WEBVTT 00:00:00.790 --> 00:00:03.190 The following content is provided under a Creative 00:00:03.190 --> 00:00:04.730 Commons license. 00:00:04.730 --> 00:00:07.030 Your support will help MIT OpenCourseWare 00:00:07.030 --> 00:00:11.390 continue to offer high-quality, educational resources for free. 00:00:11.390 --> 00:00:13.990 To make a donation, or view additional materials 00:00:13.990 --> 00:00:17.880 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.880 --> 00:00:18.850 at ocw.mit.edu. 00:00:29.957 --> 00:00:30.790 ANA BELL: All right. 00:00:30.790 --> 00:00:33.100 Let's begin. 00:00:33.100 --> 00:00:35.830 As I mentioned before, this lecture 00:00:35.830 --> 00:00:37.690 will be recorded for OCW. 00:00:37.690 --> 00:00:39.309 Again, in future lectures, if you 00:00:39.309 --> 00:00:41.350 don't want to have the back of your head show up, 00:00:41.350 --> 00:00:44.880 just don't sit in this front area here. 00:00:44.880 --> 00:00:46.630 First of all, wow, what a crowd, you guys. 00:00:46.630 --> 00:00:48.490 We're finally in 26-100. 00:00:48.490 --> 00:00:52.240 6.0001 made it big, huh? 00:00:52.240 --> 00:00:55.960 Good afternoon and welcome to the very first class of 6.0001, 00:00:55.960 --> 00:00:58.960 and also 600, this semester. 00:00:58.960 --> 00:01:00.610 My name is Ana Bell. 00:01:00.610 --> 00:01:01.840 First name, Ana. 00:01:01.840 --> 00:01:03.250 Last name, Bell. 00:01:03.250 --> 00:01:06.460 I'm a lecturer in the EECS Department. 00:01:06.460 --> 00:01:08.680 And I'll be giving some of the lectures for today, 00:01:08.680 --> 00:01:13.030 along with later on in the term, Professor Eric Grimson, who's 00:01:13.030 --> 00:01:15.820 sitting right down there, will be giving some of the lectures, 00:01:15.820 --> 00:01:17.620 as well. 00:01:17.620 --> 00:01:21.550 Today we're going to go over some basic administrivia, 00:01:21.550 --> 00:01:23.101 a little bit of course information. 00:01:23.101 --> 00:01:24.850 And then, we're going to talk a little bit 00:01:24.850 --> 00:01:26.164 about what is computation? 00:01:26.164 --> 00:01:27.580 We'll discuss at a very high level 00:01:27.580 --> 00:01:29.860 what computers do just to make sure we're all 00:01:29.860 --> 00:01:31.610 on the same page. 00:01:31.610 --> 00:01:34.180 And then, we're going to dive right into Python basics. 00:01:34.180 --> 00:01:36.763 We're going to talk a little bit about mathematical operations 00:01:36.763 --> 00:01:38.134 you can do with Python. 00:01:38.134 --> 00:01:40.300 And then, we're going to talk about Python variables 00:01:40.300 --> 00:01:42.010 and types. 00:01:42.010 --> 00:01:43.907 As I mentioned in my introductory email, all 00:01:43.907 --> 00:01:46.240 the slides and code that I'll talk about during lectures 00:01:46.240 --> 00:01:48.250 will be up before lecture, so I highly 00:01:48.250 --> 00:01:52.540 encourage you to download them and to have them open. 00:01:52.540 --> 00:01:56.020 We're going to go through some in-class exercises which will 00:01:56.020 --> 00:01:57.820 be available on those slides. 00:01:57.820 --> 00:01:59.830 And it's fun to do. 00:01:59.830 --> 00:02:07.420 And it's also great if could take notes about the code just 00:02:07.420 --> 00:02:09.210 for future reference. 00:02:09.210 --> 00:02:10.210 It's true. 00:02:10.210 --> 00:02:12.220 This is a really fast-paced course, 00:02:12.220 --> 00:02:14.110 and we ramp up really quickly. 00:02:14.110 --> 00:02:17.842 We do want to position you to succeed in this course. 00:02:17.842 --> 00:02:19.300 As I was writing this, I was trying 00:02:19.300 --> 00:02:21.160 to think about when I was first starting 00:02:21.160 --> 00:02:24.580 to program what helped me get through my very 00:02:24.580 --> 00:02:26.110 first programming course. 00:02:26.110 --> 00:02:29.470 And this is really a good list. 00:02:29.470 --> 00:02:31.840 The first thing was I just read the psets as soon 00:02:31.840 --> 00:02:37.600 as they came out, made sure that the terminology just sunk in. 00:02:37.600 --> 00:02:39.760 And then, during lectures, if the lecturer 00:02:39.760 --> 00:02:42.967 was talking about something that suddenly I remembered, 00:02:42.967 --> 00:02:45.550 oh, I saw that word in the pset and I didn't know what it was. 00:02:45.550 --> 00:02:46.720 Well, hey, now I know what it is. 00:02:46.720 --> 00:02:47.200 Right? 00:02:47.200 --> 00:02:48.158 So just give it a read. 00:02:48.158 --> 00:02:51.520 You don't need to start it. 00:02:51.520 --> 00:02:55.390 If you're new to programming, I think the key word is practice. 00:02:55.390 --> 00:02:57.880 It's like math or reading. 00:02:57.880 --> 00:03:00.260 The more you practice, the better you get at it. 00:03:00.260 --> 00:03:01.877 You're not going to absorb programming 00:03:01.877 --> 00:03:04.210 by watching me write programs because I already know how 00:03:04.210 --> 00:03:05.170 to program. 00:03:05.170 --> 00:03:07.966 You guys need to practice. 00:03:07.966 --> 00:03:09.340 Download the code before lecture. 00:03:09.340 --> 00:03:10.090 Follow along. 00:03:10.090 --> 00:03:12.430 Whatever I type, you guys can type. 00:03:12.430 --> 00:03:14.436 And I think, also, one of the big things 00:03:14.436 --> 00:03:16.060 is if you're new to programming, you're 00:03:16.060 --> 00:03:19.360 kind of afraid that you're going to break your computer. 00:03:19.360 --> 00:03:24.400 And you can't really do that just by running Anaconda 00:03:24.400 --> 00:03:26.485 and typing in some commands. 00:03:26.485 --> 00:03:28.360 So don't be afraid to just type some stuff in 00:03:28.360 --> 00:03:29.480 and see what it does. 00:03:29.480 --> 00:03:31.230 Worst case, you just restart the computer. 00:03:35.390 --> 00:03:36.411 Yeah. 00:03:36.411 --> 00:03:38.160 That's probably the big thing right there. 00:03:38.160 --> 00:03:39.743 I should have probably highlighted it, 00:03:39.743 --> 00:03:40.881 but don't be afraid. 00:03:40.881 --> 00:03:41.380 Great. 00:03:41.380 --> 00:03:46.780 So this is pretty much a roadmap of all of 6.0001 or 600 00:03:46.780 --> 00:03:47.890 as I've just explained it. 00:03:47.890 --> 00:03:51.880 There's three big things we want to get out of this course. 00:03:51.880 --> 00:03:54.130 The first thing is the knowledge of concepts, 00:03:54.130 --> 00:03:56.650 which is pretty much true of any class that you'll take. 00:03:56.650 --> 00:03:59.740 The class will teach you something through lectures. 00:03:59.740 --> 00:04:02.500 Exams will test how much you know. 00:04:02.500 --> 00:04:05.830 This is a class in programming. 00:04:05.830 --> 00:04:08.950 The other thing we want you to get out of it 00:04:08.950 --> 00:04:10.841 is programming skills. 00:04:10.841 --> 00:04:12.340 And the last thing, and I think this 00:04:12.340 --> 00:04:13.923 is what makes this class really great, 00:04:13.923 --> 00:04:16.240 is we teach you how to solve problems. 00:04:16.240 --> 00:04:18.730 And we do that through the psets. 00:04:18.730 --> 00:04:21.010 That's really how I feel the roadmap of this course 00:04:21.010 --> 00:04:22.600 looks like. 00:04:22.600 --> 00:04:25.300 And underlying all of these is just practice. 00:04:25.300 --> 00:04:29.800 You have to just type some stuff away and code a lot. 00:04:29.800 --> 00:04:33.490 And you'll succeed in this course, I think. 00:04:33.490 --> 00:04:35.590 OK. 00:04:35.590 --> 00:04:38.800 So what are the things we're going to learn in this class? 00:04:38.800 --> 00:04:41.650 I feel like the things we're going learn in this class 00:04:41.650 --> 00:04:44.470 can be divided into basically three different sections. 