:middle line:90% This is the file editor. 00:21:26.940 --> 00:21:30.240 align:middle line:84% And this is just a bunch of expressions or-- 00:21:30.240 --> 00:21:32.940 align:middle line:84% yeah, a bunch of code that we're going to type in today. 00:21:32.940 --> 00:21:36.137 align:middle line:84% And we're going to type it in the shell today, so the thing 00:21:36.137 --> 00:21:37.095 align:middle line:90% on the right-hand side. 00:21:37.095 --> 00:21:40.270 align:middle line:90% 00:21:40.270 --> 00:21:45.310 align:middle line:84% OK, so what exactly do we do when we write a program? 00:21:45.310 --> 00:21:48.220 align:middle line:84% At the base of it, we are going to create 00:21:48.220 --> 00:21:53.140 align:middle line:84% objects inside our programs, and we're going to manipulate them. 00:21:53.140 --> 00:21:54.100 align:middle line:90% That's it. 00:21:54.100 --> 00:22:00.320 align:middle line:84% That's what programming is mostly about at its core. 00:22:00.320 --> 00:22:03.370 align:middle line:84% Now, when we create objects, it's 00:22:03.370 --> 00:22:05.080 align:middle line:84% important-- this is kind of something 00:22:05.080 --> 00:22:06.455 align:middle line:84% we're going to come back to again 00:22:06.455 --> 00:22:08.770 align:middle line:84% and again in a more high-level setting. 00:22:08.770 --> 00:22:11.440 align:middle line:84% But right now what I want you to understand 00:22:11.440 --> 00:22:16.870 align:middle line:84% is that when we create an object, an object has a type. 00:22:16.870 --> 00:22:20.590 align:middle line:84% And the type that an object has tells Python 00:22:20.590 --> 00:22:23.870 align:middle line:84% the things you're allowed to do with that object. 00:22:23.870 --> 00:22:25.400 align:middle line:90% So here are two examples. 00:22:25.400 --> 00:22:29.100 align:middle line:90% The number 30, it's a number. 00:22:29.100 --> 00:22:31.460 align:middle line:84% The type we'll talk about it in a bit. 00:22:31.460 --> 00:22:32.550 align:middle line:90% The type is an integer. 00:22:32.550 --> 00:22:33.763 align:middle line:90% It's a whole number. 00:22:33.763 --> 00:22:35.180 align:middle line:84% But basically, what are the things 00:22:35.180 --> 00:22:37.580 align:middle line:84% we can do with this integer, with this number? 00:22:37.580 --> 00:22:39.110 align:middle line:90% We can add it to another number. 00:22:39.110 --> 00:22:40.670 align:middle line:84% We can subtract it to another number. 00:22:40.670 --> 00:22:43.110 align:middle line:90% We can take it to another power. 00:22:43.110 --> 00:22:47.510 align:middle line:84% We can take some other number to this power of 30. 00:22:47.510 --> 00:22:50.240 align:middle line:84% A bunch of mathematical operations, 00:22:50.240 --> 00:22:52.260 align:middle line:90% as you would expect. 00:22:52.260 --> 00:22:53.940 align:middle line:84% So that's pretty straightforward. 00:22:53.940 --> 00:22:57.770 align:middle line:84% What about this one here, this quotation capital 00:22:57.770 --> 00:22:59.190 align:middle line:90% A, lowercase a-- 00:22:59.190 --> 00:23:01.590 align:middle line:84% lowercase n, lowercase a quotation? 00:23:01.590 --> 00:23:04.100 align:middle line:84% So this is something we'll talk about next lecture. 00:23:04.100 --> 00:23:05.270 align:middle line:90% It's called a string. 00:23:05.270 --> 00:23:07.640 align:middle line:84% And it's a sequence of characters. 00:23:07.640 --> 00:23:10.710 align:middle line:84% The quotations tell Python it's a sequence of characters. 00:23:10.710 --> 00:23:13.550 align:middle line:84% And the characters part of it are capital A, lowercase n, 00:23:13.550 --> 00:23:15.500 align:middle line:90% and lowercase a. 00:23:15.500 --> 00:23:19.040 align:middle line:84% The kinds of things I can do with this string 00:23:19.040 --> 00:23:20.990 align:middle line:84% are not the same kinds of things I'm allowed 00:23:20.990 --> 00:23:22.198 align:middle line:90% to do with the number, right? 00:23:22.198 --> 00:23:25.490 align:middle line:84% If I tried to take Ana and divide it 00:23:25.490 --> 00:23:28.790 align:middle line:84% by the sequence of characters Bob, 00:23:28.790 --> 00:23:31.340 align:middle line:84% Python would complain very much because you 00:23:31.340 --> 00:23:33.058 align:middle line:84% can't divide a string by another string, 00:23:33.058 --> 00:23:34.100 align:middle line:90% a sequence of characters. 00:23:34.100 --> 00:23:35.630 align:middle line:84% It doesn't make sense to divide it 00:23:35.630 --> 00:23:37.700 align:middle line:84% by another sequence of characters. 00:23:37.700 --> 00:23:40.790 align:middle line:84% Similarly, I can't take Ana to some power. 00:23:40.790 --> 00:23:42.950 align:middle line:90% I can't multiply-- 00:23:42.950 --> 00:23:45.420 align:middle line:84% I can't multiply by itself, things like that. 00:23:45.420 --> 00:23:47.270 align:middle line:84% But the kinds of things that I am allowed 00:23:47.270 --> 00:23:48.860 align:middle line:84% to do on a sequence of characters 00:23:48.860 --> 00:23:50.402 align:middle line:84% is different than the kinds of things 00:23:50.402 --> 00:23:51.720 align:middle line:90% I'm allowed to do on a number. 00:23:51.720 --> 00:23:53.887 align:middle line:84% So the things I can do with a sequence of characters 00:23:53.887 --> 00:23:56.240 align:middle line:84% is I can say, well, what's the character 00:23:56.240 --> 00:23:58.010 align:middle line:90% at the first position? 00:23:58.010 --> 00:23:59.780 align:middle line:90% What's the middle character? 00:23:59.780 --> 00:24:02.090 align:middle line:84% How long is the sequence of characters? 00:24:02.090 --> 00:24:04.460 align:middle line:90% How many characters do I have? 00:24:04.460 --> 00:24:07.352 align:middle line:84% And so now you can see that the type of the object 00:24:07.352 --> 00:24:08.560 align:middle line:90% is actually really important. 00:24:08.560 --> 00:24:11.950 align:middle line:84% Python uses it to know the kinds of operations 00:24:11.950 --> 00:24:13.400 align:middle line:90% you're allowed to do with it. 00:24:13.400 --> 00:24:16.490 align:middle line:84% And so there's actually scalar objects, 00:24:16.490 --> 00:24:19.930 align:middle line:84% and these are Python's primitives, numbers 00:24:19.930 --> 00:24:21.670 align:middle line:90% and truth values. 00:24:21.670 --> 00:24:23.320 align:middle line:90% And there are nonscalar objects. 00:24:23.320 --> 00:24:24.920 align:middle line:84% We're not talking about these yet. 00:24:24.920 --> 00:24:26.628 align:middle line:84% We'll talk about these in a few lectures. 00:24:26.628 --> 00:24:28.570 align:middle line:84% But these have some sort of structure. 00:24:28.570 --> 00:24:30.790 align:middle line:84% So for example, a list of numbers 00:24:30.790 --> 00:24:33.862 align:middle line:84% has a structure because there's a number at the beginning 00:24:33.862 --> 00:24:36.070 align:middle line:84% of the list, there's a number at the end of the list, 00:24:36.070 --> 00:24:36.778 align:middle line:90% things like that. 00:24:36.778 --> 00:24:39.470 align:middle line:84% But a number itself doesn't have a structure. 00:24:39.470 --> 00:24:42.410 align:middle line:90% It's just the number. 00:24:42.410 --> 00:24:44.912 align:middle line:84% So what are the types of the scalar objects? 00:24:44.912 --> 00:24:46.870 align:middle line:84% What are the types of the primitives in Python? 00:24:46.870 --> 00:24:52.660 align:middle line:84% Integers, so number 5, 0, negative 100, 1 million. 00:24:52.660 --> 00:24:55.000 align:middle line:90% Float is another type. 00:24:55.000 --> 00:24:58.870 align:middle line:84% It represents all the real numbers, so 3.27. 00:24:58.870 --> 00:25:02.590 align:middle line:84% 2.0 is a float because it has a decimal number even 00:25:02.590 --> 00:25:04.790 align:middle line:90% though to us that just means 2. 00:25:04.790 --> 00:25:07.310 align:middle line:84% But to Python, if you put in 2.0, 00:25:07.310 --> 00:25:09.020 align:middle line:90% it says that's a type float. 00:25:09.020 --> 00:25:12.350 align:middle line:84% Negative 3.14159, things like that. 00:25:12.350 --> 00:25:13.850 align:middle line:90% Bool is a Boolean. 00:25:13.850 --> 00:25:15.620 align:middle line:90% It represents truth values. 00:25:15.620 --> 00:25:20.440 align:middle line:84% And there's only two possible values that a Boolean type has, 00:25:20.440 --> 00:25:21.470 align:middle line:90% True and False. 00:25:21.470 --> 00:25:26.690 align:middle line:84% And it has to be capital T True and capital F False. 00:25:26.690 --> 00:25:31.670 align:middle line:84% And the last one is this NoneType type. 00:25:31.670 --> 00:25:33.560 align:middle line:90% It's literally called NoneType. 00:25:33.560 --> 00:25:36.178 align:middle line:84% And it has only one special value, None. 00:25:36.178 --> 00:25:37.970 align:middle line:84% We're not going to talk about it for a bit, 00:25:37.970 --> 00:25:41.240 align:middle line:84% but we will sometime in the future. 00:25:41.240 --> 00:25:43.390 align:middle line:84% So to figure out the type of an object 00:25:43.390 --> 00:25:47.360 align:middle line:84% when you create that object, you use the type command. 00:25:47.360 --> 00:25:50.440 align:middle line:84% So we can say something like type parentheses. 00:25:50.440 --> 00:25:51.670 align:middle line:90% And this is a command. 00:25:51.670 --> 00:25:54.048 align:middle line:84% And inside the parentheses, you say, what do you 00:25:54.048 --> 00:25:55.090 align:middle line:90% want to find the type of? 00:25:55.090 --> 00:25:59.120 align:middle line:84% So if we do type of 7, it tells me it's an int. 00:25:59.120 --> 00:26:01.100 align:middle line:84% And if you want to do the same command again, 00:26:01.100 --> 00:26:02.870 align:middle line:84% I hit the up arrow, and it automatically 00:26:02.870 --> 00:26:04.580 align:middle line:90% puts in what I wrote previously. 00:26:04.580 --> 00:26:07.460 align:middle line:84% And then if I want to do type of 0.0, 00:26:07.460 --> 00:26:09.485 align:middle line:84% it's a float because there's a decimal point. 