WEBVTT 00:00.080 --> 00:04.880 One of the most important things to learn first is how to use variables with Python. 00:04.880 --> 00:08.240 And first let's understand why you need variables. 00:08.270 --> 00:10.730 And so I'm starting with a very simple example. 00:10.760 --> 00:14.360 Here we just print the number four okay. 00:14.360 --> 00:17.660 So here we don't have quotes because we can also print a number. 00:17.660 --> 00:20.390 We're going to come back to data types just a bit later. 00:20.390 --> 00:23.960 So we print four and we do this three times okay. 00:24.170 --> 00:28.940 Here the goal is not to focus on the code and logic because yes this could be improved. 00:29.000 --> 00:31.970 But here we are going to focus on variables. 00:31.970 --> 00:34.700 So we print four and then we print four and we print four. 00:34.730 --> 00:38.210 The result is going to be four four and four. 00:38.240 --> 00:43.430 But you can see that's not really convenient because what if I want to change the value instead. 00:43.430 --> 00:46.250 I don't want to print four but I want to print seven. 00:46.280 --> 00:49.100 I will have to modify it three times. 00:49.130 --> 00:50.480 And I just have three lines. 00:50.510 --> 00:54.380 Imagine now that I have this 100 times in the code. 00:54.380 --> 00:57.170 So I run again and it's seven, seven, seven. 00:57.170 --> 01:01.460 And if we just do this well, we are going to make a lot of mistakes. 01:01.470 --> 01:08.160 and as the code becomes bigger, it will be impossible to scale it and it's going to become a real mess. 01:08.160 --> 01:10.980 So what we're going to do is to use variables. 01:10.980 --> 01:12.600 I'm just going to start. 01:12.690 --> 01:16.380 I'm going to create a variable here and then explain how to do it. 01:16.380 --> 01:22.260 So I'm going to write number equals and then seven. 01:22.260 --> 01:23.730 So that's a variable. 01:23.730 --> 01:30.930 And then instead of writing seven here I'm going to write number I'm also going to write number here. 01:31.080 --> 01:33.900 And I'm going to write number here. 01:33.900 --> 01:37.350 So now what will happen if I click on play. 01:37.380 --> 01:39.930 You can see we have 777 okay. 01:39.960 --> 01:42.510 We don't print the word number. 01:42.510 --> 01:45.840 We print the variable number which is equal to seven. 01:45.870 --> 01:52.800 Now if I want to go to let's say I want to change to 12, I run the script again and you see 12, 12, 01:52.830 --> 01:53.190 12. 01:53.190 --> 01:56.340 So I only had to modify one value. 01:56.340 --> 02:01.080 And then it was modified everywhere where I used the variable number. 02:01.110 --> 02:01.680 Okay. 02:01.680 --> 02:04.040 So now let's create a new file. 02:04.400 --> 02:08.540 Let's click on file and you can do a new file here. 02:08.570 --> 02:12.380 We're going to start from an empty program and start from the beginning. 02:12.380 --> 02:14.450 So what is a variable. 02:14.480 --> 02:20.510 Well a variable is basically a container where you store an information to use later. 02:20.540 --> 02:21.950 A variable has a name. 02:21.950 --> 02:25.580 So this is easier for you and your computer to recognize it. 02:25.790 --> 02:31.490 Basically almost all values that you will use in your programs will be stored in variables. 02:31.490 --> 02:33.740 You can give any name you want to a variable. 02:33.740 --> 02:40.670 For example, let's create a new variable here called a just a and that will store a number. 02:40.670 --> 02:43.760 So to create a variable you first need to write. 02:43.790 --> 02:47.600 So on the left you will write the name of the variable. 02:47.600 --> 02:51.170 Then you have an equal sign and then the value. 02:51.200 --> 02:53.120 So for example 45. 02:53.150 --> 02:55.370 Note that I use spaces here. 02:55.370 --> 03:00.020 I don't have to use spaces, but it's much better for readability. 