00:04:44.470 --> 00:04:50.290 The first one is related to these first two items here. 00:04:50.290 --> 00:04:53.380 It's really about learning how to program. 00:04:53.380 --> 00:04:55.330 Learning how to program, part of it 00:04:55.330 --> 00:04:58.990 is figuring out what objects to create. 00:04:58.990 --> 00:05:00.394 You'll learn about these later. 00:05:00.394 --> 00:05:02.560 How do you represent knowledge with data structures? 00:05:02.560 --> 00:05:04.529 That's sort of the broad term for that. 00:05:04.529 --> 00:05:06.070 And then, as you're writing programs, 00:05:06.070 --> 00:05:08.830 you need to-- programs aren't just linear. 00:05:08.830 --> 00:05:10.300 Sometimes programs jump around. 00:05:10.300 --> 00:05:11.260 They make decisions. 00:05:11.260 --> 00:05:13.690 There's some control flow to programs. 00:05:13.690 --> 00:05:18.620 That's what the second line is going to be about. 00:05:18.620 --> 00:05:20.900 The second big part of this course 00:05:20.900 --> 00:05:24.590 is a little bit more abstract, and it 00:05:24.590 --> 00:05:29.000 deals with how do you write good code, good style, 00:05:29.000 --> 00:05:30.530 code that's readable. 00:05:30.530 --> 00:05:33.020 When you write code, you want to write it such 00:05:33.020 --> 00:05:35.480 that-- you're in big company, other people will read it, 00:05:35.480 --> 00:05:37.970 other people will use it, so it has 00:05:37.970 --> 00:05:40.220 to be readable and understandable by others. 00:05:40.220 --> 00:05:41.900 To that end, you need to write code 00:05:41.900 --> 00:05:48.820 that's well organized, modular, easy to understand. 00:05:48.820 --> 00:05:50.570 And not only that, not only will your code 00:05:50.570 --> 00:05:53.577 be read by other people, but next year, maybe, 00:05:53.577 --> 00:05:55.160 you'll take another course, and you'll 00:05:55.160 --> 00:05:56.868 want to look back at some of the problems 00:05:56.868 --> 00:05:58.320 that you wrote in this class. 00:05:58.320 --> 00:06:00.740 You want to be able to reread your code. 00:06:00.740 --> 00:06:03.950 If it's a big mess, you might not be able to understand-- 00:06:03.950 --> 00:06:06.530 or reunderstand-- what you were doing. 00:06:06.530 --> 00:06:08.630 So writing readable code and organizing code 00:06:08.630 --> 00:06:10.360 is also a big part. 00:06:10.360 --> 00:06:15.680 And the last section is going to deal with-- the first two 00:06:15.680 --> 00:06:19.100 are actually part of the programming in Introduction 00:06:19.100 --> 00:06:21.440 to Programming and Computer Science in Python. 00:06:21.440 --> 00:06:26.000 And the last one deals mostly with the computer science part 00:06:26.000 --> 00:06:29.540 in Introduction to Programming and Computer Science in Python. 00:06:29.540 --> 00:06:31.970 We're going to talk about, once you have learned 00:06:31.970 --> 00:06:33.650 how to write programs in Python, how do 00:06:33.650 --> 00:06:35.540 you compare programs in Python? 00:06:35.540 --> 00:06:38.162 How do you know that one program is better than the other? 00:06:38.162 --> 00:06:39.620 How do you know that one program is 00:06:39.620 --> 00:06:41.084 more efficient than the other? 00:06:41.084 --> 00:06:42.500 How do you know that one algorithm 00:06:42.500 --> 00:06:45.230 is better than the other? 00:06:45.230 --> 00:06:47.480 That's what we're going to talk about in the last part 00:06:47.480 --> 00:06:48.850 of the course. 00:06:48.850 --> 00:06:50.250 OK. 00:06:50.250 --> 00:06:52.920 That's all for the administrative part 00:06:52.920 --> 00:06:54.230 of the course. 00:06:54.230 --> 00:06:59.640 Let's start by talking at a high level what does a computer do. 00:06:59.640 --> 00:07:03.130 Fundamentally, it does two things. 00:07:03.130 --> 00:07:05.960 One, performs calculations. 00:07:05.960 --> 00:07:07.770 It performs a lot of calculations. 00:07:07.770 --> 00:07:09.680 Computers these days are really, really fast, 00:07:09.680 --> 00:07:15.240 a billion calculations per second is probably not far off. 00:07:15.240 --> 00:07:16.920 It performs these calculations and it 00:07:16.920 --> 00:07:18.550 has to store them somewhere. 00:07:18.550 --> 00:07:19.050 Right? 00:07:19.050 --> 00:07:21.030 Stores them in computer memory. 00:07:21.030 --> 00:07:24.070 So a computer also has to remember results. 00:07:24.070 --> 00:07:26.280 And these days, it's not uncommon to find computers 00:07:26.280 --> 00:07:30.390 with hundreds of gigabytes of storage. 00:07:30.390 --> 00:07:34.457 The kinds of calculations that computers do, 00:07:34.457 --> 00:07:35.290 there are two kinds. 00:07:35.290 --> 00:07:37.590 One are calculations that are built into the language. 00:07:37.590 --> 00:07:39.450 These are the very low level types 00:07:39.450 --> 00:07:41.890 of calculations, things like addition, 00:07:41.890 --> 00:07:45.610 subtraction, multiplication, and so on. 00:07:45.610 --> 00:07:47.790 And once you have a language that 00:07:47.790 --> 00:07:53.070 has these primitive calculation types, you, as a programmer, 00:07:53.070 --> 00:07:55.890 can put these types together and then define 00:07:55.890 --> 00:07:57.680 your own calculations. 00:07:57.680 --> 00:08:00.210 You can create new types of calculations. 00:08:00.210 --> 00:08:04.570 And the computer will be able to perform those, as well. 00:08:04.570 --> 00:08:07.810 I think, one thing I want to stress-- 00:08:07.810 --> 00:08:09.930 and we're going to come back to this 00:08:09.930 --> 00:08:12.300 again during this entire lecture, actually-- 00:08:12.300 --> 00:08:15.690 is computers only know what you tell them. 00:08:15.690 --> 00:08:18.220 Computers only do what you tell them to do. 00:08:18.220 --> 00:08:19.860 They're not magical. 00:08:19.860 --> 00:08:22.667 They don't have a mind. 00:08:22.667 --> 00:08:24.750 They just know how to perform calculations really, 00:08:24.750 --> 00:08:26.340 really quickly. 00:08:26.340 --> 00:08:29.936 But you have to tell them what calculations to do. 00:08:32.940 --> 00:08:34.500 Computers don't know anything. 00:08:34.500 --> 00:08:35.220 All right. 00:08:35.220 --> 00:08:36.050 We've come to that. 00:08:40.059 --> 00:08:44.320 Let's go into the types of knowledge. 00:08:44.320 --> 00:08:48.220 The first type of knowledge is declarative knowledge. 00:08:48.220 --> 00:08:50.730 And those are things like statements of fact. 00:08:50.730 --> 00:08:53.080 And this is where my email came into play. 00:08:53.080 --> 00:08:55.500 If you read it all the way to the bottom, 00:08:55.500 --> 00:08:57.960 you would have entered a raffle. 00:08:57.960 --> 00:08:59.890 So a statement of fact for today's lecture 00:08:59.890 --> 00:09:03.280 is, someone will win a prize before class ends. 00:09:03.280 --> 00:09:06.820 And the prize was a Google Cardboard. 00:09:06.820 --> 00:09:09.890 Google state-of-the-art virtual reality glasses. 00:09:09.890 --> 00:09:11.474 And I have them right here. 00:09:14.400 --> 00:09:15.950 Yea. 00:09:15.950 --> 00:09:18.650 I delivered on my promise. 00:09:18.650 --> 00:09:20.770 That's a statement of fact. 00:09:20.770 --> 00:09:22.600 So pretend I'm a machine. 00:09:22.600 --> 00:09:23.480 OK? 00:09:23.480 --> 00:09:26.990 I don't know anything except what you tell me. 00:09:26.990 --> 00:09:28.100 I don't know. 00:09:28.100 --> 00:09:30.320 I know that you tell me this statement. 00:09:30.320 --> 00:09:31.847 I'm like, OK. 00:09:31.847 --> 00:09:33.930 But how is someone going to win a Google Cardboard 00:09:33.930 --> 00:09:35.459 before class ends, right? 00:09:35.459 --> 00:09:37.250 That's where imperative knowledge comes in. 00:09:37.250 --> 00:09:39.530 Imperative knowledge is the recipe, or the how-to, 00:09:39.530 --> 00:09:42.230 or the sequence of steps. 