00:26:09.485 --> 00:26:13.110 align:middle line:90% 00:26:13.110 --> 00:26:14.540 align:middle line:84% So this is basically what I said. 00:26:14.540 --> 00:26:16.460 align:middle line:90% So we type this in the shell. 00:26:16.460 --> 00:26:23.290 align:middle line:84% And the shell tells us what the output is. 00:26:23.290 --> 00:26:28.170 align:middle line:84% So just to reiterate, int, float, bool, 00:26:28.170 --> 00:26:31.870 align:middle line:84% and NoneType are types of objects. 00:26:31.870 --> 00:26:34.690 align:middle line:84% And there can be many different objects 00:26:34.690 --> 00:26:36.850 align:middle line:90% you can create of that type. 00:26:36.850 --> 00:26:38.980 align:middle line:84% So if you think about it, ints and floats, 00:26:38.980 --> 00:26:41.140 align:middle line:84% we basically have an infinite number 00:26:41.140 --> 00:26:44.020 align:middle line:84% of objects we can create of those types 00:26:44.020 --> 00:26:48.700 align:middle line:84% because we can have 0, 1, 2, 3, 100, 200, 300, 1 million, 00:26:48.700 --> 00:26:49.840 align:middle line:90% and all the negatives. 00:26:49.840 --> 00:26:54.490 align:middle line:84% There's almost an infinite number of values or objects 00:26:54.490 --> 00:26:56.830 align:middle line:84% that we can create of type int and float. 00:26:56.830 --> 00:27:01.450 align:middle line:84% But bool, there's only two, the truth values True or False. 00:27:01.450 --> 00:27:04.930 align:middle line:84% And the NoneType, there's only one, this None. 00:27:04.930 --> 00:27:07.660 align:middle line:84% So that's the type, and these are the possible values, 00:27:07.660 --> 00:27:11.210 align:middle line:90% possible objects we can create. 00:27:11.210 --> 00:27:12.350 align:middle line:90% You try it. 00:27:12.350 --> 00:27:14.700 align:middle line:84% So you can just yell out the answers. 00:27:14.700 --> 00:27:17.750 align:middle line:84% There's nothing to type unless you want to check yourself so 00:27:17.750 --> 00:27:20.198 align:middle line:90% what is the type of 1234? 00:27:20.198 --> 00:27:21.540 align:middle line:90% AUDIENCE: Int. 00:27:21.540 --> 00:27:22.600 align:middle line:90% ANA BELL: Int. 00:27:22.600 --> 00:27:24.730 align:middle line:90% Type of 8.99? 00:27:24.730 --> 00:27:25.510 align:middle line:90% Float. 00:27:25.510 --> 00:27:27.220 align:middle line:90% Type of 9.0? 00:27:27.220 --> 00:27:27.880 align:middle line:90% Float. 00:27:27.880 --> 00:27:29.780 align:middle line:90% Type of True? 00:27:29.780 --> 00:27:30.280 align:middle line:90% Bool. 00:27:30.280 --> 00:27:31.720 align:middle line:90% And type of False? 00:27:31.720 --> 00:27:32.440 align:middle line:90% Bool. 00:27:32.440 --> 00:27:33.400 align:middle line:90% Perfect. 00:27:33.400 --> 00:27:35.800 align:middle line:84% If you ever wonder what the type of something is, 00:27:35.800 --> 00:27:37.425 align:middle line:90% you type it in here. 00:27:37.425 --> 00:27:38.650 align:middle line:90% You guys are doing well. 00:27:38.650 --> 00:27:39.700 align:middle line:90% Type is bool. 00:27:39.700 --> 00:27:43.870 align:middle line:84% Type of lowercase t true is an error, 00:27:43.870 --> 00:27:46.390 align:middle line:84% just wanted to point that out just to reiterate the fact 00:27:46.390 --> 00:27:49.000 align:middle line:84% that capitalization matters in Python. 00:27:49.000 --> 00:27:51.280 align:middle line:84% This is our first error, by the way, guys. 00:27:51.280 --> 00:27:52.600 align:middle line:90% Very exciting. 00:27:52.600 --> 00:27:55.210 align:middle line:84% The error is a NameError, and this is 00:27:55.210 --> 00:28:00.150 align:middle line:90% the message associated with it. 00:28:00.150 --> 00:28:04.490 align:middle line:84% You also know that it's something special in Python 00:28:04.490 --> 00:28:06.500 align:middle line:90% when you have color-coded stuff. 00:28:06.500 --> 00:28:10.430 align:middle line:84% So you see capital T True, capital F False are 00:28:10.430 --> 00:28:13.310 align:middle line:84% this dark blue here, whereas anything 00:28:13.310 --> 00:28:17.900 align:middle line:84% that's not special in Python is just black. 00:28:17.900 --> 00:28:19.910 align:middle line:90% So type is a special command. 00:28:19.910 --> 00:28:23.700 align:middle line:84% This is a float, so you see they're color coded. 00:28:23.700 --> 00:28:25.110 align:middle line:90% OK. 00:28:25.110 --> 00:28:27.660 align:middle line:84% So once we create objects, one thing 00:28:27.660 --> 00:28:30.300 align:middle line:84% we can do with these objects is to cast them 00:28:30.300 --> 00:28:31.680 align:middle line:90% to a different type. 00:28:31.680 --> 00:28:36.300 align:middle line:84% Now, this is a little bit maybe confusing 00:28:36.300 --> 00:28:39.660 align:middle line:84% because we're not actually changing the object once we've 00:28:39.660 --> 00:28:40.720 align:middle line:90% created it. 00:28:40.720 --> 00:28:46.210 align:middle line:84% So once we create the integer 3, it's there in memory. 00:28:46.210 --> 00:28:49.900 align:middle line:84% If we cast that integer to a float version of it, 00:28:49.900 --> 00:28:52.240 align:middle line:84% we're creating a new object in memory. 00:28:52.240 --> 00:28:53.290 align:middle line:90% We're not changing the 3. 00:28:53.290 --> 00:28:55.310 align:middle line:90% The 3 already exists. 00:28:55.310 --> 00:28:57.640 align:middle line:84% We're just getting the float version of it 00:28:57.640 --> 00:29:00.440 align:middle line:84% and storing it as a new object in memory. 00:29:00.440 --> 00:29:03.790 align:middle line:84% So when we do float 3, this is a command 00:29:03.790 --> 00:29:08.140 align:middle line:84% that gets for me the float version of the integer 3. 00:29:08.140 --> 00:29:10.720 align:middle line:90% So that will give me 3.0. 00:29:10.720 --> 00:29:16.750 align:middle line:84% So for example, this is what I had, float 3. 00:29:16.750 --> 00:29:18.970 align:middle line:90% The output is 3.0. 00:29:18.970 --> 00:29:26.120 align:middle line:84% If I do int of 5.2, it truncates it, 00:29:26.120 --> 00:29:29.090 align:middle line:84% and it gives me the integer portion of this float. 00:29:29.090 --> 00:29:33.582 align:middle line:84% If I do int of 5.9, it still truncates it 00:29:33.582 --> 00:29:35.540 align:middle line:84% and gives me the integer version of this float. 00:29:35.540 --> 00:29:37.670 align:middle line:90% It doesn't round. 00:29:37.670 --> 00:29:42.200 align:middle line:84% I'm just asking for the integer version of this float. 00:29:42.200 --> 00:29:45.780 align:middle line:84% Some operations like round is an operation 00:29:45.780 --> 00:29:48.870 align:middle line:84% we can do has an implicit cast in it. 00:29:48.870 --> 00:29:53.570 align:middle line:84% So if I round 5.9, it's actually going to round it to 6.0 00:29:53.570 --> 00:29:55.710 align:middle line:90% and then cast it to an integer. 00:29:55.710 --> 00:29:58.550 align:middle line:84% So notice it doesn't give me as an output 6.0. 00:29:58.550 --> 00:30:00.620 align:middle line:90% It then rounds it to just six. 00:30:00.620 --> 00:30:03.560 align:middle line:90% 00:30:03.560 --> 00:30:08.100 align:middle line:84% So that's basically what I said in the example. 00:30:08.100 --> 00:30:10.940 align:middle line:90% So let's have you try this. 00:30:10.940 --> 00:30:12.800 align:middle line:84% What are the types of the following? 00:30:12.800 --> 00:30:14.820 align:middle line:84% I don't need the values but the types. 00:30:14.820 --> 00:30:23.640 align:middle line:84% So if I get type of float of 123, what is the type of that? 00:30:23.640 --> 00:30:24.990 align:middle line:90% Float, yeah, exactly. 00:30:24.990 --> 00:30:26.850 align:middle line:90% Yep. 00:30:26.850 --> 00:30:28.830 align:middle line:90% What if I round 7.9? 00:30:28.830 --> 00:30:30.180 align:middle line:90% What's the type of the result? 00:30:30.180 --> 00:30:31.080 align:middle line:90% Int, yep. 00:30:31.080 --> 00:30:35.700 align:middle line:84% What if I create a float of the round of 7.2? 00:30:35.700 --> 00:30:36.390 align:middle line:90% AUDIENCE: Float. 00:30:36.390 --> 00:30:37.920 align:middle line:90% ANA BELL: Yes, good. 00:30:37.920 --> 00:30:39.300 align:middle line:90% Float would be 7.0. 00:30:39.300 --> 00:30:41.550 align:middle line:90% And the int of 7.2? 00:30:41.550 --> 00:30:42.924 align:middle line:90% AUDIENCE: 7. 00:30:42.924 --> 00:30:43.840 align:middle line:90% Int. 00:30:43.840 --> 00:30:45.040 align:middle line:90% ANA BELL: Int, yes, exactly. 00:30:45.040 --> 00:30:46.510 align:middle line:90% I want the type not the value. 00:30:46.510 --> 00:30:51.490 align:middle line:84% And the int of 7.9 is an int, exactly. 00:30:51.490 --> 00:30:54.060 align:middle line:90% Awesome, good. 00:30:54.060 --> 00:30:57.075 align:middle line:84% OK, so we've created a bunch of objects. 00:30:57.075 --> 00:30:58.950 align:middle line:84% We know that we can create a bunch of objects 00:30:58.950 --> 00:30:59.670 align:middle line:90% in our programs. 00:30:59.670 --> 00:31:01.180 align:middle line:90% What do we do with them? 00:31:01.180 --> 00:31:05.380 align:middle line:84% Well, we can combine them into expressions. 00:31:05.380 --> 00:31:08.790 align:middle line:90% So let's say we have 3 plus 2. 00:31:08.790 --> 00:31:11.490 align:middle line:84% I've got object, operator, object. 00:31:11.490 --> 00:31:13.530 align:middle line:84% Cool, syntactically valid in Python 00:31:13.530 --> 00:31:15.910 align:middle line:84% and has no static semantic error. 00:31:15.910 --> 00:31:21.120 align:middle line:84% So if I do that in Python, it's going to be OK. 00:31:21.120 --> 00:31:25.100 align:middle line:90% 3 plus 2, 5. 00:31:25.100 --> 00:31:31.420 align:middle line:84% And the type of 3 plus 2 is an integer. 00:31:31.420 --> 00:31:32.980 align:middle line:84% So basically what I've done here, 00:31:32.980 --> 00:31:38.320 align:middle line:84% I've put an expression within this type command. 