03:00.050 --> 03:00.320 Okay. 03:00.410 --> 03:06.810 So what will happen when Python when the Python interpreter goes to line one, it will see this and 03:06.810 --> 03:08.610 it will first read. 03:08.640 --> 03:13.590 Okay, here's the value 45 that we put inside the variable a. 03:13.620 --> 03:13.890 Okay. 03:13.920 --> 03:15.870 So A becomes 45. 03:15.870 --> 03:21.570 And then after I have created my variable and put a value to it I can print. 03:21.570 --> 03:23.130 So I will use print. 03:23.160 --> 03:28.350 Open parentheses the name of the variable and then close parenthesis. 03:28.350 --> 03:32.340 And you can see here we don't print the letter A. 03:32.370 --> 03:35.640 We're going to print the value 45. 03:35.670 --> 03:38.010 All right so you can click on play. 03:38.040 --> 03:41.640 You can also if you want save the file. 03:42.390 --> 03:44.910 Okay let's name it variables. 03:44.970 --> 03:45.390 Okay. 03:45.390 --> 03:48.180 So for each lesson I'm not necessarily going to do that myself. 03:48.180 --> 03:51.480 But for each lesson you can just create a new file. 03:51.510 --> 03:56.340 You name it with the name of the lesson so that it will be easier for you to follow the course. 03:56.340 --> 04:04.790 So here, for example, I go to my Python programs folder and I name my file variables dot P y. 04:04.820 --> 04:07.370 Always use the p y extension. 04:07.430 --> 04:08.570 Click on okay. 04:08.570 --> 04:11.360 And now my file is viable dot p y. 04:11.390 --> 04:11.780 Great. 04:11.780 --> 04:15.740 So we have created a variable and we print the variable. 04:15.740 --> 04:16.880 So we don't print the variable. 04:16.910 --> 04:21.470 Actually we print the value that is inside the variable. 04:21.470 --> 04:27.110 Now here what we've done is just to say that a is equal to this number. 04:27.110 --> 04:30.920 We can also assign a value with a computation. 04:30.920 --> 04:34.760 With the sign plus minus we can do a multiplication. 04:34.760 --> 04:36.350 We can do a division. 04:36.350 --> 04:45.620 So for example if I say that a is equal to ten times two and once again I use spaces, there is no need. 04:45.620 --> 04:47.900 But it's much better for readability. 04:47.900 --> 04:55.040 So if you had to guess what will be printed here on line two, well, this is going to be 20 simply 04:55.040 --> 05:00.020 because we first evaluate what's here on the right of the equal sign. 05:00.020 --> 05:01.790 This is ten times two. 05:01.820 --> 05:02.870 So it's 20. 05:02.870 --> 05:08.400 And then we assign that to the variable a, and after that we print the variable A. 05:08.400 --> 05:10.620 And note that the order is very important. 05:10.620 --> 05:15.780 So let's say that I'm going to just copy I'm going to cut this and paste it here. 05:15.810 --> 05:16.380 Okay. 05:16.380 --> 05:20.430 So here we print a and then we assign a what's going to happen. 05:21.240 --> 05:26.040 You see we have an error I'm just going to reduce this okay. 05:26.280 --> 05:28.170 We don't really need the assistant. 05:28.170 --> 05:31.020 And you can see that A is not defined. 05:31.050 --> 05:32.850 Okay I have defined A here. 05:32.880 --> 05:38.850 But once again the order of the commands is very important. 05:38.850 --> 05:40.560 So we start on line one here. 05:40.560 --> 05:42.030 There is nothing I could remove it. 05:42.030 --> 05:44.460 Even so we start on line one. 05:44.460 --> 05:48.900 We print a but a has not been defined yet. 05:48.930 --> 05:51.000 Okay A is defined on line two. 05:51.030 --> 05:57.150 So at this point the Python interpreter only knows what has been done before. 05:57.150 --> 06:00.690 So that's why you have to do this in this order. 