00:09:42.230 --> 00:09:43.780 Sorry. 00:09:43.780 --> 00:09:47.210 That's just my funny for that one. 00:09:47.210 --> 00:09:53.530 So the sequence of steps is imperative knowledge. 00:09:53.530 --> 00:09:57.930 If I'm a machine, you need to tell me 00:09:57.930 --> 00:10:00.910 how someone will win a Google Cardboard before class. 00:10:00.910 --> 00:10:02.680 If I follow these steps, then technically, 00:10:02.680 --> 00:10:06.460 I should reach a conclusion. 00:10:06.460 --> 00:10:08.950 Step one, I think we've already done that. 00:10:08.950 --> 00:10:11.590 Whoever wanted to sign up has signed up. 00:10:11.590 --> 00:10:13.180 Now I'm going to open my IDE. 00:10:13.180 --> 00:10:14.680 I'm just basically being a machine 00:10:14.680 --> 00:10:17.740 and following the steps that you've told me. 00:10:17.740 --> 00:10:21.631 The IDE that we're using in this class is called Anaconda. 00:10:21.631 --> 00:10:25.330 I'm just scrolling down to the bottom. 00:10:25.330 --> 00:10:28.660 Hopefully, you've installed it in problem set zero. 00:10:28.660 --> 00:10:30.610 I've opened my IDE. 00:10:30.610 --> 00:10:34.012 I'm going to follow the next set of instructions. 00:10:34.012 --> 00:10:36.220 I'm going to choose a random number between the first 00:10:36.220 --> 00:10:39.460 and the nth responder. 00:10:39.460 --> 00:10:42.340 Now, I'm going to actually use Python to do this . 00:10:42.340 --> 00:10:44.620 And this is also an example of how just 00:10:44.620 --> 00:10:46.310 a really simple task in your life, 00:10:46.310 --> 00:10:48.742 you can use computers or programming to do that. 00:10:48.742 --> 00:10:50.200 Because if I chose a random number, 00:10:50.200 --> 00:10:51.824 I might be biased because, for example, 00:10:51.824 --> 00:10:53.860 I might like the number 8. 00:10:53.860 --> 00:10:57.399 To choose a random number, I'm going to go and say, OK, 00:10:57.399 --> 00:10:58.690 where's the list of responders? 00:10:58.690 --> 00:10:59.800 It starts at 15. 00:10:59.800 --> 00:11:03.070 Actually, it starts at 16 because that's me. 00:11:03.070 --> 00:11:05.440 We're going to choose a random number between 16 00:11:05.440 --> 00:11:09.210 and the end person 266. 00:11:09.210 --> 00:11:11.190 Oh, we just got-- oh. 00:11:11.190 --> 00:11:13.070 OK. 00:11:13.070 --> 00:11:13.570 OK. 00:11:13.570 --> 00:11:15.028 I'm going to cut it off right here. 00:11:15.028 --> 00:11:15.541 271. 00:11:15.541 --> 00:11:16.040 OK. 00:11:16.040 --> 00:11:18.490 16 and 271. 00:11:18.490 --> 00:11:19.330 Perfect. 00:11:19.330 --> 00:11:20.380 OK. 00:11:20.380 --> 00:11:21.880 I'm going to choose a random number. 00:11:21.880 --> 00:11:22.900 I'm going to go to my IDE. 00:11:22.900 --> 00:11:24.490 And you don't need to know how to do this yet, 00:11:24.490 --> 00:11:26.115 but by the end of this class, you will. 00:11:26.115 --> 00:11:29.191 I'm just going to use Python. 00:11:29.191 --> 00:11:31.690 I'm just going to get the random number package that's going 00:11:31.690 --> 00:11:32.950 to give me a random number. 00:11:32.950 --> 00:11:35.820 I'm going to say random.randint. 00:11:35.820 --> 00:11:40.510 And I'm going to choose a random number between 16 and 272, 00:11:40.510 --> 00:11:41.380 OK. 00:11:41.380 --> 00:11:42.440 75. 00:11:42.440 --> 00:11:43.690 OK. 00:11:43.690 --> 00:11:44.800 Great. 00:11:44.800 --> 00:11:45.880 I chose a random number. 00:11:45.880 --> 00:11:48.557 And I'm going to find the number in the responder's sheet. 00:11:48.557 --> 00:11:49.640 What was the number again? 00:11:49.640 --> 00:11:51.040 Sorry. 00:11:51.040 --> 00:11:52.000 75. 00:11:52.000 --> 00:11:54.020 OK. 00:11:54.020 --> 00:11:56.080 Up we go. 00:11:56.080 --> 00:11:57.710 There we go. 00:11:57.710 --> 00:12:01.300 Lauren Z-O-V. Yeah. 00:12:01.300 --> 00:12:02.060 Nice. 00:12:02.060 --> 00:12:02.710 You're here. 00:12:14.180 --> 00:12:16.110 Awesome. 00:12:16.110 --> 00:12:17.810 All right. 00:12:17.810 --> 00:12:21.200 That's an example of me being a machine and also, 00:12:21.200 --> 00:12:23.870 at the same time, using Python in my everyday life, 00:12:23.870 --> 00:12:28.650 just lecturing, to find a random number. 00:12:28.650 --> 00:12:30.350 Try to use Python wherever you can. 00:12:30.350 --> 00:12:31.725 And that just gives you practice. 00:12:34.580 --> 00:12:35.770 That was fun. 00:12:35.770 --> 00:12:37.190 But we're at MIT. 00:12:37.190 --> 00:12:39.110 We're MIT students. 00:12:39.110 --> 00:12:41.900 And we love numbers here at MIT. 00:12:41.900 --> 00:12:44.749 Here's a numerical example that shows 00:12:44.749 --> 00:12:46.790 the difference between declarative and imperative 00:12:46.790 --> 00:12:50.000 knowledge. 00:12:50.000 --> 00:12:51.470 An example of declarative knowledge 00:12:51.470 --> 00:12:54.530 is the square root of a number x is y such that y times y 00:12:54.530 --> 00:12:57.020 is equal to x. 00:12:57.020 --> 00:12:59.010 That's just a statement of fact It's true. 00:13:02.300 --> 00:13:05.872 Computers don't know what to do with that. 00:13:05.872 --> 00:13:07.830 They don't know what to do with that statement. 00:13:07.830 --> 00:13:11.270 But computers do know how to follow a recipe. 00:13:11.270 --> 00:13:13.250 Here's a well-known algorithm. 00:13:13.250 --> 00:13:16.550 To find the square root of a number x, 00:13:16.550 --> 00:13:23.070 let's say x is originally 16, if a computer follows 00:13:23.070 --> 00:13:26.640 this algorithm, it's going to start with a guess, g, 00:13:26.640 --> 00:13:28.620 let's say, 3. 00:13:28.620 --> 00:13:30.940 We're trying to find the square root of 16. 00:13:30.940 --> 00:13:34.320 We're going to calculate g times g is 9. 00:13:34.320 --> 00:13:36.120 And we're going to ask is if g times g 00:13:36.120 --> 00:13:39.670 is close enough to x, then stop and say, g is the answer. 00:13:39.670 --> 00:13:42.699 I'm not really happy with 9 being really close to 16. 00:13:42.699 --> 00:13:44.490 So I'm going to say, I'm not stopping here. 00:13:44.490 --> 00:13:47.130 I'm going to keep going. 00:13:47.130 --> 00:13:48.690 If it's not close enough, then I'm 00:13:48.690 --> 00:13:52.920 going to make a new guess by averaging g and x over g. 00:13:52.920 --> 00:13:54.480 That's x over g here. 00:13:54.480 --> 00:13:57.150 And that's the average over there. 00:13:57.150 --> 00:14:00.120 And the new average is going to be my new guess. 00:14:04.320 --> 00:14:05.430 And that's what it says. 00:14:05.430 --> 00:14:07.680 And then, the last step is using the new guess, 00:14:07.680 --> 00:14:08.790 repeat the process. 00:14:08.790 --> 00:14:11.610 Then we go back to the beginning and repeat the whole process 00:14:11.610 --> 00:14:13.484 over and over again. 00:14:13.484 --> 00:14:15.150 And that's what the rest of the rows do. 00:14:15.150 --> 00:14:16.590 And you keep doing this until you decide 00:14:16.590 --> 00:14:17.631 that you're close enough. 00:14:23.340 --> 00:14:25.140 What we saw for the imperative knowledge 00:14:25.140 --> 00:14:26.610 in the previous numerical example 00:14:26.610 --> 00:14:31.604 was the recipe for how to find the square root of x. 00:14:31.604 --> 00:14:33.270 What were the three parts of the recipe? 00:14:33.270 --> 00:14:36.090 One was a simple sequence of steps. 00:14:36.090 --> 00:14:39.270 There were four steps. 00:14:39.270 --> 00:14:42.660 The other was a flow of control, so there were 00:14:42.660 --> 00:14:45.360 parts where we made decisions. 00:14:45.360 --> 00:14:46.980 Are we close enough? 00:14:46.980 --> 00:14:49.320 There were parts where we repeated some steps. 