00:31:38.320 --> 00:31:39.760 align:middle line:90% And that's OK. 00:31:39.760 --> 00:31:41.530 align:middle line:84% That's, in fact, encouraged in Python. 00:31:41.530 --> 00:31:44.665 align:middle line:84% You don't just want to calculate and then stick in. 00:31:44.665 --> 00:31:46.040 align:middle line:84% That would be very, very tedious. 00:31:46.040 --> 00:31:50.360 align:middle line:84% So you can insert expressions in many, many different places. 00:31:50.360 --> 00:31:53.510 align:middle line:84% So here we have 3 plus 2, 5 divided by 3. 00:31:53.510 --> 00:31:56.630 align:middle line:84% Again, we've got 5 divided by 3 has this decimal value. 00:31:56.630 --> 00:31:59.480 align:middle line:90% And the result has a float-- 00:31:59.480 --> 00:32:01.830 align:middle line:90% is of type float. 00:32:01.830 --> 00:32:03.380 align:middle line:84% So the important thing to remember 00:32:03.380 --> 00:32:08.390 align:middle line:84% when we're doing expressions is Python reads the expression, 00:32:08.390 --> 00:32:12.680 align:middle line:84% but it does not store the expression in memory. 00:32:12.680 --> 00:32:15.080 align:middle line:84% What it does is it reads the expression, 00:32:15.080 --> 00:32:18.330 align:middle line:84% evaluates it to one single value, 00:32:18.330 --> 00:32:22.050 align:middle line:84% and then it stores the result value in memory. 00:32:22.050 --> 00:32:23.690 align:middle line:84% So it never stores the expression. 00:32:23.690 --> 00:32:27.650 align:middle line:84% It evaluates the expression and then stores the value. 00:32:27.650 --> 00:32:30.680 align:middle line:84% And so this is the syntax for an expression-- object, operator, 00:32:30.680 --> 00:32:33.180 align:middle line:90% object, as we just saw. 00:32:33.180 --> 00:32:36.650 align:middle line:84% And that's really-- and the idea I said before, 00:32:36.650 --> 00:32:40.340 align:middle line:84% where Python stores values of expressions, 00:32:40.340 --> 00:32:45.480 align:middle line:84% not the expressions themselves, is really, really important. 00:32:45.480 --> 00:32:48.420 align:middle line:84% So this is my first big idea slide. 00:32:48.420 --> 00:32:50.610 align:middle line:84% I decided to insert these because I 00:32:50.610 --> 00:32:53.590 align:middle line:84% think they stress the importance of several concepts. 00:32:53.590 --> 00:32:55.240 align:middle line:90% So I hope this is one. 00:32:55.240 --> 00:32:57.690 align:middle line:90% So we're taking expressions. 00:32:57.690 --> 00:32:59.320 align:middle line:84% They can be as complex as you'd like. 00:32:59.320 --> 00:33:01.860 align:middle line:84% We can use parentheses, a bunch of-- it 00:33:01.860 --> 00:33:05.070 align:middle line:84% doesn't just have to be object, operator, object. 00:33:05.070 --> 00:33:07.440 align:middle line:84% It can be more complex than that. 00:33:07.440 --> 00:33:10.590 align:middle line:84% But basically, however complex that expression 00:33:10.590 --> 00:33:14.730 align:middle line:84% is, we evaluate it, and we replace it with one value. 00:33:14.730 --> 00:33:18.350 align:middle line:84% And the expression can be something like this. 00:33:18.350 --> 00:33:20.920 align:middle line:84% It doesn't just have to be something that's mathematical. 00:33:20.920 --> 00:33:22.660 align:middle line:84% This was a mathematical expression, 00:33:22.660 --> 00:33:24.400 align:middle line:90% but this is also an expression. 00:33:24.400 --> 00:33:25.660 align:middle line:90% And it evaluates. 00:33:25.660 --> 00:33:31.240 align:middle line:84% So this entire thing evaluates to this word, this word which 00:33:31.240 --> 00:33:34.270 align:middle line:90% represents the type integer. 00:33:34.270 --> 00:33:36.660 align:middle line:90% So here are some more examples. 00:33:36.660 --> 00:33:39.480 align:middle line:90% 3 plus 2, again. 00:33:39.480 --> 00:33:42.270 align:middle line:84% We've got these examples with the parentheses, 00:33:42.270 --> 00:33:44.220 align:middle line:84% 4 plus 2 times 6 minus 1 obviously 00:33:44.220 --> 00:33:46.140 align:middle line:90% gives us the number, 35. 00:33:46.140 --> 00:33:49.120 align:middle line:84% And then we can insert expressions wherever we'd like. 00:33:49.120 --> 00:33:51.930 align:middle line:84% So here I'm inserting that specific expression 00:33:51.930 --> 00:33:54.220 align:middle line:90% in the type command. 00:33:54.220 --> 00:33:56.940 align:middle line:84% And this is also an expression, like I just said. 00:33:56.940 --> 00:34:00.570 align:middle line:90% And its result is int. 00:34:00.570 --> 00:34:04.080 align:middle line:84% And similarly, we can also insert that expression here. 00:34:04.080 --> 00:34:06.900 align:middle line:84% And then we can wrap that around cast. 00:34:06.900 --> 00:34:08.489 align:middle line:90% And it gives us a float. 00:34:08.489 --> 00:34:09.796 align:middle line:90% Yes? 00:34:09.796 --> 00:34:11.949 align:middle line:84% AUDIENCE: So when you're inserting expressions 00:34:11.949 --> 00:34:15.035 align:middle line:84% [INAUDIBLE] include the operators-- 00:34:15.035 --> 00:34:16.253 align:middle line:90% [INAUDIBLE] operators in? 00:34:16.253 --> 00:34:17.920 align:middle line:84% ANA BELL: When you're inserting-- sorry, 00:34:17.920 --> 00:34:19.299 align:middle line:90% when you're inserting what? 00:34:19.299 --> 00:34:21.424 align:middle line:84% AUDIENCE: Well, since you said they're expressions, 00:34:21.424 --> 00:34:24.550 align:middle line:84% and you said that you need like object, operator, object, 00:34:24.550 --> 00:34:25.969 align:middle line:90% expression, type. 00:34:25.969 --> 00:34:28.090 align:middle line:84% What would be the operators in this case? 00:34:28.090 --> 00:34:30.157 align:middle line:90% ANA BELL: Oh, I see. 00:34:30.157 --> 00:34:31.449 align:middle line:90% AUDIENCE: How are they defined? 00:34:31.449 --> 00:34:34.429 align:middle line:84% ANA BELL: Yeah, that's a good-- that's a good question. 00:34:34.429 --> 00:34:41.909 align:middle line:84% So in this particular case, the type and the float are not-- 00:34:41.909 --> 00:34:44.949 align:middle line:84% there is no operator I guess in this particular case. 00:34:44.949 --> 00:34:48.210 align:middle line:84% It's more like a command that gives us an output. 00:34:48.210 --> 00:34:51.210 align:middle line:90% But there is still some-- 00:34:51.210 --> 00:34:54.510 align:middle line:84% there is still an output that it gives us. 00:34:54.510 --> 00:34:56.940 align:middle line:84% So we can then take the result of this 00:34:56.940 --> 00:35:00.240 align:middle line:90% and save it somewhere else. 00:35:00.240 --> 00:35:04.140 align:middle line:84% Sorry, yeah, I guess the example I gave on the previous slide 00:35:04.140 --> 00:35:06.360 align:middle line:84% was just an example of an expression 00:35:06.360 --> 00:35:08.695 align:middle line:84% where we could do object, operator, object. 00:35:08.695 --> 00:35:09.195 align:middle line:90% Yeah. 00:35:09.195 --> 00:35:14.830 align:middle line:90% 00:35:14.830 --> 00:35:16.510 align:middle line:90% Yeah, so when we have these-- 00:35:16.510 --> 00:35:19.480 align:middle line:84% I guess it works for mathematical expressions. 00:35:19.480 --> 00:35:21.760 align:middle line:84% Mathematical expressions work left to right, just 00:35:21.760 --> 00:35:22.600 align:middle line:90% like in math. 00:35:22.600 --> 00:35:29.150 align:middle line:84% Parentheses can override certain precedents. 00:35:29.150 --> 00:35:33.280 align:middle line:84% If we have commands that have computations, 00:35:33.280 --> 00:35:36.310 align:middle line:84% then we have this command with the parentheses. 00:35:36.310 --> 00:35:39.140 align:middle line:84% And we evaluate what's inside the parentheses first. 00:35:39.140 --> 00:35:42.310 align:middle line:84% So we work our way in to out in that particular case. 00:35:42.310 --> 00:35:45.130 align:middle line:90% 00:35:45.130 --> 00:35:46.570 align:middle line:90% So here are some examples. 00:35:46.570 --> 00:35:47.700 align:middle line:90% Let's have you try these. 00:35:47.700 --> 00:35:50.520 align:middle line:84% So we can type these in our console. 00:35:50.520 --> 00:35:53.560 align:middle line:84% What are the values of the following expressions? 00:35:53.560 --> 00:35:57.640 align:middle line:84% So 13 minus 4 divided by 12 times 12. 00:35:57.640 --> 00:35:58.635 align:middle line:90% So we can try that. 00:35:58.635 --> 00:36:03.310 align:middle line:90% 00:36:03.310 --> 00:36:04.810 align:middle line:84% I don't know off the top of my head, 00:36:04.810 --> 00:36:05.977 align:middle line:90% so we'll have to type it in. 00:36:05.977 --> 00:36:07.720 align:middle line:90% 0.0625, OK. 00:36:07.720 --> 00:36:10.960 align:middle line:84% So the value of that expression is a float, right? 00:36:10.960 --> 00:36:12.160 align:middle line:90% 0.0625. 00:36:12.160 --> 00:36:17.512 align:middle line:84% What's the value of the expression type 4 times 3? 00:36:17.512 --> 00:36:18.500 align:middle line:90% AUDIENCE: Int. 00:36:18.500 --> 00:36:19.340 align:middle line:90% ANA BELL: Int, yeah. 00:36:19.340 --> 00:36:23.690 align:middle line:84% What about the type of the expression 4.0 times 3? 00:36:23.690 --> 00:36:24.420 align:middle line:90% AUDIENCE: Float. 00:36:24.420 --> 00:36:26.370 align:middle line:90% ANA BELL: Yes, exactly. 00:36:26.370 --> 00:36:27.210 align:middle line:90% That's very good. 00:36:27.210 --> 00:36:30.000 align:middle line:90% So type of 4 times 3 is int. 00:36:30.000 --> 00:36:32.550 align:middle line:90% But 4.0 times 3 is a float. 00:36:32.550 --> 00:36:33.510 align:middle line:90% Good. 00:36:33.510 --> 00:36:37.260 align:middle line:84% And then what about int of a half or of 1 over 2? 00:36:37.260 --> 00:36:40.515 align:middle line:90% 00:36:40.515 --> 00:36:42.380 align:middle line:90% AUDIENCE: So it's 0. 00:36:42.380 --> 00:36:45.040 align:middle line:90% ANA BELL: Yeah, exactly, it's 0. 00:36:45.040 --> 00:36:50.220 align:middle line:84% Yep, because it's 0.5, and we truncate to 0. 