06:00.990 --> 06:01.290 Okay. 06:01.320 --> 06:02.820 I'm just going to remove that line. 06:02.940 --> 06:09.620 You first declare a valuable and you define it with a value and then you can use it. 06:09.650 --> 06:11.480 Now it's going to work. 06:11.480 --> 06:14.930 And we have an internal I don't know what's that. 06:15.170 --> 06:15.800 It's okay. 06:15.800 --> 06:18.500 You can just ignore you just press again okay. 06:18.530 --> 06:19.520 And you have 20. 06:19.550 --> 06:21.740 After you create a variable you can also modify it. 06:21.740 --> 06:24.740 So let's say do a is equal to three. 06:25.280 --> 06:27.860 And then I print a again. 06:28.130 --> 06:30.170 So what's going to happen here. 06:30.200 --> 06:31.670 First time a is 20. 06:31.670 --> 06:33.740 But then we change the value inside A. 06:33.770 --> 06:37.940 So after line three A is going to be equal to three. 06:37.970 --> 06:40.220 Once again order is very important. 06:40.220 --> 06:43.220 So you can create a variable and you can modify it. 06:43.250 --> 06:45.140 We can also create another variable. 06:45.140 --> 06:46.910 Let's name this one b. 06:47.330 --> 06:53.030 And here instead of just setting a value I'm going to say that b is equal to a. 06:53.060 --> 06:54.410 What does that mean. 06:54.410 --> 07:01.700 It means that on line six, A is going to be evaluated to its current value, which is here three, 07:01.700 --> 07:04.310 and then B is going to be assigned this value. 07:04.310 --> 07:06.810 So B is going to be equal to three. 07:06.840 --> 07:08.640 Let's verify that. 07:08.670 --> 07:13.740 Let's print b and you can see b is equal to three. 07:13.770 --> 07:17.220 I can also use computations for example plus one. 07:17.430 --> 07:22.470 And then b here is equal to four because it's going to be three plus one okay. 07:22.500 --> 07:33.150 And the same thing as I said before, if you try to set B for example before okay you set A you're going 07:33.180 --> 07:34.560 to have an error okay. 07:34.590 --> 07:37.440 Because A is not defined here on line one. 07:37.470 --> 07:41.100 You cannot use A if you have not defined it before. 07:41.940 --> 07:43.620 So I'll put it back here. 07:43.620 --> 07:45.750 And now it's going to work correctly. 07:45.780 --> 07:46.230 All right. 07:46.230 --> 07:49.470 And finally well here we use A and B for names. 07:49.470 --> 07:52.650 This is fine if you just make a quick example. 07:52.710 --> 07:54.930 Or if you do computation with two numbers for example. 07:54.930 --> 07:59.010 But in your programs you are going to use names that are more meaningful. 07:59.040 --> 07:59.520 Okay. 07:59.520 --> 08:07.880 So if you want to store a temperature you're going to write temperature for example is equal to. 08:07.910 --> 08:09.170 I don't know, 25. 08:10.040 --> 08:13.310 If you want to store a counter, you're going to write counter. 08:13.430 --> 08:14.000 Okay. 08:14.180 --> 08:19.160 And maybe start at zero if you want to store a user age. 08:19.190 --> 08:21.590 You're going to write user age like this. 08:22.220 --> 08:22.490 Okay. 08:22.520 --> 08:23.810 For example 30. 08:23.810 --> 08:29.270 And you can note that here we have we have basically two worlds into one. 08:29.270 --> 08:34.670 So I could just write like this user age, but it's more readable if I add an underscore. 08:34.670 --> 08:36.560 One thing for sure is you should not. 08:36.560 --> 08:43.100 So this is not valid to add a space in a variable name, and you would probably get an error anyway. 08:43.100 --> 08:45.320 You see we have a syntax error here. 08:46.580 --> 08:52.880 Okay, so if you want to separate different worlds inside a variable then simply use an underscore. 08:52.880 --> 08:56.600 And with this you have three variables with a meaningful name. 08:56.600 --> 09:03.710 So to recap, a variable is a container that you can use to store a value and reuse that value later 09:03.710 --> 09:04.880 in your program. 09:04.880 --> 09:09.290 To create a variable, you must give it a name and assign a value to it.