00:14:49.320 --> 00:14:52.890 At the end, we said, repeat steps 1, 2, 3. 00:14:52.890 --> 00:14:55.320 That's the flow of control. 00:14:55.320 --> 00:14:58.340 And the last part of the recipe was a way to stop. 00:14:58.340 --> 00:15:00.660 You don't want a program that keeps going and going. 00:15:00.660 --> 00:15:03.869 Or for a recipe, you don't want to keep baking bread forever. 00:15:03.869 --> 00:15:05.160 You want to stop at some point. 00:15:05.160 --> 00:15:07.800 Like 10 breads is enough, right? 00:15:07.800 --> 00:15:10.230 So you have to have a way of stopping. 00:15:10.230 --> 00:15:12.780 In the previous example, the way of stopping 00:15:12.780 --> 00:15:15.240 was that we decided we were close enough. 00:15:15.240 --> 00:15:20.261 Close enough was maybe being within .01, .001, 00:15:20.261 --> 00:15:21.010 whatever you pick. 00:15:23.830 --> 00:15:26.550 This recipe is there for an algorithm. 00:15:26.550 --> 00:15:29.248 In computer science speak, it's going to be an algorithm. 00:15:29.248 --> 00:15:31.622 And that's what we're going to learn about in this class. 00:15:34.532 --> 00:15:35.740 We're dealing with computers. 00:15:35.740 --> 00:15:37.990 And we actually want to capture a recipe 00:15:37.990 --> 00:15:41.095 inside a computer, a computer being a mechanical process. 00:15:49.640 --> 00:15:52.952 Historically, there were two different types of computers. 00:15:55.540 --> 00:15:57.730 Originally, there were these things 00:15:57.730 --> 00:15:59.800 called fixed-program computers. 00:15:59.800 --> 00:16:02.110 And I'm old enough to have used something 00:16:02.110 --> 00:16:06.710 like this, where there's just numbers and plus, minus, 00:16:06.710 --> 00:16:08.920 multiplication, divide, and equal. 00:16:08.920 --> 00:16:11.350 But calculators these days are a lot more complicated. 00:16:11.350 --> 00:16:15.280 But way back then, an example of a fixed-program computer 00:16:15.280 --> 00:16:16.210 is this calculator. 00:16:16.210 --> 00:16:19.190 It only knows how to do addition, multiplication, 00:16:19.190 --> 00:16:20.740 subtraction, division. 00:16:20.740 --> 00:16:22.480 If you want to plot something, you can't. 00:16:22.480 --> 00:16:27.040 If you want to go on the internet, send email with it, 00:16:27.040 --> 00:16:27.780 you can't. 00:16:27.780 --> 00:16:31.040 It can only do this one thing. 00:16:31.040 --> 00:16:33.610 And if you wanted to create a machine that did another thing, 00:16:33.610 --> 00:16:37.360 then you'd have to create another fixed-program computer 00:16:37.360 --> 00:16:39.970 that did a completely separate test. 00:16:39.970 --> 00:16:41.890 That's not very great. 00:16:41.890 --> 00:16:45.910 That's when stored-program computers came into play. 00:16:45.910 --> 00:16:47.710 And these were machines that could store 00:16:47.710 --> 00:16:50.530 a sequence of instructions. 00:16:50.530 --> 00:16:54.729 And these machines could execute the sequence of instructions. 00:16:54.729 --> 00:16:56.770 And you could change the sequence of instructions 00:16:56.770 --> 00:17:00.220 and execute this different sequence of instructions. 00:17:00.220 --> 00:17:03.490 You could do different tasks in the same machine. 00:17:03.490 --> 00:17:07.930 And that's the computer as we know it these days. 00:17:07.930 --> 00:17:11.119 The central processing unit is where all of these decisions 00:17:11.119 --> 00:17:11.619 get made. 00:17:11.619 --> 00:17:16.339 And these are all the peripherals. 00:17:16.339 --> 00:17:20.030 The basic machine architecture-- at the heart of every computer 00:17:20.030 --> 00:17:25.280 there's just this basic architecture-- 00:17:25.280 --> 00:17:28.760 and it contains, I guess, four main parts. 00:17:28.760 --> 00:17:31.550 The first is the memory. 00:17:31.550 --> 00:17:34.700 Input and output is the other one. 00:17:34.700 --> 00:17:39.980 The ALU is where all of the operations are done. 00:17:39.980 --> 00:17:41.570 And the operations that the ALU can do 00:17:41.570 --> 00:17:44.570 are really primitive operations, addition, subtraction, 00:17:44.570 --> 00:17:46.460 and so on. 00:17:46.460 --> 00:17:52.610 What the memory contains is a bunch of data 00:17:52.610 --> 00:17:54.854 and your sequence of instructions. 00:18:00.540 --> 00:18:03.870 Interacting with the Arithmetic Logic Unit is the Control Unit. 00:18:03.870 --> 00:18:07.110 And the Control Unit contains one program counter. 00:18:07.110 --> 00:18:09.900 When you load a sequence of instructions, 00:18:09.900 --> 00:18:12.630 the program counter starts at the first sequence. 00:18:15.540 --> 00:18:18.510 It starts at the sequence, at the first instruction. 00:18:18.510 --> 00:18:22.200 It gets what the instruction is, and it sends it to the ALU. 00:18:22.200 --> 00:18:25.990 The ALU asks, what are we doing operations on here? 00:18:25.990 --> 00:18:27.330 What's happening? 00:18:27.330 --> 00:18:29.150 It might get some data. 00:18:29.150 --> 00:18:31.400 If you're adding two numbers, it might get two numbers 00:18:31.400 --> 00:18:33.300 from memory. 00:18:33.300 --> 00:18:34.680 It might do some operations. 00:18:34.680 --> 00:18:37.650 And it might store data back into memory. 00:18:37.650 --> 00:18:41.517 And after it's done, the ALU is going to go back, 00:18:41.517 --> 00:18:43.350 and the program counter is going to increase 00:18:43.350 --> 00:18:45.360 by 1, which means that we're going 00:18:45.360 --> 00:18:50.200 to go to the next sequence in the instruction set. 00:18:50.200 --> 00:18:53.840 And it just goes linearly, instruction by instruction. 00:18:53.840 --> 00:18:56.330 There might be one particular instruction 00:18:56.330 --> 00:18:58.100 that does some sort of test. 00:18:58.100 --> 00:19:07.040 It's going to say, is this particular value 00:19:07.040 --> 00:19:11.330 greater or equal to or the same as this other particular value? 00:19:11.330 --> 00:19:13.200 That's a test, an example of a test. 00:19:13.200 --> 00:19:17.630 And the test is going to either return true or false. 00:19:17.630 --> 00:19:20.810 And depending on the result of that test, 00:19:20.810 --> 00:19:24.530 you might either go to the next instruction, 00:19:24.530 --> 00:19:26.150 or you might set the program counter 00:19:26.150 --> 00:19:29.810 to go all the way back to the beginning, and so on. 00:19:29.810 --> 00:19:32.262 You're not just linearly stepping 00:19:32.262 --> 00:19:33.470 through all the instructions. 00:19:33.470 --> 00:19:35.300 There might be some control flow involved, 00:19:35.300 --> 00:19:36.800 where you might skip an instruction, 00:19:36.800 --> 00:19:39.730 or start from the beginning, or so on. 00:19:39.730 --> 00:19:42.410 And after you're done, when you finished 00:19:42.410 --> 00:19:44.540 executing the last instruction, then you 00:19:44.540 --> 00:19:47.180 might output something. 00:19:47.180 --> 00:19:53.420 That's really the basic way that a computer works. 00:19:53.420 --> 00:19:55.790 Just to recap, you have the stored program computer 00:19:55.790 --> 00:19:59.140 that contains these sequences of instructions. 00:19:59.140 --> 00:20:00.950 The primitive operations that it can do 00:20:00.950 --> 00:20:06.050 are addition, subtraction, logic operations, tests-- 00:20:06.050 --> 00:20:08.840 which are something equal to something else, something 00:20:08.840 --> 00:20:10.790 less than, and so on-- and moving data, 00:20:10.790 --> 00:20:14.390 so storing data, moving data around, and things like that. 00:20:14.390 --> 00:20:19.836 And the interpreter goes through every instruction 00:20:19.836 --> 00:20:22.460 and decides whether you're going to go to the next instruction, 00:20:22.460 --> 00:20:28.830 skip instructions, or repeat instructions, and so on. 00:20:28.830 --> 00:20:30.570 So we've talked about primitives. 