00:36:50.220 --> 00:36:52.770 align:middle line:84% The reason I had this here is because it leads nicely 00:36:52.770 --> 00:36:54.070 align:middle line:90% into this slide. 00:36:54.070 --> 00:36:56.310 align:middle line:84% You don't have to memorize these rules. 00:36:56.310 --> 00:36:59.340 align:middle line:84% You can always check it out in the console. 00:36:59.340 --> 00:37:01.500 align:middle line:84% But there are some rules for the resulting 00:37:01.500 --> 00:37:03.220 align:middle line:90% types when we do operations. 00:37:03.220 --> 00:37:06.990 align:middle line:84% So when we do operations with numbers, 00:37:06.990 --> 00:37:11.730 align:middle line:84% addition, subtraction, and multiplication always 00:37:11.730 --> 00:37:15.540 align:middle line:84% yield an integer if both of the operators are integers. 00:37:15.540 --> 00:37:17.400 align:middle line:84% If one is a float or both are floats, 00:37:17.400 --> 00:37:19.410 align:middle line:90% then it gives me a float. 00:37:19.410 --> 00:37:20.820 align:middle line:90% Division is different. 00:37:20.820 --> 00:37:24.510 align:middle line:84% No matter what types you divide, you will always get a float. 00:37:24.510 --> 00:37:27.190 align:middle line:90% 00:37:27.190 --> 00:37:30.090 align:middle line:84% Now what about this // and this percent? 00:37:30.090 --> 00:37:32.250 align:middle line:84% These are actually useful operations. 00:37:32.250 --> 00:37:35.110 align:middle line:84% They kind of go hand in hand with division. 00:37:35.110 --> 00:37:41.726 align:middle line:84% So when I do 5 divided by 3, it's this 1.667. 00:37:41.726 --> 00:37:48.110 align:middle line:84% // is basically a floor or getting the integer portion 00:37:48.110 --> 00:37:49.250 align:middle line:90% of the division. 00:37:49.250 --> 00:37:52.460 align:middle line:90% So 5//3 gives me one. 00:37:52.460 --> 00:37:57.620 align:middle line:90% It truncates the fraction. 00:37:57.620 --> 00:38:01.140 align:middle line:84% The percent gives me the remainder. 00:38:01.140 --> 00:38:05.630 align:middle line:84% So 5%3 gives me the remainder when I divide 5 by 3. 00:38:05.630 --> 00:38:09.530 align:middle line:84% So it's going to give me-- give it to me in a whole number. 00:38:09.530 --> 00:38:12.440 align:middle line:84% So that's going to be 2 because there's 2 left over when 00:38:12.440 --> 00:38:15.440 align:middle line:90% I divide 5 by 3. 00:38:15.440 --> 00:38:17.810 align:middle line:84% So these are pretty useful operations, 00:38:17.810 --> 00:38:23.210 align:middle line:84% the // and the percent, when we do mathematical programs. 00:38:23.210 --> 00:38:25.910 align:middle line:84% The last thing is the ** is how we 00:38:25.910 --> 00:38:30.320 align:middle line:84% denote power, exponentiation, kind of different 00:38:30.320 --> 00:38:32.960 align:middle line:84% than you might be used to in math. 00:38:32.960 --> 00:38:36.980 align:middle line:90% So 2 to the power of 3, 8. 00:38:36.980 --> 00:38:39.620 align:middle line:90% 2 to the power of 3.0, 8.0. 00:38:39.620 --> 00:38:45.080 align:middle line:84% And the rules for integer division, percent, 00:38:45.080 --> 00:38:48.960 align:middle line:84% and exponentiation are just like addition, subtraction, 00:38:48.960 --> 00:38:49.610 align:middle line:90% multiplication. 00:38:49.610 --> 00:38:52.580 align:middle line:84% If one is a float, then the result will be a float as well. 00:38:52.580 --> 00:38:57.040 align:middle line:90% 00:38:57.040 --> 00:38:57.540 align:middle line:90% Yeah. 00:38:57.540 --> 00:39:00.270 align:middle line:90% 00:39:00.270 --> 00:39:04.260 align:middle line:84% OK, and we talked about the type of output. 00:39:04.260 --> 00:39:06.110 align:middle line:84% So I think I briefly mentioned this. 00:39:06.110 --> 00:39:09.290 align:middle line:84% The operator precedence is exponentiation and then 00:39:09.290 --> 00:39:12.920 align:middle line:84% multiplication, division, percent or remainder 00:39:12.920 --> 00:39:14.550 align:middle line:84% at the next level, and then addition, 00:39:14.550 --> 00:39:15.720 align:middle line:90% subtraction at the bottom. 00:39:15.720 --> 00:39:20.510 align:middle line:84% But you can always override these using parentheses. 00:39:20.510 --> 00:39:24.810 align:middle line:84% OK, questions so far before we move on? 00:39:24.810 --> 00:39:25.520 align:middle line:90% Yes. 00:39:25.520 --> 00:39:27.770 align:middle line:84% AUDIENCE: So why does division-- why does it always 00:39:27.770 --> 00:39:30.110 align:middle line:84% result in float if you have 9 by 3 00:39:30.110 --> 00:39:32.330 align:middle line:84% and that's [INAUDIBLE] why does it [INAUDIBLE]?? 00:39:32.330 --> 00:39:34.640 align:middle line:84% ANA BELL: Yeah, so the question is, why does it always 00:39:34.640 --> 00:39:36.410 align:middle line:90% result in a float? 00:39:36.410 --> 00:39:40.520 align:middle line:84% If it didn't, I think it would the operation itself would have 00:39:40.520 --> 00:39:43.760 align:middle line:84% to do extra work to figure out whether it's a whole number 00:39:43.760 --> 00:39:44.490 align:middle line:90% or not. 00:39:44.490 --> 00:39:49.400 align:middle line:84% So I think it's just easier that it gives us always a float, 00:39:49.400 --> 00:39:50.390 align:middle line:90% I guess. 00:39:50.390 --> 00:39:54.230 align:middle line:84% Previous versions of Python, the / was actually, 00:39:54.230 --> 00:39:58.100 align:middle line:84% I think, integer division, which is super counterintuitive 00:39:58.100 --> 00:40:00.047 align:middle line:84% because you would use that in your program. 00:40:00.047 --> 00:40:01.880 align:middle line:84% And then you would basically integer divide, 00:40:01.880 --> 00:40:03.320 align:middle line:90% and things would go wrong. 00:40:03.320 --> 00:40:08.310 align:middle line:84% But again, just a design choice on behalf of the programmers. 00:40:08.310 --> 00:40:11.630 align:middle line:90% Other questions so far? 00:40:11.630 --> 00:40:15.110 align:middle line:90% OK, so we have a lot of objects. 00:40:15.110 --> 00:40:18.890 align:middle line:84% Objects have different types, again, floats, integers, 00:40:18.890 --> 00:40:20.120 align:middle line:90% Booleans. 00:40:20.120 --> 00:40:22.310 align:middle line:90% What can we do with them? 00:40:22.310 --> 00:40:24.740 align:middle line:84% So far, they're kind of just sitting in there, 00:40:24.740 --> 00:40:26.600 align:middle line:84% and we can get properties about them. 00:40:26.600 --> 00:40:28.730 align:middle line:84% But what we'd like to do is write programs, 00:40:28.730 --> 00:40:30.590 align:middle line:84% basically trying to automate some things 00:40:30.590 --> 00:40:32.930 align:middle line:84% about these objects, manipulate them 00:40:32.930 --> 00:40:36.740 align:middle line:84% to help us achieve a more complicated and interesting 00:40:36.740 --> 00:40:38.370 align:middle line:90% program. 00:40:38.370 --> 00:40:42.770 align:middle line:84% So what we can do to get to that end 00:40:42.770 --> 00:40:48.260 align:middle line:84% is to start assigning names to some of these objects. 00:40:48.260 --> 00:40:54.860 align:middle line:84% If I create an object for pi in my program to 20 decimal places 00:40:54.860 --> 00:40:57.800 align:middle line:84% somehow, and I have that number in my program, 00:40:57.800 --> 00:40:59.390 align:middle line:90% that float in my program-- 00:40:59.390 --> 00:41:02.480 align:middle line:84% if I want to use that number in many different places 00:41:02.480 --> 00:41:04.670 align:middle line:84% in my program, I'd have to copy and paste 00:41:04.670 --> 00:41:10.330 align:middle line:84% it a whole bunch of times so far, which is very tedious, 00:41:10.330 --> 00:41:12.640 align:middle line:90% lots of errors will happen. 00:41:12.640 --> 00:41:13.730 align:middle line:90% I don't want to do that. 00:41:13.730 --> 00:41:16.360 align:middle line:84% So instead what I can do is I can give a name 00:41:16.360 --> 00:41:21.100 align:middle line:84% to this ridiculously long value of pi called pi. 00:41:21.100 --> 00:41:23.470 align:middle line:84% And then I can just use this name anywhere 00:41:23.470 --> 00:41:26.290 align:middle line:84% I want to grab that ridiculously long value for pi 00:41:26.290 --> 00:41:27.490 align:middle line:90% in my program. 00:41:27.490 --> 00:41:29.860 align:middle line:90% It's a lot easier to read. 00:41:29.860 --> 00:41:32.350 align:middle line:84% It's a lot easier for me to write this program. 00:41:32.350 --> 00:41:38.780 align:middle line:84% And it leads to a really nice and neat program. 00:41:38.780 --> 00:41:40.930 align:middle line:84% So what we can do is we can start 00:41:40.930 --> 00:41:45.610 align:middle line:84% saying that the float 0.001 will be referenced by the name 00:41:45.610 --> 00:41:52.790 align:middle line:84% "small" or the 100.4 will be referenced by the name "temp." 00:41:52.790 --> 00:41:56.110 align:middle line:84% So what we want to do is create these things called variables. 00:41:56.110 --> 00:42:00.220 align:middle line:84% And a variable is different in computer science 00:42:00.220 --> 00:42:02.800 align:middle line:84% from a mathematical variable or variables 00:42:02.800 --> 00:42:04.360 align:middle line:84% that you've known so far in math. 00:42:04.360 --> 00:42:06.580 align:middle line:84% So math variables come back to the idea 00:42:06.580 --> 00:42:09.530 align:middle line:84% of declarative knowledge, a declarative statement. 00:42:09.530 --> 00:42:12.470 align:middle line:84% You can have something like a plus b is equal to b minus 1 00:42:12.470 --> 00:42:16.850 align:middle line:84% in math, or x is equal to-- or x times x is equal to y, 00:42:16.850 --> 00:42:19.040 align:middle line:90% and that's perfectly OK. 00:42:19.040 --> 00:42:22.670 align:middle line:84% In math, we basically say that variable x represents 00:42:22.670 --> 00:42:24.970 align:middle line:90% all the square roots of y. 00:42:24.970 --> 00:42:27.805 align:middle line:84% That's not going to fly in computer science. 00:42:27.805 --> 00:42:30.700 align:middle line:84% In computer science, we don't have-- 00:42:30.700 --> 00:42:32.275 align:middle line:84% we don't do declarative knowledge. 00:42:32.275 --> 00:42:33.400 align:middle line:90% We do imperative knowledge. 00:42:33.400 --> 00:42:35.770 align:middle line:84% And so what we're working with in computer science 00:42:35.770 --> 00:42:39.790 align:middle line:84% is a bunch of assignment statements. 00:42:39.790 --> 00:42:42.370 align:middle line:84% So what we can do in computer science 00:42:42.370 --> 00:42:47.750 align:middle line:84% is we're going to basically bind a value to a variable. 00:42:47.750 --> 00:42:49.780 align:middle line:84% So we're going to say this variable name is 00:42:49.780 --> 00:42:50.860 align:middle line:90% bound to this value. 00:42:50.860 --> 00:42:52.630 align:middle line:84% Every time I want to grab this value, 00:42:52.630 --> 00:42:55.100 align:middle line:84% I'm going to invoke this variable name. 00:42:55.100 --> 00:42:56.560 align:middle line:90% So here are some examples. 00:42:56.560 --> 00:42:59.900 align:middle line:90% I've got a is equal to b plus 1. 00:42:59.900 --> 00:43:03.260 align:middle line:84% The thing on the right-hand side will evaluate to some value 00:43:03.260 --> 00:43:09.050 align:middle line:84% as long as I have something that b has a value for. 00:43:09.050 --> 00:43:11.540 align:middle line:90% I've got here m is equal to 10. 00:43:11.540 --> 00:43:12.920 align:middle line:90% So m is a variable. 00:43:12.920 --> 00:43:14.870 align:middle line:90% Its value is 10. 00:43:14.870 --> 00:43:19.010 align:middle line:84% I've got F is equal to m times 9.98. 00:43:19.010 --> 00:43:21.620 align:middle line:84% So again, I have an expression on the right-hand side, 00:43:21.620 --> 00:43:22.730 align:middle line:90% and that's OK. 00:43:22.730 --> 00:43:27.320 align:middle line:84% I'm going to use the value of 10, so F's value will be 99.8. 00:43:27.320 --> 00:43:28.205 align:middle line:90% Yeah. 00:43:28.205 --> 00:43:32.570 align:middle line:84% AUDIENCE: Can you put it so that for F-- 00:43:32.570 --> 00:43:34.310 align:middle line:90% is it like this one value of m? 00:43:34.310 --> 00:43:35.900 align:middle line:84% Or can you have it so it's going to be 00:43:35.900 --> 00:43:38.365 align:middle line:90% whatever m assigned recently? 00:43:38.365 --> 00:43:38.990 align:middle line:90% ANA BELL: Yeah. 00:43:38.990 --> 00:43:42.630 align:middle line:84% The question is, can you have m whatever it recently is? 00:43:42.630 --> 00:43:45.350 align:middle line:84% So in this particular case, I just have these two lines. 00:43:45.350 --> 00:43:47.240 align:middle line:90% And m will be whatever 10 is. 00:43:47.240 --> 00:43:51.020 align:middle line:84% But we'll see in a couple lectures that we can write 00:43:51.020 --> 00:43:53.750 align:middle line:90% a loop where you change m. 00:43:53.750 --> 00:43:58.070 align:middle line:84% And then every time you change m, you immediately calculate F. 00:43:58.070 --> 00:44:01.320 align:middle line:84% And then it'll calculate F based on the new value of m. 00:44:01.320 --> 00:44:05.430 align:middle line:84% But if we just have these two lines, that's all there is. 00:44:05.430 --> 00:44:07.190 align:middle line:90% It just uses 10. 00:44:07.190 --> 00:44:08.315 align:middle line:90% Was there another question? 00:44:08.315 --> 00:44:11.820 align:middle line:90% 00:44:11.820 --> 00:44:15.300 align:middle line:84% So in computer science, you have only one variable 00:44:15.300 --> 00:44:18.540 align:middle line:84% to the left of this equal sign, called the assignment operator. 00:44:18.540 --> 00:44:21.420 align:middle line:84% And you have a value to the right-hand side 00:44:21.420 --> 00:44:24.220 align:middle line:84% of the equal sign, the assignment operator. 00:44:24.220 --> 00:44:29.670 align:middle line:84% So one variable basically maps to or binds to one value. 00:44:29.670 --> 00:44:34.220 align:middle line:84% So the equal sign is an assignment statement. 00:44:34.220 --> 00:44:35.460 align:middle line:90% It's not equality. 00:44:35.460 --> 00:44:37.760 align:middle line:84% It's not a solve for x type of situation. 00:44:37.760 --> 00:44:39.260 align:middle line:90% It's just an assignment. 00:44:39.260 --> 00:44:42.850 align:middle line:84% It binds this name to this value. 00:44:42.850 --> 00:44:46.950 align:middle line:84% So the way that we figure out the name with the value 00:44:46.950 --> 00:44:49.770 align:middle line:84% is, well, if we have this assignment statement here, 00:44:49.770 --> 00:44:51.807 align:middle line:84% we first look at the right-hand side. 00:44:51.807 --> 00:44:53.640 align:middle line:84% So we always start with the right-hand side. 00:44:53.640 --> 00:44:55.540 align:middle line:90% And we evaluate it. 00:44:55.540 --> 00:44:58.120 align:middle line:84% Remember, we have an expression on the right. 00:44:58.120 --> 00:45:00.970 align:middle line:84% We have to evaluate it to one value. 00:45:00.970 --> 00:45:04.950 align:middle line:84% So this will be 3.14, whatever it is, 1.159. 00:45:04.950 --> 00:45:09.810 align:middle line:84% And then we take that value and bind it to the name pi. 00:45:09.810 --> 00:45:14.160 align:middle line:84% So anytime I type in p-i, "pi," in my program from now on, 00:45:14.160 --> 00:45:18.450 align:middle line:84% Python will automatically grab 3.14159 from memory. 00:45:18.450 --> 00:45:21.430 align:middle line:90% So it's bound to that value now. 00:45:21.430 --> 00:45:23.410 align:middle line:90% OK, there are some rules. 00:45:23.410 --> 00:45:24.910 align:middle line:84% Did I have them on the previous one? 00:45:24.910 --> 00:45:32.270 align:middle line:84% Yes, there are some rules to variable names, 00:45:32.270 --> 00:45:33.920 align:middle line:84% but we'll talk about that in a bit. 00:45:33.920 --> 00:45:36.820 align:middle line:84% For now, I want you to tell me if any of the following 00:45:36.820 --> 00:45:37.720 align:middle line:90% are allowed. 00:45:37.720 --> 00:45:40.190 align:middle line:84% If I do x is equal to 6, is that allowed in Python? 00:45:40.190 --> 00:45:41.630 align:middle line:90% AUDIENCE: Yes. 00:45:41.630 --> 00:45:42.550 align:middle line:90% ANA BELL: Yes, it is. 00:45:42.550 --> 00:45:42.820 align:middle line:90% Good. 00:45:42.820 --> 00:45:45.250 align:middle line:84% Because I have one variable name bound to one value, 6. 00:45:45.250 --> 00:45:46.480 align:middle line:90% What about 6 equals x? 00:45:46.480 --> 00:45:47.530 align:middle line:90% It's just backward. 00:45:47.530 --> 00:45:48.130 align:middle line:90% AUDIENCE: No. 00:45:48.130 --> 00:45:49.300 align:middle line:90% ANA BELL: OK, good. 00:45:49.300 --> 00:45:53.500 align:middle line:90% 6 equals x is bad, syntax error. 00:45:53.500 --> 00:45:56.560 align:middle line:84% How about x times y equals 3 plus 4? 00:45:56.560 --> 00:45:57.320 align:middle line:90% AUDIENCE: No. 00:45:57.320 --> 00:45:59.560 align:middle line:84% ANA BELL: No, exactly, because the thing on the left 00:45:59.560 --> 00:46:00.970 align:middle line:90% has an operator in it. 00:46:00.970 --> 00:46:02.830 align:middle line:90% And operators are special. 00:46:02.830 --> 00:46:06.040 align:middle line:84% So it can't have-- you can't have a variable with that * 00:46:06.040 --> 00:46:06.910 align:middle line:90% as a name. 00:46:06.910 --> 00:46:09.790 align:middle line:90% How about xy equals 3 plus 4? 00:46:09.790 --> 00:46:10.457 align:middle line:90% AUDIENCE: Yes. 00:46:10.457 --> 00:46:11.790 align:middle line:90% ANA BELL: Allowed, yes, exactly. 00:46:11.790 --> 00:46:14.160 align:middle line:84% I was hoping to get you guys with that, but I didn't. 00:46:14.160 --> 00:46:17.730 align:middle line:90% Xy equals 3 plus 4 is OK. 00:46:17.730 --> 00:46:18.730 align:middle line:90% There was no error. 00:46:18.730 --> 00:46:21.810 align:middle line:84% And then I can invoke the name of the variable I just 00:46:21.810 --> 00:46:23.980 align:middle line:90% created simply by typing it in. 00:46:23.980 --> 00:46:27.690 align:middle line:84% So if I type in xy, it gives me 7. 00:46:27.690 --> 00:46:32.250 align:middle line:84% And then I can do operations with it, xy plus 1 is 8. 00:46:32.250 --> 00:46:32.820 align:middle line:90% Yeah. 00:46:32.820 --> 00:46:34.695 align:middle line:84% AUDIENCE: Before you were putting the strings 00:46:34.695 --> 00:46:35.760 align:middle line:90% with apostrophes. 00:46:35.760 --> 00:46:37.420 align:middle line:90% So wouldn't you need that? 00:46:37.420 --> 00:46:40.360 align:middle line:84% ANA BELL: So those are strings, right, sequences of characters. 00:46:40.360 --> 00:46:42.140 align:middle line:90% Here, these are variables. 00:46:42.140 --> 00:46:47.280 align:middle line:84% So these are names that I am giving as a variable. 00:46:47.280 --> 00:46:48.530 align:middle line:90% Yeah, that's a great question. 00:46:48.530 --> 00:46:51.010 align:middle line:90% So this is going to be a string. 00:46:51.010 --> 00:46:52.840 align:middle line:90% And you notice it changed color. 00:46:52.840 --> 00:46:55.390 align:middle line:90% It has some meaning in Python. 00:46:55.390 --> 00:46:58.690 align:middle line:84% But xy is a variable that I create. 00:46:58.690 --> 00:47:03.760 align:middle line:90% 00:47:03.760 --> 00:47:08.080 align:middle line:84% OK, so why do we want to give names to variables? 00:47:08.080 --> 00:47:10.990 align:middle line:84% Because as I showed you with the pi example, 00:47:10.990 --> 00:47:14.530 align:middle line:84% it's a lot easier to write readable code 00:47:14.530 --> 00:47:18.470 align:middle line:84% if you have variable names within your programs. 00:47:18.470 --> 00:47:20.560 align:middle line:90% So when you grab-- 00:47:20.560 --> 00:47:23.110 align:middle line:84% when you write programs, it's important to choose 00:47:23.110 --> 00:47:25.060 align:middle line:90% variable names wisely. 00:47:25.060 --> 00:47:27.472 align:middle line:84% You don't want to use just single letters. 