00:20:30.570 --> 00:20:35.267 And in fact, Alan Turing, who was a really great computer 00:20:35.267 --> 00:20:37.350 scientist, he showed that you can compute anything 00:20:37.350 --> 00:20:38.880 using the six primitives. 00:20:38.880 --> 00:20:46.590 And the six primitives are move left, move right, read, write, 00:20:46.590 --> 00:20:49.050 scan, and do nothing. 00:20:49.050 --> 00:20:54.390 Using those six instructions and the piece of tape, 00:20:54.390 --> 00:20:57.330 he showed that you can compute anything. 00:20:57.330 --> 00:21:00.390 And using those six instructions, 00:21:00.390 --> 00:21:03.090 programming languages came about that 00:21:03.090 --> 00:21:05.100 created a more convenient set of primitives. 00:21:05.100 --> 00:21:08.520 You don't have to program in only these six commands. 00:21:11.490 --> 00:21:16.080 And one interesting thing, or one really important thing, 00:21:16.080 --> 00:21:19.230 that came about from these six primitives 00:21:19.230 --> 00:21:22.290 is that if you can compute something in Python, 00:21:22.290 --> 00:21:25.770 let's say-- if you write a program that computes something 00:21:25.770 --> 00:21:28.950 in Python, then, in theory, you can 00:21:28.950 --> 00:21:31.260 write a program that computes the exact same thing 00:21:31.260 --> 00:21:32.610 in any other language. 00:21:32.610 --> 00:21:36.720 And that's a really powerful statement. 00:21:36.720 --> 00:21:39.660 Think about that today when you review your slides. 00:21:39.660 --> 00:21:40.860 Think about that again. 00:21:40.860 --> 00:21:41.910 That's really powerful. 00:21:45.240 --> 00:21:49.980 Once you have your set of primitives 00:21:49.980 --> 00:21:53.839 for a particular language, you can start creating expressions. 00:21:53.839 --> 00:21:55.380 And these expressions are going to be 00:21:55.380 --> 00:22:00.510 combinations of the primitives in the programming language. 00:22:00.510 --> 00:22:02.790 And the expressions are going to have some value. 00:22:02.790 --> 00:22:05.740 And they're going up some meaning in the programming 00:22:05.740 --> 00:22:08.440 language. 00:22:08.440 --> 00:22:10.630 Let's do a little bit of a parallel with English 00:22:10.630 --> 00:22:14.020 just so you see what I mean. 00:22:14.020 --> 00:22:15.520 In English, the primitive constructs 00:22:15.520 --> 00:22:17.230 are going to be words. 00:22:17.230 --> 00:22:19.190 There's a lot of words in the English language. 00:22:19.190 --> 00:22:23.230 Programming languages-- in Python, there are primitives, 00:22:23.230 --> 00:22:25.110 but there aren't as many of them. 00:22:25.110 --> 00:22:28.900 There are floats, Booleans, these 00:22:28.900 --> 00:22:31.660 are numbers, strings, and simple operators, 00:22:31.660 --> 00:22:35.170 like addition, subtraction, and so on. 00:22:35.170 --> 00:22:38.230 So we have primitive constructs. 00:22:38.230 --> 00:22:39.820 Using these primitive constructs, 00:22:39.820 --> 00:22:48.167 we can start creating, in English, phrases, sentences, 00:22:48.167 --> 00:22:49.750 and the same in programming languages. 00:22:49.750 --> 00:22:54.370 In English, we can say something like, "cat, dog, boy. 00:22:54.370 --> 00:22:58.360 That, we say, is not syntactically valid. 00:22:58.360 --> 00:23:00.170 That's bad syntax. 00:23:00.170 --> 00:23:01.920 That's noun, noun, noun. 00:23:01.920 --> 00:23:05.160 That doesn't make sense. 00:23:05.160 --> 00:23:08.940 What does have good syntax in English is noun, verb, noun. 00:23:08.940 --> 00:23:12.790 So, "cat, hugs boy" is syntactically valid. 00:23:12.790 --> 00:23:14.680 Similarly, in a programming language, 00:23:14.680 --> 00:23:18.195 something like this-- in Python, in this case-- a word 00:23:18.195 --> 00:23:20.320 and then the number five doesn't really make sense. 00:23:20.320 --> 00:23:22.000 It's not syntactically valid. 00:23:22.000 --> 00:23:28.030 But something like operator, operand, operator is OK. 00:23:28.030 --> 00:23:34.480 So once you've created these phrases, or these expressions, 00:23:34.480 --> 00:23:37.240 that are syntactically valid, you 00:23:37.240 --> 00:23:41.730 have to think about the static semantics of your phrase, 00:23:41.730 --> 00:23:45.130 or of your expression. 00:23:45.130 --> 00:23:51.280 For example, in English, "I are hungry" is good syntax. 00:23:51.280 --> 00:23:55.330 But it's weird to say. 00:23:55.330 --> 00:23:58.270 We have a pronoun, a verb, and an adjective, which 00:23:58.270 --> 00:23:59.770 doesn't really make sense. 00:23:59.770 --> 00:24:03.490 "I am hungry" is better. 00:24:03.490 --> 00:24:07.270 This does not have good static semantics. 00:24:07.270 --> 00:24:09.010 Similarly, in programming languages-- 00:24:09.010 --> 00:24:11.440 and you'll get the hang of this the more 00:24:11.440 --> 00:24:15.950 you do it-- something like this, "3.2 times 5, is OK. 00:24:15.950 --> 00:24:17.390 But what does it mean? 00:24:17.390 --> 00:24:22.330 What's the meaning to have a word added to a number? 00:24:22.330 --> 00:24:25.660 There's no meaning behind that. 00:24:25.660 --> 00:24:28.030 Its syntax is OK, because you have 00:24:28.030 --> 00:24:30.120 operator, operand, operator. 00:24:30.120 --> 00:24:32.770 But it doesn't really make sense to add a number to a word, 00:24:32.770 --> 00:24:33.300 for example. 00:24:36.640 --> 00:24:39.970 Once you have created these expressions that 00:24:39.970 --> 00:24:44.940 are syntactically correct and static, semantically correct, 00:24:44.940 --> 00:24:48.940 in English, for example, you think about the semantics. 00:24:48.940 --> 00:24:50.760 What's the meaning of the phrase? 00:24:50.760 --> 00:24:52.780 In English, you can actually have more than one 00:24:52.780 --> 00:24:56.140 meaning to an entire phrase. 00:24:56.140 --> 00:25:01.284 In this case, "flying planes can be dangerous" 00:25:01.284 --> 00:25:02.200 can have two meanings. 00:25:02.200 --> 00:25:04.990 It's the act of flying a plane is dangerous, 00:25:04.990 --> 00:25:08.860 or the plane that is in the air is dangerous. 00:25:08.860 --> 00:25:10.650 And this might be a cuter example. 00:25:10.650 --> 00:25:12.490 "This reading lamp hasn't uttered a word 00:25:12.490 --> 00:25:13.690 since I bought it. 00:25:13.690 --> 00:25:15.160 What's going on?" 00:25:15.160 --> 00:25:16.380 So that has two meanings. 00:25:16.380 --> 00:25:21.130 It's playing on the word "reading lamp." 00:25:21.130 --> 00:25:21.880 That's in English. 00:25:21.880 --> 00:25:23.338 In English, you can have a sentence 00:25:23.338 --> 00:25:25.140 that has more than one meaning, that's 00:25:25.140 --> 00:25:28.150 syntactically correct and static, semantically correct. 00:25:28.150 --> 00:25:31.180 But in programming languages, the program that you write, 00:25:31.180 --> 00:25:34.085 the set of instructions that you write, only has one meaning. 00:25:34.085 --> 00:25:35.710 Remember, we're coming back to the fact 00:25:35.710 --> 00:25:40.120 that the computer only does what you tell it to do. 00:25:40.120 --> 00:25:42.250 It's not going to suddenly decide 00:25:42.250 --> 00:25:46.390 to add another variable for some reason. 00:25:46.390 --> 00:25:49.060 It's just going to execute whatever statements you've 00:25:49.060 --> 00:25:50.715 put up. 00:25:50.715 --> 00:25:52.840 In programming languages, there's only one meaning. 00:25:52.840 --> 00:25:58.099 But the problem that comes into play in programming languages 00:25:58.099 --> 00:25:59.890 is it's not the meaning that you might have 00:25:59.890 --> 00:26:03.130 intended, as the programmer. 