00:47:27.472 --> 00:47:28.930 align:middle line:84% You don't want to name it something 00:47:28.930 --> 00:47:31.305 align:middle line:84% that doesn't have something to do with the program you're 00:47:31.305 --> 00:47:33.640 align:middle line:84% writing, because you're going to want 00:47:33.640 --> 00:47:36.940 align:middle line:84% to reread these programs sometime in the future. 00:47:36.940 --> 00:47:39.490 align:middle line:84% Or others might want to read your programs sometime 00:47:39.490 --> 00:47:40.730 align:middle line:90% in the future. 00:47:40.730 --> 00:47:43.240 align:middle line:84% So here's an example of a nice program. 00:47:43.240 --> 00:47:46.120 align:middle line:84% It's just basically four assignment statements 00:47:46.120 --> 00:47:48.350 align:middle line:90% that do some calculations. 00:47:48.350 --> 00:47:51.050 align:middle line:84% The first line of the program is not really a line. 00:47:51.050 --> 00:47:52.950 align:middle line:90% It's called a comment. 00:47:52.950 --> 00:47:54.700 align:middle line:84% You can have as many of these as you like. 00:47:54.700 --> 00:47:56.020 align:middle line:90% They start with a hash. 00:47:56.020 --> 00:47:57.520 align:middle line:84% It's a line that starts with a hash. 00:47:57.520 --> 00:48:00.880 align:middle line:84% And it's basically a text that you 00:48:00.880 --> 00:48:05.690 align:middle line:84% write that helps you or others figure out what 00:48:05.690 --> 00:48:07.020 align:middle line:90% the code is supposed to do. 00:48:07.020 --> 00:48:10.430 align:middle line:84% And usually we comment large chunks of code at a time, 00:48:10.430 --> 00:48:13.700 align:middle line:90% not line by line. 00:48:13.700 --> 00:48:17.790 align:middle line:84% Then we have these four assignment statements. 00:48:17.790 --> 00:48:22.160 align:middle line:84% So here I'm defining variable named pi bound to the value 00:48:22.160 --> 00:48:25.430 align:middle line:84% here, so not the division but 3.14159. 00:48:25.430 --> 00:48:28.820 align:middle line:84% Variable named radius bound to this float 2.2. 00:48:28.820 --> 00:48:30.740 align:middle line:84% And then I have a variable named area 00:48:30.740 --> 00:48:34.580 align:middle line:84% which is bound to the result of this expression. 00:48:34.580 --> 00:48:38.330 align:middle line:84% So when Python sees my pi and my radius, 00:48:38.330 --> 00:48:42.260 align:middle line:84% it grabs them from memory, replaces them with the values, 00:48:42.260 --> 00:48:45.680 align:middle line:84% evaluates the expression, grabs that one 00:48:45.680 --> 00:48:49.280 align:middle line:84% value that we evaluated to 15-point-something, whatever 00:48:49.280 --> 00:48:52.760 align:middle line:84% this is, and binds the 15-point-something to the name 00:48:52.760 --> 00:48:54.200 align:middle line:90% area. 00:48:54.200 --> 00:48:55.760 align:middle line:90% Same with circumference. 00:48:55.760 --> 00:48:59.578 align:middle line:90% 00:48:59.578 --> 00:49:01.370 align:middle line:84% Code style is something that we're actually 00:49:01.370 --> 00:49:03.270 align:middle line:84% going to look at in your problem sets. 00:49:03.270 --> 00:49:05.540 align:middle line:84% So I just wanted to quickly talk about that. 00:49:05.540 --> 00:49:09.560 align:middle line:84% Here is a program that has really bad style. 00:49:09.560 --> 00:49:10.940 align:middle line:90% Actually, that shouldn't be meh. 00:49:10.940 --> 00:49:13.370 align:middle line:84% It should be terrible or something like that. 00:49:13.370 --> 00:49:15.830 align:middle line:84% But in case you haven't noticed, it's 00:49:15.830 --> 00:49:17.900 align:middle line:84% the same program as on the previous slide. 00:49:17.900 --> 00:49:20.600 align:middle line:84% But if I gave you this program straight off the bat, 00:49:20.600 --> 00:49:23.030 align:middle line:84% you probably wouldn't know what it's doing. 00:49:23.030 --> 00:49:27.020 align:middle line:84% It's reusing 355 over 113 twice here. 00:49:27.020 --> 00:49:30.140 align:middle line:84% It's using just a and c as variable names. 00:49:30.140 --> 00:49:33.720 align:middle line:84% Its description is "do calculations." 00:49:33.720 --> 00:49:35.060 align:middle line:90% So pretty bad. 00:49:35.060 --> 00:49:36.660 align:middle line:90% This is a little bit better. 00:49:36.660 --> 00:49:40.980 align:middle line:84% I've recognized that 355 over 113 is being used twice. 00:49:40.980 --> 00:49:43.050 align:middle line:90% So I'm saving it as a variable. 00:49:43.050 --> 00:49:47.930 align:middle line:84% But my variables are still single characters. 00:49:47.930 --> 00:49:51.530 align:middle line:90% And my comments are pretty bad. 00:49:51.530 --> 00:49:54.620 align:middle line:84% I'm basically saying what the code is doing. 00:49:54.620 --> 00:49:56.670 align:middle line:90% Please don't do that. 00:49:56.670 --> 00:50:01.410 align:middle line:84% We can see that a equals p times r times r. 00:50:01.410 --> 00:50:03.850 align:middle line:84% I see that I'm multiplying p with r squared. 00:50:03.850 --> 00:50:07.020 align:middle line:84% I don't need to read that in English. 00:50:07.020 --> 00:50:10.590 align:middle line:84% What I would like to see is a comment like this. 00:50:10.590 --> 00:50:13.920 align:middle line:84% Here I'm commenting a chunk of code. 00:50:13.920 --> 00:50:17.340 align:middle line:84% And someone who doesn't want to read this chunk of code 00:50:17.340 --> 00:50:19.020 align:middle line:84% just reads the comment, and I already 00:50:19.020 --> 00:50:21.960 align:middle line:84% know that I'm calculating the area and circumference using 00:50:21.960 --> 00:50:23.670 align:middle line:90% an approximation for pi. 00:50:23.670 --> 00:50:27.960 align:middle line:84% That's a pretty nice comment there and good descriptive 00:50:27.960 --> 00:50:31.200 align:middle line:90% names and all that. 00:50:31.200 --> 00:50:34.640 align:middle line:84% So we can actually-- once we create an object, 00:50:34.640 --> 00:50:37.370 align:middle line:84% a variable-- sorry, once we create an object and bind it 00:50:37.370 --> 00:50:41.270 align:middle line:84% to a variable, we can change the bindings. 00:50:41.270 --> 00:50:43.370 align:middle line:84% So we can take that variable name 00:50:43.370 --> 00:50:46.620 align:middle line:84% and bind it to a completely different value. 00:50:46.620 --> 00:50:49.880 align:middle line:84% This might not be useful right now, 00:50:49.880 --> 00:50:53.000 align:middle line:84% but it will be useful when we introduce 00:50:53.000 --> 00:50:55.640 align:middle line:90% control flow in our programs. 00:50:55.640 --> 00:50:59.390 align:middle line:84% So to rebind a variable what that means is 00:50:59.390 --> 00:51:01.640 align:middle line:84% we're going to take the name, we're 00:51:01.640 --> 00:51:03.860 align:middle line:84% going to lose the binding to the previous value, 00:51:03.860 --> 00:51:05.970 align:middle line:84% and we're going to rebind it to a new value. 00:51:05.970 --> 00:51:08.870 align:middle line:84% So I'm going to show you how this looks like in memory. 00:51:08.870 --> 00:51:11.660 align:middle line:84% I'm going to use this sort of cloud picture 00:51:11.660 --> 00:51:14.750 align:middle line:84% to represent what happens behind the scenes 00:51:14.750 --> 00:51:16.040 align:middle line:90% whenever we write programs. 00:51:16.040 --> 00:51:18.980 align:middle line:84% And it's like a little animation to help you understand line 00:51:18.980 --> 00:51:20.760 align:middle line:90% by line what's going on. 00:51:20.760 --> 00:51:24.080 align:middle line:90% So here we have pi equals 3.14. 00:51:24.080 --> 00:51:27.920 align:middle line:84% So the green 3.14 is my value in memory. 00:51:27.920 --> 00:51:28.700 align:middle line:90% Cloud is memory. 00:51:28.700 --> 00:51:29.960 align:middle line:90% That's my value in memory. 00:51:29.960 --> 00:51:32.450 align:middle line:90% And it's bound to this name pi. 00:51:32.450 --> 00:51:35.440 align:middle line:90% So this is my variable name. 00:51:35.440 --> 00:51:38.590 align:middle line:84% The next line, radius equals 2.2, same thing. 00:51:38.590 --> 00:51:41.740 align:middle line:84% I've got 2.2 as my value in memory, my object. 00:51:41.740 --> 00:51:44.930 align:middle line:84% And radius is the name for that object. 00:51:44.930 --> 00:51:47.190 align:middle line:84% Area equals pi times radius squared. 00:51:47.190 --> 00:51:49.100 align:middle line:84% So what happens behind the scenes 00:51:49.100 --> 00:51:51.517 align:middle line:90% is it calculates this value. 00:51:51.517 --> 00:51:52.850 align:middle line:90% It doesn't store the expression. 00:51:52.850 --> 00:51:55.550 align:middle line:84% It stores the value resulting from the calculation, 00:51:55.550 --> 00:51:58.430 align:middle line:90% and then it saves it-- 00:51:58.430 --> 00:52:01.530 align:middle line:90% or binds it to the name area. 00:52:01.530 --> 00:52:03.090 align:middle line:90% OK, everything OK so far? 00:52:03.090 --> 00:52:04.360 align:middle line:90% We've seen this code before. 00:52:04.360 --> 00:52:04.860 align:middle line:90% Cool. 00:52:04.860 --> 00:52:06.930 align:middle line:84% So now what happens when we do this, 00:52:06.930 --> 00:52:09.720 align:middle line:90% radius equals radius plus 1? 00:52:09.720 --> 00:52:15.610 align:middle line:84% In math, that would say 0 equals 1. 00:52:15.610 --> 00:52:16.840 align:middle line:90% But we're not in math here. 00:52:16.840 --> 00:52:20.620 align:middle line:84% We're in computer science, and this is perfectly valid. 00:52:20.620 --> 00:52:25.180 align:middle line:84% We're following the rule when we have an assignment that 00:52:25.180 --> 00:52:29.920 align:middle line:84% says look at the right-hand side first and evaluate it 00:52:29.920 --> 00:52:32.570 align:middle line:84% and then bind it to the left-hand side. 00:52:32.570 --> 00:52:37.422 align:middle line:84% So if we look at the right-hand side first, we see radius. 00:52:37.422 --> 00:52:38.380 align:middle line:90% Well, what's the value? 