00:26:03.130 --> 00:26:05.926 That's where things can go wrong. 00:26:05.926 --> 00:26:07.300 And there's going to be a lecture 00:26:07.300 --> 00:26:10.010 on debugging a little bit later in the course. 00:26:10.010 --> 00:26:12.880 But this is here just to tell you 00:26:12.880 --> 00:26:16.090 that if you see an error pop up in your program, 00:26:16.090 --> 00:26:18.410 it's just some text that says, error. 00:26:21.160 --> 00:26:28.330 For example, if we do something like this, 00:26:28.330 --> 00:26:30.900 this is syntactically correct. 00:26:30.900 --> 00:26:31.400 Incorrect. 00:26:31.400 --> 00:26:32.230 Syntactically incorrect. 00:26:32.230 --> 00:26:32.730 See? 00:26:32.730 --> 00:26:36.050 There's some angry text right here. 00:26:36.050 --> 00:26:37.719 What is going on? 00:26:37.719 --> 00:26:39.260 The more you program, the more you'll 00:26:39.260 --> 00:26:41.210 get the hang of reading these errors. 00:26:41.210 --> 00:26:43.550 But this is basically telling me the line 00:26:43.550 --> 00:26:45.890 that I wrote is syntactically incorrect. 00:26:45.890 --> 00:26:49.190 And it's pointing to the exact line and says, this is wrong, 00:26:49.190 --> 00:26:52.450 so I can go back and fix it as a programmer. 00:26:55.480 --> 00:26:57.905 Syntax errors are actually really easily caught by Python. 00:27:00.800 --> 00:27:02.870 That was an example of a syntax error. 00:27:02.870 --> 00:27:04.640 Static semantic errors can also be 00:27:04.640 --> 00:27:09.830 caught by Python as long as, if your program has some decisions 00:27:09.830 --> 00:27:13.970 to make, as long as you've gone down the branch where 00:27:13.970 --> 00:27:18.080 the static semantic error happens. 00:27:18.080 --> 00:27:22.160 And this is probably going to be the most frustrating one, 00:27:22.160 --> 00:27:23.767 especially as you're starting out. 00:27:23.767 --> 00:27:25.850 The program might do something different than what 00:27:25.850 --> 00:27:27.740 you expected it to do. 00:27:27.740 --> 00:27:32.300 And that's not because the program suddenly-- for example, 00:27:32.300 --> 00:27:34.460 you expected the program to give you an output of 0 00:27:34.460 --> 00:27:37.826 for a certain test case, and the output that you got was 10. 00:27:37.826 --> 00:27:39.200 Well, the program didn't suddenly 00:27:39.200 --> 00:27:42.350 decide to change its answer to 10. 00:27:42.350 --> 00:27:45.410 It just executed the program that you wrote. 00:27:48.710 --> 00:27:50.660 That's the case where the program gave you 00:27:50.660 --> 00:27:53.210 a different answer than expected. 00:27:53.210 --> 00:27:55.580 Programs might crash, which means they stop running. 00:27:55.580 --> 00:27:57.800 That's OK. 00:27:57.800 --> 00:28:00.440 Just go back to your code and figure out what was wrong. 00:28:00.440 --> 00:28:03.650 And another example of a different meaning 00:28:03.650 --> 00:28:06.790 than what you intended was maybe the program won't stop. 00:28:06.790 --> 00:28:07.580 It's also OK. 00:28:07.580 --> 00:28:10.070 There are ways to stop it besides restarting 00:28:10.070 --> 00:28:12.950 the computer. 00:28:12.950 --> 00:28:17.770 So then Python programs are going 00:28:17.770 --> 00:28:20.260 to be sequences of definitions and commands. 00:28:20.260 --> 00:28:25.090 We're going to have expressions that are going to be evaluated 00:28:25.090 --> 00:28:33.820 and commands that tell the interpreter to do something. 00:28:33.820 --> 00:28:35.410 If you've done problem set 0, you'll 00:28:35.410 --> 00:28:37.930 see that you can type commands directly 00:28:37.930 --> 00:28:40.720 in the shell here, which is the part on the right where 00:28:40.720 --> 00:28:44.500 I did some really simple things, 2 plus 4. 00:28:44.500 --> 00:28:49.480 Or you can type commands up in here, on the left-hand side, 00:28:49.480 --> 00:28:52.100 and then run your program. 00:28:52.100 --> 00:28:54.907 Notice that, well, we'll talk about this-- I 00:28:54.907 --> 00:28:55.990 won't talk about this now. 00:28:55.990 --> 00:28:59.920 But these are-- on the right-hand side, typically, 00:28:59.920 --> 00:29:01.870 you write very simple commands just if you're 00:29:01.870 --> 00:29:03.076 testing something out. 00:29:03.076 --> 00:29:04.950 And on the left-hand side here in the editor, 00:29:04.950 --> 00:29:10.105 you write more lines and more complicated programs. 00:29:15.130 --> 00:29:18.700 Now we're going to start talking about Python. 00:29:18.700 --> 00:29:20.710 And in Python, we're going to come back to this, 00:29:20.710 --> 00:29:23.530 everything is an object. 00:29:23.530 --> 00:29:27.910 And Python programs manipulate these data objects. 00:29:27.910 --> 00:29:30.880 All objects in Python are going to have a type. 00:29:30.880 --> 00:29:34.000 And the type is going to tell Python the kinds of operations 00:29:34.000 --> 00:29:37.630 that you can do on these objects. 00:29:37.630 --> 00:29:39.950 If an object is the number five, for example, 00:29:39.950 --> 00:29:42.749 you can add the number to another number, 00:29:42.749 --> 00:29:45.040 subtract the number, take it to the power of something, 00:29:45.040 --> 00:29:47.260 and so on. 00:29:47.260 --> 00:29:51.520 As a more general example, for example, I am a human. 00:29:51.520 --> 00:29:52.930 So that's my type. 00:29:52.930 --> 00:29:55.700 And I can walk, speak English, et cetera. 00:29:55.700 --> 00:29:59.860 Chewbacca is going to be a type Wookie. 00:29:59.860 --> 00:30:02.834 He can walk, do that sound that I can't do. 00:30:02.834 --> 00:30:04.000 He can do that, but I can't. 00:30:04.000 --> 00:30:06.683 I'm not even going to try, and so on. 00:30:09.640 --> 00:30:11.200 Once you have these Python objects, 00:30:11.200 --> 00:30:14.934 everything is an object in Python. 00:30:14.934 --> 00:30:16.600 There are actually two types of objects. 00:30:16.600 --> 00:30:18.100 One are scalar objects. 00:30:18.100 --> 00:30:21.370 That means these are very basic objects in Python from which 00:30:21.370 --> 00:30:24.760 everything can be made. 00:30:24.760 --> 00:30:26.050 These are scalar objects. 00:30:26.050 --> 00:30:28.480 That can't be subdivided. 00:30:28.480 --> 00:30:31.210 The other type of object is a non-scalar object. 00:30:31.210 --> 00:30:33.700 And these are objects that have some internal structure. 00:30:33.700 --> 00:30:36.730 For example, the number five is a scalar 00:30:36.730 --> 00:30:39.070 object because it can't be subdivided. 00:30:39.070 --> 00:30:41.710 But a list of numbers, for example, 5, 6, 00:30:41.710 --> 00:30:45.370 7,8, is going to be a non-scalar object 00:30:45.370 --> 00:30:46.600 because you can subdivide it. 00:30:46.600 --> 00:30:53.120 You can subdivide it into-- you can find parts to it. 00:30:53.120 --> 00:30:55.130 It's made up of a sequence of numbers. 00:30:58.700 --> 00:31:01.220 Here's the list of all of the scalar objects in Python. 00:31:01.220 --> 00:31:05.870 We have integers, for example, all of the whole numbers. 00:31:05.870 --> 00:31:10.130 Floats, which are all of the real numbers, anything 00:31:10.130 --> 00:31:11.510 with a decimal. 00:31:11.510 --> 00:31:13.760 Bools are Booleans. 00:31:13.760 --> 00:31:16.100 There's only two values to Booleans. 00:31:16.100 --> 00:31:18.500 That's True and False. 00:31:18.500 --> 00:31:23.060 Note the capitalization, capital T and capital F. 00:31:23.060 --> 00:31:24.805 And this other thing called NoneType. 00:31:24.805 --> 00:31:26.510 It's special. 00:31:26.510 --> 00:31:28.590 It has only one value called None. 00:31:28.590 --> 00:31:30.500 And it represents the absence of a type. 00:31:30.500 --> 00:31:34.640 And it sometimes comes in handy for some programs. 