00:52:38.380 --> 00:52:40.060 align:middle line:90% 2.2. 00:52:40.060 --> 00:52:43.870 align:middle line:90% We see add 1 to it, 3.2. 00:52:43.870 --> 00:52:45.610 align:middle line:90% Save that in memory. 00:52:45.610 --> 00:52:47.840 align:middle line:90% And then we see the assignment. 00:52:47.840 --> 00:52:52.000 align:middle line:84% Now save it with the name radius. 00:52:52.000 --> 00:52:56.260 align:middle line:84% OK, so we can only have one variable assigned 00:52:56.260 --> 00:52:58.030 align:middle line:90% to one value at a time. 00:52:58.030 --> 00:52:59.590 align:middle line:90% This is not math. 00:52:59.590 --> 00:53:00.850 align:middle line:90% This is computer science. 00:53:00.850 --> 00:53:05.380 align:middle line:84% So you can only have radius point to one thing at a time. 00:53:05.380 --> 00:53:08.590 align:middle line:84% With this line of code, radius equals radius plus 1. 00:53:08.590 --> 00:53:12.700 align:middle line:84% We've lost the binding to 2.2, this object in memory, 00:53:12.700 --> 00:53:17.520 align:middle line:84% and we've rebound it to the value 3.2. 00:53:17.520 --> 00:53:19.320 align:middle line:90% And that's perfectly fine. 00:53:19.320 --> 00:53:21.330 align:middle line:84% 2.2 is now just sitting in memory. 00:53:21.330 --> 00:53:25.380 align:middle line:84% We can't get back to it unless we say maybe radius equals 2.2. 00:53:25.380 --> 00:53:27.480 align:middle line:84% It just sits in memory and then might 00:53:27.480 --> 00:53:29.910 align:middle line:84% be collected later on by-- or reclaimed 00:53:29.910 --> 00:53:31.840 align:middle line:84% by garbage collection or something like that. 00:53:31.840 --> 00:53:34.470 align:middle line:84% But for now, we can't get back to it. 00:53:34.470 --> 00:53:39.800 align:middle line:84% Now, what's the value for area at the end of these lines? 00:53:39.800 --> 00:53:43.640 align:middle line:84% Well, according to this, it's 15.1976. 00:53:43.640 --> 00:53:47.700 align:middle line:84% So it's using the old 2.2 value for radius. 00:53:47.700 --> 00:53:52.890 align:middle line:84% And that's OK because the program never told-- 00:53:52.890 --> 00:53:55.950 align:middle line:84% never had a line that said recalculate area 00:53:55.950 --> 00:53:59.380 align:middle line:90% after we changed the radius. 00:53:59.380 --> 00:54:02.830 align:middle line:84% It's just following, dumb, line by line. 00:54:02.830 --> 00:54:05.410 align:middle line:84% It doesn't know that, hey, if I change the radius, 00:54:05.410 --> 00:54:08.050 align:middle line:84% the user might want the area changed. 00:54:08.050 --> 00:54:09.980 align:middle line:84% It doesn't make those connections. 00:54:09.980 --> 00:54:12.190 align:middle line:84% It's just following instructions. 00:54:12.190 --> 00:54:13.330 align:middle line:90% And that's OK. 00:54:13.330 --> 00:54:15.350 align:middle line:84% If we want it to change the area, 00:54:15.350 --> 00:54:18.400 align:middle line:84% we would have to copy this line and paste it 00:54:18.400 --> 00:54:20.020 align:middle line:90% after we've changed the radius. 00:54:20.020 --> 00:54:24.215 align:middle line:84% And then the area would change as well. 00:54:24.215 --> 00:54:25.090 align:middle line:90% Does that make sense? 00:54:25.090 --> 00:54:28.320 align:middle line:84% That's kind of an important part of this lecture. 00:54:28.320 --> 00:54:30.410 align:middle line:90% OK, cool. 00:54:30.410 --> 00:54:33.493 align:middle line:84% So big idea here is our lines are evaluated one 00:54:33.493 --> 00:54:34.160 align:middle line:90% after the other. 00:54:34.160 --> 00:54:35.060 align:middle line:90% We're not skipping. 00:54:35.060 --> 00:54:36.500 align:middle line:90% We're not repeating things. 00:54:36.500 --> 00:54:38.610 align:middle line:84% That's something we're going to learn about later. 00:54:38.610 --> 00:54:41.820 align:middle line:90% But for now, line by line. 00:54:41.820 --> 00:54:44.390 align:middle line:90% So here's a little you try it. 00:54:44.390 --> 00:54:46.340 align:middle line:84% These three lines are executed in order. 00:54:46.340 --> 00:54:49.490 align:middle line:84% What are the values for meters and feet 00:54:49.490 --> 00:54:51.230 align:middle line:90% variables at each line? 00:54:51.230 --> 00:54:54.770 align:middle line:84% So how about at the first line, what's the value for meters 00:54:54.770 --> 00:54:57.450 align:middle line:90% after we execute the first line? 00:54:57.450 --> 00:54:57.950 align:middle line:90% 100. 00:54:57.950 --> 00:54:58.715 align:middle line:90% What about feet? 00:54:58.715 --> 00:55:01.620 align:middle line:90% 00:55:01.620 --> 00:55:05.930 align:middle line:84% So at the end of the first line, there is no value for feet yet. 00:55:05.930 --> 00:55:07.730 align:middle line:90% How about after the second line? 00:55:07.730 --> 00:55:09.320 align:middle line:90% 328.08. 00:55:09.320 --> 00:55:09.980 align:middle line:90% Right? 00:55:09.980 --> 00:55:11.960 align:middle line:90% How about the value for meters? 00:55:11.960 --> 00:55:12.680 align:middle line:90% AUDIENCE: 100. 00:55:12.680 --> 00:55:13.970 align:middle line:90% ANA BELL: 100 still. 00:55:13.970 --> 00:55:15.950 align:middle line:84% And what about after the third line? 00:55:15.950 --> 00:55:20.360 align:middle line:90% I'm changing meters to 200. 00:55:20.360 --> 00:55:21.380 align:middle line:90% Exactly, yeah. 00:55:21.380 --> 00:55:26.240 align:middle line:84% Meters is 200, but feet is still 328.08. 00:55:26.240 --> 00:55:29.980 align:middle line:84% And this is something I want to show you guys today. 00:55:29.980 --> 00:55:31.510 align:middle line:84% And we're going to use this Python 00:55:31.510 --> 00:55:34.870 align:middle line:90% Tutor a lot more in the future. 00:55:34.870 --> 00:55:37.450 align:middle line:84% Python Tutor is a nice website that allows 00:55:37.450 --> 00:55:39.460 align:middle line:90% you to step in your code-- 00:55:39.460 --> 00:55:41.600 align:middle line:84% step through your code step by step. 00:55:41.600 --> 00:55:44.830 align:middle line:84% So at each line that you execute, 00:55:44.830 --> 00:55:47.950 align:middle line:84% you get to see the values of all the variables in the code. 00:55:47.950 --> 00:55:49.790 align:middle line:84% It's a very useful debugging tool. 00:55:49.790 --> 00:55:52.990 align:middle line:84% I hope you'll try it out today and on Monday, maybe, 00:55:52.990 --> 00:55:55.810 align:middle line:84% for the finger exercises if you're having trouble. 00:55:55.810 --> 00:55:59.350 align:middle line:84% And you can use it for quizzes to help you debug. 00:55:59.350 --> 00:56:00.700 align:middle line:90% But I can just show you. 00:56:00.700 --> 00:56:03.740 align:middle line:84% It's pretty simple here because it's just a step by step. 00:56:03.740 --> 00:56:05.470 align:middle line:90% So we step through. 00:56:05.470 --> 00:56:08.320 align:middle line:84% So the red says the line I'm going to execute. 00:56:08.320 --> 00:56:10.310 align:middle line:84% Green is the line I just executed. 00:56:10.310 --> 00:56:12.230 align:middle line:90% So I just executed meters 100. 00:56:12.230 --> 00:56:16.120 align:middle line:84% So here I have my meters variable with the value 100. 00:56:16.120 --> 00:56:17.630 align:middle line:90% Step through next. 00:56:17.630 --> 00:56:19.450 align:middle line:84% So I just executed feet equals this. 00:56:19.450 --> 00:56:24.770 align:middle line:84% So I now have a variable named feet with a value 328.08. 00:56:24.770 --> 00:56:26.590 align:middle line:90% Meters still 100. 00:56:26.590 --> 00:56:33.760 align:middle line:84% And then meters 200, feet remained 328.08. 00:56:33.760 --> 00:56:35.710 align:middle line:84% So obviously, this is a pretty simple program 00:56:35.710 --> 00:56:37.240 align:middle line:84% to run the Python Tutor on, but you 00:56:37.240 --> 00:56:44.500 align:middle line:84% can imagine using it in more complex settings. 00:56:44.500 --> 00:56:45.700 align:middle line:90% How about one more? 00:56:45.700 --> 00:56:47.770 align:middle line:90% And this is my last example. 00:56:47.770 --> 00:56:49.990 align:middle line:84% I want you to try to write a program that 00:56:49.990 --> 00:56:51.910 align:middle line:90% swaps the values of x and y. 00:56:51.910 --> 00:56:53.800 align:middle line:84% So originally-- and I'll draw this, 00:56:53.800 --> 00:56:55.610 align:middle line:90% the memory diagram real quick. 00:56:55.610 --> 00:56:58.330 align:middle line:90% So we have-- this is our memory. 00:56:58.330 --> 00:57:00.970 align:middle line:90% We have x is bound to 1. 00:57:00.970 --> 00:57:03.550 align:middle line:90% Y is bound to 2. 00:57:03.550 --> 00:57:08.110 align:middle line:84% And what I want to do without saying x equals 2, 00:57:08.110 --> 00:57:10.750 align:middle line:84% y equals 1, what I want to do is swap the values. 00:57:10.750 --> 00:57:14.980 align:middle line:84% I want x to be associated with 2 and y to be associated with 1 00:57:14.980 --> 00:57:19.000 align:middle line:84% but only using commands like this. 00:57:19.000 --> 00:57:22.780 align:middle line:90% And so the code here is buggy. 00:57:22.780 --> 00:57:23.980 align:middle line:90% That means it's wrong. 00:57:23.980 --> 00:57:26.650 align:middle line:90% It has an error in it. 00:57:26.650 --> 00:57:28.270 align:middle line:84% Well, let's step through-- let's step 00:57:28.270 --> 00:57:29.710 align:middle line:90% through a little bit at a time. 00:57:29.710 --> 00:57:32.110 align:middle line:90% Y equals x. 00:57:32.110 --> 00:57:34.630 align:middle line:84% What do I do when y equals x here? 00:57:34.630 --> 00:57:37.610 align:middle line:90% 00:57:37.610 --> 00:57:43.110 align:middle line:84% Yeah, exactly, y is going to move from 2 to 1. 00:57:43.110 --> 00:57:45.285 align:middle line:84% Now, what happens when I do x equals y? 00:57:45.285 --> 00:57:47.840 align:middle line:90% 00:57:47.840 --> 00:57:49.820 align:middle line:90% Yes, x stays the same. 00:57:49.820 --> 00:57:54.260 align:middle line:84% My first line, y equals x, lost the binding to 2. 