00:31:34.640 --> 00:31:36.390 If you want to find the type of an object, 00:31:36.390 --> 00:31:39.532 you can use this special command called type. 00:31:39.532 --> 00:31:41.240 And then in the parentheses, you put down 00:31:41.240 --> 00:31:44.840 what you want to find the type of. 00:31:44.840 --> 00:31:47.480 You can write into the shell "type of 5," 00:31:47.480 --> 00:31:49.566 and the shell will tell you, that's an integer. 00:31:52.990 --> 00:31:56.290 If you happen to want to convert between two different types, 00:31:56.290 --> 00:31:58.180 Python allows you to do that. 00:31:58.180 --> 00:32:01.450 And to do that, you put the type that you 00:32:01.450 --> 00:32:03.820 want to convert to right before the object 00:32:03.820 --> 00:32:05.270 that you want to convert to. 00:32:05.270 --> 00:32:12.700 So float(3) will convert the integer 3 to the float 3.0. 00:32:12.700 --> 00:32:16.870 And similarly, you can convert any float into an integer. 00:32:16.870 --> 00:32:20.170 And converting to an integer just truncates. 00:32:20.170 --> 00:32:22.180 It just takes away the decimal and whatever's 00:32:22.180 --> 00:32:26.150 after it-- it does not round-- and keeps just the integer 00:32:26.150 --> 00:32:26.650 part. 00:32:30.119 --> 00:32:31.910 For this slide, I'm going to talk about it. 00:32:31.910 --> 00:32:35.060 But if you'd like if you have the slides up, 00:32:35.060 --> 00:32:37.200 go to go to this exercise. 00:32:37.200 --> 00:32:41.000 And after I'm done talking about the slide, 00:32:41.000 --> 00:32:45.532 we'll see what people think for that exercise. 00:32:45.532 --> 00:32:47.240 One of the most important things that you 00:32:47.240 --> 00:32:51.280 can do in basically any programming, 00:32:51.280 --> 00:32:55.520 in Python also, is to print things out. 00:32:55.520 --> 00:33:00.292 Printing out is how you interact with the user. 00:33:00.292 --> 00:33:02.250 To print things out, you use the print command. 00:33:04.850 --> 00:33:07.560 If you're in the shell, if you simply type "3 plus 2," 00:33:07.560 --> 00:33:09.700 you do see a value here. 00:33:09.700 --> 00:33:11.720 Five, right? 00:33:11.720 --> 00:33:13.850 But that's not actually printing something out. 00:33:13.850 --> 00:33:18.380 And that becomes apparent when you actually 00:33:18.380 --> 00:33:19.640 type things into the editor. 00:33:19.640 --> 00:33:23.160 If you just do "3 plus 2," and you run the program-- that's 00:33:23.160 --> 00:33:26.270 the green button here-- you see on the right-hand side here, 00:33:26.270 --> 00:33:27.080 it ran my program. 00:33:27.080 --> 00:33:30.200 But it didn't actually print anything. 00:33:30.200 --> 00:33:32.150 If you type this into the console, 00:33:32.150 --> 00:33:33.920 it does show you this value, but that's 00:33:33.920 --> 00:33:39.470 just like peeking into the value for you as a programmer. 00:33:39.470 --> 00:33:41.412 It's not actually printing it out to anyone. 00:33:41.412 --> 00:33:42.870 If you want to print something out, 00:33:42.870 --> 00:33:47.990 you have to use the print statement like that. 00:33:47.990 --> 00:33:50.630 In this case, this is actually going to print this number 00:33:50.630 --> 00:33:52.290 five to the console. 00:33:58.296 --> 00:33:59.545 That's basically what it says. 00:33:59.545 --> 00:34:01.794 It just tells you it's an interaction within the shell 00:34:01.794 --> 00:34:02.410 only. 00:34:02.410 --> 00:34:04.529 It's not interacting with anyone else. 00:34:04.529 --> 00:34:06.070 And if you don't have any "Out," that 00:34:06.070 --> 00:34:09.489 means it got printed out to the console. 00:34:09.489 --> 00:34:09.989 All right. 00:34:09.989 --> 00:34:13.320 We talked a little bit about objects. 00:34:13.320 --> 00:34:16.050 Once you have objects, you can combine objects and operators 00:34:16.050 --> 00:34:17.170 to form these expressions. 00:34:17.170 --> 00:34:19.440 And each expression is going to have a value. 00:34:19.440 --> 00:34:22.949 So an expression evaluates to a value. 00:34:22.949 --> 00:34:24.600 The syntax for an expression is going 00:34:24.600 --> 00:34:30.449 to be object, operator, object, like that. 00:34:30.449 --> 00:34:34.409 And these are some operators you can do on ints and floats. 00:34:34.409 --> 00:34:36.580 There's the typical ones, addition, subtraction, 00:34:36.580 --> 00:34:38.710 multiplication, and division. 00:34:38.710 --> 00:34:43.104 If, for the first three, the answer 00:34:43.104 --> 00:34:45.270 that you get-- the type of the answer that you get-- 00:34:45.270 --> 00:34:48.480 is going to depend on the type of your variables. 00:34:48.480 --> 00:34:52.769 If both of the variables of the operands are integers, 00:34:52.769 --> 00:34:55.060 then the result you're going to get is of type integer. 00:34:55.060 --> 00:34:56.893 But if at least one of them is a float, then 00:34:56.893 --> 00:34:58.940 the result you're going to get is a float. 00:34:58.940 --> 00:35:02.340 Division is a little bit special in that 00:35:02.340 --> 00:35:04.690 no matter what the operands are, the result 00:35:04.690 --> 00:35:05.940 is always going to be a float. 00:35:09.760 --> 00:35:14.480 The other operations you can do, and these are also useful, 00:35:14.480 --> 00:35:19.140 are the remainder, so the percent sign. 00:35:19.140 --> 00:35:22.960 If you use the percent sign between two operands, 00:35:22.960 --> 00:35:25.540 that's going to give you the remainder when you divide i 00:35:25.540 --> 00:35:28.024 by j. 00:35:28.024 --> 00:35:30.190 And raising something to the power of something else 00:35:30.190 --> 00:35:32.770 is using the star star operator. 00:35:32.770 --> 00:35:36.050 And i star stars j is going to take i to the power of j. 00:35:41.390 --> 00:35:43.940 These operations have the typical precedence 00:35:43.940 --> 00:35:47.060 that you might expect in math, for example. 00:35:47.060 --> 00:35:49.075 And if you'd like to put precedence 00:35:49.075 --> 00:35:50.450 toward some other operations, you 00:35:50.450 --> 00:35:52.270 can use parentheses to do that. 00:36:01.490 --> 00:36:03.990 All right. 00:36:03.990 --> 00:36:07.050 So we have ways of creating expressions. 00:36:07.050 --> 00:36:13.500 And we have operations we can do on objects. 00:36:13.500 --> 00:36:19.140 But what's going to be useful is to be able to save values 00:36:19.140 --> 00:36:21.390 to some name. 00:36:21.390 --> 00:36:24.780 And the name is going to be something that you pick. 00:36:24.780 --> 00:36:27.840 And it should be a descriptive name. 00:36:27.840 --> 00:36:32.310 And when you save the value to a name, 00:36:32.310 --> 00:36:36.030 you're going to be able to access that value later 00:36:36.030 --> 00:36:37.950 on in your program. 00:36:37.950 --> 00:36:40.440 And that's very useful. 00:36:40.440 --> 00:36:45.270 To save a value to a variable name, you use the equal sign. 00:36:49.180 --> 00:36:51.520 And the equal sign is an assignment. 00:36:51.520 --> 00:36:53.080 It assigns the right-hand side, which 00:36:53.080 --> 00:36:55.540 is a value, to the left-hand side, which 00:36:55.540 --> 00:36:56.898 is going to be a variable name. 00:36:59.500 --> 00:37:03.000 In this case, I assigned the float 3.14159 00:37:03.000 --> 00:37:05.590 to the variable pi. 00:37:05.590 --> 00:37:08.200 And in the second line, I'm going 00:37:08.200 --> 00:37:10.750 to take this expression, 22 divided by 7, 00:37:10.750 --> 00:37:13.090 I'm going to evaluate it. 00:37:13.090 --> 00:37:16.060 It's going to come up with some decimal number. 00:37:16.060 --> 00:37:19.960 And I'm going to save it into the variable pi_approx. 00:37:19.960 --> 00:37:21.520 values are stored in memory. 00:37:21.520 --> 00:37:22.960 And this assignment in Python, we 00:37:22.960 --> 00:37:26.950 say the assignment binds the name to the value. 00:37:26.950 --> 00:37:30.850 When you use that name later on in your program, 00:37:30.850 --> 00:37:35.650 you're going to be referring to the value in memory. 00:37:35.650 --> 00:37:37.879 And if you ever want to refer to the value 00:37:37.879 --> 00:37:39.670 later on in your code, you just simply type 00:37:39.670 --> 00:37:44.470 the name of the variable that you've assigned it to. 00:37:44.470 --> 00:37:48.250 So why do we want to give names to expressions? 00:37:48.250 --> 00:37:51.670 Well, you want to reuse the names instead of the values. 00:37:51.670 --> 00:37:54.340 And it makes your code look a lot nicer. 00:37:54.340 --> 00:37:56.830 This is a piece of code that calculates 00:37:56.830 --> 00:37:59.390 the area of a circle. 00:37:59.390 --> 00:38:03.110 And notice, I've assigned a variable pi to 3.14159. 00:38:03.110 --> 00:38:07.360 I've assigned another variable called radius to be 2.2. 00:38:07.360 --> 00:38:09.700 And then, later on in my code, I have another line 00:38:09.700 --> 00:38:12.940 that says area-- this is another variable-- is 00:38:12.940 --> 00:38:15.400 equal to-- this is an assignment-- 00:38:15.400 --> 00:38:17.290 to this expression. 00:38:17.290 --> 00:38:23.080 And this expression is referring to these variable names, pi 00:38:23.080 --> 00:38:24.490 and radius. 00:38:24.490 --> 00:38:26.410 And it's going look up their values in memory. 00:38:26.410 --> 00:38:29.080 And it's going to replace these variable names 00:38:29.080 --> 00:38:30.100 with those values. 00:38:30.100 --> 00:38:32.850 And it's going to do the calculation for me. 00:38:32.850 --> 00:38:34.450 And in the end, this whole expression 00:38:34.450 --> 00:38:36.402 is going to be replaced by one number. 00:38:36.402 --> 00:38:37.693 And it's going to be the float. 00:38:40.420 --> 00:38:45.280 Here's another exercise, while I'm talking about the slide. 00:38:45.280 --> 00:38:49.090 I do want to make a note about programming versus math. 00:38:49.090 --> 00:38:55.320 In math, you're often presented with a problem 00:38:55.320 --> 00:38:57.580 that says, solve for x. 00:38:57.580 --> 00:39:00.260 x plus y is equal to something something. 00:39:00.260 --> 00:39:03.760 Solve for x, for example. 00:39:03.760 --> 00:39:09.550 That's coming back to the fact that computers don't 00:39:09.550 --> 00:39:10.720 know what to do with that. 00:39:10.720 --> 00:39:12.659 Computers need to be told what to do. 00:39:12.659 --> 00:39:14.450 In programming, if you want to solve for x, 00:39:14.450 --> 00:39:18.530 you need to tell the computer exactly how to solve for x. 00:39:18.530 --> 00:39:20.350 You need to figure out what formula 00:39:20.350 --> 00:39:22.900 you need to give the computer in order to be 00:39:22.900 --> 00:39:25.690 able to solve for x. 00:39:25.690 --> 00:39:29.530 That means always in programming the right-hand side is 00:39:29.530 --> 00:39:33.040 going to be an expression. 00:39:33.040 --> 00:39:35.980 It's something that's going to be evaluated to a value. 00:39:35.980 --> 00:39:39.099 And the left-hand side is always a variable. 00:39:39.099 --> 00:39:40.390 It's going to be an assignment. 00:39:40.390 --> 00:39:43.900 The equal sign is not like in math 00:39:43.900 --> 00:39:45.820 where you can have a lot of things to the left 00:39:45.820 --> 00:39:47.380 and a lot of things to the right of the equal sign. 00:39:47.380 --> 00:39:49.060 There's only one thing to the left of the equal sign. 00:39:49.060 --> 00:39:50.770 And that's going to be a variable. 00:39:50.770 --> 00:39:53.710 An equal sign stands for an assignment. 00:39:53.710 --> 00:39:57.130 Once we've created expressions, and we have these assignments, 00:39:57.130 --> 00:39:59.470 you can rebind variable names using new assignment 00:39:59.470 --> 00:40:00.250 statements. 00:40:03.160 --> 00:40:05.020 Let's look at an example for that. 00:40:05.020 --> 00:40:07.840 Let's say this is our memory. 00:40:07.840 --> 00:40:13.420 Let's type back in the example with finding the radius. 00:40:13.420 --> 00:40:19.030 Let's say, pi is equal to 3.14. 00:40:19.030 --> 00:40:22.210 In memory, we're going to create this value 3.14. 00:40:22.210 --> 00:40:28.180 We're going to bind it to the variable named pi. 00:40:28.180 --> 00:40:31.180 Next line, radius is equal to 2.2. 00:40:31.180 --> 00:40:35.200 In memory, we're creating this value 2.2. 00:40:35.200 --> 00:40:40.630 And we're going to bind it to the variable named radius. 00:40:40.630 --> 00:40:44.590 Then we have this expression here. 00:40:44.590 --> 00:40:46.630 It's going to substitute the values for pi 00:40:46.630 --> 00:40:49.950 from memory and the value for radius from memory. 00:40:49.950 --> 00:40:54.730 It's going to calculate the value that this expression 00:40:54.730 --> 00:40:56.310 evaluates to. 00:40:56.310 --> 00:40:57.940 It's going to pop that into the memory. 00:40:57.940 --> 00:40:59.925 And it's going to assign-- because we're 00:40:59.925 --> 00:41:01.300 using the equal sign-- it's going 00:41:01.300 --> 00:41:05.630 to assign that value to that variable area. 00:41:08.310 --> 00:41:14.940 Now, let's say we rebind radius to be something else. 00:41:14.940 --> 00:41:21.540 Radius i is bound to the value 2.2. 00:41:21.540 --> 00:41:24.540 But when we do this line, radius is equal to radius plus 1, 00:41:24.540 --> 00:41:27.600 we're going to take away the binding to 2.2. 00:41:27.600 --> 00:41:29.970 We're going to do this calculation. 00:41:29.970 --> 00:41:31.320 The new value is 3.2. 00:41:31.320 --> 00:41:38.070 And we're going to rebind that value to that same variable. 00:41:38.070 --> 00:41:40.110 In memory, notice we're still going 00:41:40.110 --> 00:41:43.860 to have this value, 2.2, floating around. 00:41:43.860 --> 00:41:46.770 But we've lost the handle for it. 00:41:46.770 --> 00:41:48.060 There's no way to get it back. 00:41:48.060 --> 00:41:52.050 It's just in memory sitting there. 00:41:52.050 --> 00:41:55.080 At some point, it might get collected by what 00:41:55.080 --> 00:41:56.550 we call the garbage collector. 00:41:56.550 --> 00:42:00.960 In Python, And it'll retrieve these lost values, 00:42:00.960 --> 00:42:05.910 and it'll reuse them for new values, and things like that. 00:42:05.910 --> 00:42:08.520 But radius now points to the new value. 00:42:08.520 --> 00:42:13.800 We can never get back 2.2. 00:42:13.800 --> 00:42:16.340 And that's it. 00:42:16.340 --> 00:42:18.980 The value of area-- notice, this is very important. 00:42:18.980 --> 00:42:20.450 The value of area did not change. 00:42:23.150 --> 00:42:26.000 And it did not change because these are all the instructions 00:42:26.000 --> 00:42:28.460 we told the computer to do. 00:42:28.460 --> 00:42:33.290 We just told it to change radius to be radius plus 1. 00:42:33.290 --> 00:42:37.490 We never told it to recalculate the value of area. 00:42:37.490 --> 00:42:41.600 If I copied that line down here, then the value of area 00:42:41.600 --> 00:42:42.830 would change. 00:42:42.830 --> 00:42:44.430 But we never told it to do that. 00:42:44.430 --> 00:42:46.954 The computer only does what we tell it to do. 00:42:46.954 --> 00:42:47.870 That's the last thing. 00:42:47.870 --> 00:42:51.150 Next lecture, we're going to talk about adding control 00:42:51.150 --> 00:42:53.570 flow to our programs, so how do you tell the computer 00:42:53.570 --> 00:42:55.550 to do one thing or another? 00:42:55.550 --> 00:42:57.160 All right.