00:57:54.260 --> 00:57:58.470 align:middle line:84% And now it's all messed up because I can't get it back. 00:57:58.470 --> 00:58:00.860 align:middle line:84% So instead-- so if you didn't understand this, 00:58:00.860 --> 00:58:03.710 align:middle line:84% you can click Python Tutor and just step through step 00:58:03.710 --> 00:58:05.510 align:middle line:90% by step on your own. 00:58:05.510 --> 00:58:07.943 align:middle line:90% But how can we fix this? 00:58:07.943 --> 00:58:09.755 align:middle line:84% AUDIENCE: Create a third variable. 00:58:09.755 --> 00:58:11.600 align:middle line:84% ANA BELL: Create a third variable? 00:58:11.600 --> 00:58:12.820 align:middle line:90% Yeah, that's a great idea. 00:58:12.820 --> 00:58:14.540 align:middle line:84% Yeah, we can create a third variable. 00:58:14.540 --> 00:58:16.960 align:middle line:90% So x is 1, y is 1-- 00:58:16.960 --> 00:58:17.863 align:middle line:90% y is 2. 00:58:17.863 --> 00:58:19.280 align:middle line:84% So we can create a third variable. 00:58:19.280 --> 00:58:23.230 align:middle line:84% What do you want to make the variable equal to? 00:58:23.230 --> 00:58:25.090 align:middle line:90% X or y? 00:58:25.090 --> 00:58:27.280 align:middle line:90% Yeah, either one. 00:58:27.280 --> 00:58:30.190 align:middle line:90% I made it y, so let's do y. 00:58:30.190 --> 00:58:33.490 align:middle line:84% So here I've got a temporary variable called "temp," 00:58:33.490 --> 00:58:36.970 align:middle line:90% and I made it equal to 2. 00:58:36.970 --> 00:58:38.395 align:middle line:90% And now what can I do? 00:58:38.395 --> 00:58:41.890 align:middle line:90% 00:58:41.890 --> 00:58:43.410 align:middle line:90% Which one can I reassign now? 00:58:43.410 --> 00:58:47.390 align:middle line:90% X equals y, or y equals x? 00:58:47.390 --> 00:58:49.880 align:middle line:84% Exactly, y equals-- if I do x equals y, 00:58:49.880 --> 00:58:53.430 align:middle line:84% I lose my binding to 1, and it messed up again. 00:58:53.430 --> 00:58:55.610 align:middle line:90% So y equals x is OK to do. 00:58:55.610 --> 00:58:59.290 align:middle line:84% So I'm going to lose the binding from y from 2 00:58:59.290 --> 00:59:01.120 align:middle line:90% and bind it up to 1. 00:59:01.120 --> 00:59:04.590 align:middle line:90% And now what do I do? 00:59:04.590 --> 00:59:10.960 align:middle line:84% Yeah, now I can safely reassign x to temp. 00:59:10.960 --> 00:59:14.830 align:middle line:84% So I can say x is equal to temp because temp points to 2. 00:59:14.830 --> 00:59:19.470 align:middle line:84% And I want to make x point to 2 as well. 00:59:19.470 --> 00:59:23.297 align:middle line:84% So in terms of code, that's sort of the diagram. 00:59:23.297 --> 00:59:24.380 align:middle line:90% But we can write the code. 00:59:24.380 --> 00:59:27.680 align:middle line:90% So you don't-- let's see. 00:59:27.680 --> 00:59:31.040 align:middle line:84% We don't write it in here, but on your own, 00:59:31.040 --> 00:59:32.828 align:middle line:84% you can write it in here if you'd like. 00:59:32.828 --> 00:59:33.870 align:middle line:90% Or we can do it together. 00:59:33.870 --> 00:59:34.970 align:middle line:90% So x is equal-- 00:59:34.970 --> 00:59:35.480 align:middle line:90% oops. 00:59:35.480 --> 00:59:39.050 align:middle line:90% X equals 1, y equals 2. 00:59:39.050 --> 00:59:41.780 align:middle line:90% And then we had temp. 00:59:41.780 --> 00:59:45.020 align:middle line:84% We wanted to assign it to whatever y was. 00:59:45.020 --> 00:59:48.120 align:middle line:90% So we say temp is equal to y. 00:59:48.120 --> 00:59:50.640 align:middle line:84% And if you want to check the values of the variables, 00:59:50.640 --> 00:59:52.350 align:middle line:90% you can just invoke the names. 00:59:52.350 --> 00:59:59.380 align:middle line:84% So x is 1, y is 2, and temp should be whatever y is, 2. 00:59:59.380 --> 01:00:00.980 align:middle line:90% OK, good so far. 01:00:00.980 --> 01:00:07.210 align:middle line:84% So now I'm at the step here, I think, right? 01:00:07.210 --> 01:00:08.480 align:middle line:90% I've just created this. 01:00:08.480 --> 01:00:10.120 align:middle line:84% And then the last thing I need to do 01:00:10.120 --> 01:00:15.730 align:middle line:84% is lose the binding from x to whatever temp is. 01:00:15.730 --> 01:00:18.320 align:middle line:84% So I want to do this operation here, 01:00:18.320 --> 01:00:23.540 align:middle line:84% which means I want to assign x to be equal to temp. 01:00:23.540 --> 01:00:26.960 align:middle line:90% So now x is 2, y is 1. 01:00:26.960 --> 01:00:29.720 align:middle line:90% 01:00:29.720 --> 01:00:32.850 align:middle line:90% What did I do? 01:00:32.850 --> 01:00:36.120 align:middle line:90% Yeah, so this happens sometimes. 01:00:36.120 --> 01:00:39.210 align:middle line:90% We can just start all over. 01:00:39.210 --> 01:00:41.670 align:middle line:90% So y equals temp. 01:00:41.670 --> 01:00:42.540 align:middle line:90% Sorry. 01:00:42.540 --> 01:00:44.910 align:middle line:90% Temp equals y. 01:00:44.910 --> 01:00:49.610 align:middle line:90% 01:00:49.610 --> 01:00:52.360 align:middle line:90% Y equals x. 01:00:52.360 --> 01:00:53.620 align:middle line:90% Y is one. 01:00:53.620 --> 01:00:54.670 align:middle line:90% X is 1. 01:00:54.670 --> 01:00:58.060 align:middle line:90% And then x equals temp. 01:00:58.060 --> 01:01:00.940 align:middle line:90% Y is one, x is 2. 01:01:00.940 --> 01:01:04.480 align:middle line:90% So it's OK if things go wrong. 01:01:04.480 --> 01:01:05.870 align:middle line:90% They will go wrong. 01:01:05.870 --> 01:01:08.080 align:middle line:84% We can just start all over in this particular case 01:01:08.080 --> 01:01:10.300 align:middle line:84% by redefining our variables and just 01:01:10.300 --> 01:01:11.960 align:middle line:90% trying it out all over again. 01:01:11.960 --> 01:01:13.730 align:middle line:84% So that's kind of what the shell is for. 01:01:13.730 --> 01:01:14.660 align:middle line:90% That's what I use it for. 01:01:14.660 --> 01:01:16.827 align:middle line:84% That's what we're going to use it for in the future, 01:01:16.827 --> 01:01:20.950 align:middle line:84% just to do quick things like this and also things 01:01:20.950 --> 01:01:24.070 align:middle line:84% like checking the types and other commands we've 01:01:24.070 --> 01:01:27.700 align:middle line:90% done earlier. 01:01:27.700 --> 01:01:30.910 align:middle line:84% OK, so any questions before we do the summary? 01:01:30.910 --> 01:01:33.640 align:middle line:84% Was this all right pace or was it too fast? 01:01:33.640 --> 01:01:35.510 align:middle line:90% Or it was OK? 01:01:35.510 --> 01:01:36.700 align:middle line:90% OK, good. 01:01:36.700 --> 01:01:37.700 align:middle line:90% Thumbs up is good. 01:01:37.700 --> 01:01:40.280 align:middle line:90% So let's do a quick summary. 01:01:40.280 --> 01:01:43.420 align:middle line:84% We saw that we can create programs 01:01:43.420 --> 01:01:45.160 align:middle line:90% by manipulating objects. 01:01:45.160 --> 01:01:47.500 align:middle line:90% We created objects in Python. 01:01:47.500 --> 01:01:50.080 align:middle line:84% And we saw that objects have a particular type. 01:01:50.080 --> 01:01:53.470 align:middle line:84% The type that the object has tells Python the things 01:01:53.470 --> 01:01:57.260 align:middle line:84% that you can do with that object. 01:01:57.260 --> 01:02:00.770 align:middle line:84% We can combine objects in expressions. 01:02:00.770 --> 01:02:04.000 align:middle line:84% And these expressions evaluate or boil down 01:02:04.000 --> 01:02:06.310 align:middle line:90% to one particular value. 01:02:06.310 --> 01:02:09.850 align:middle line:84% Objects or values can be stored in variables. 01:02:09.850 --> 01:02:13.510 align:middle line:84% And these variables allow us to access these values 01:02:13.510 --> 01:02:15.820 align:middle line:84% with nicer names later on in our program. 01:02:15.820 --> 01:02:20.470 align:middle line:84% And then we're able to write neater, more legible programs 01:02:20.470 --> 01:02:21.790 align:middle line:90% as well. 01:02:21.790 --> 01:02:22.600 align:middle line:90% So the equal sign-- 01:02:22.600 --> 01:02:25.360 align:middle line:84% I showed you a couple of differences between math 01:02:25.360 --> 01:02:26.290 align:middle line:90% and computer science. 01:02:26.290 --> 01:02:29.840 align:middle line:84% The equal sign was one notable difference. 01:02:29.840 --> 01:02:31.910 align:middle line:84% The equal sign in math is declarative, 01:02:31.910 --> 01:02:34.910 align:middle line:84% and the equal sign in computer science is an assignment. 01:02:34.910 --> 01:02:38.600 align:middle line:84% You're basically saying this is associated with this. 01:02:38.600 --> 01:02:44.230 align:middle line:84% And we're not doing any sort of equality in computer science. 01:02:44.230 --> 01:02:48.640 align:middle line:84% And yes, computers do what you tell them to do. 01:02:48.640 --> 01:02:51.340 align:middle line:84% That's kind of the big thing here. 01:02:51.340 --> 01:02:54.010 align:middle line:84% Line by line, it executes starting from the top, 01:02:54.010 --> 01:02:55.880 align:middle line:90% goes line by line. 01:02:55.880 --> 01:02:58.960 align:middle line:84% So far, we haven't seen any places where 01:02:58.960 --> 01:03:00.610 align:middle line:90% the computer makes a decision. 01:03:00.610 --> 01:03:02.560 align:middle line:84% But next lecture, we will see how 01:03:02.560 --> 01:03:06.130 align:middle line:84% we can insert decision points in our programs 01:03:06.130 --> 01:03:10.420 align:middle line:84% for the computer to either execute one set of code 01:03:10.420 --> 01:03:12.380 align:middle line:90% or another set of code. 01:03:12.380 --> 01:03:14.870 align:middle line:84% All right, so that's the end of today's lecture. 01:03:14.870 --> 01:03:16.070 align:middle line:90% Thank you all for joining. 01:03:16.070 --> 01:03:18.330 align:middle line:90% I will see you on Monday. 01:03:18.330 --> 01:03:30.000 align:middle line:90%