WEBVTT 00:00.080 --> 00:05.840 This is almost the end of the Python programming part of this course, so congratulations if you've 00:05.840 --> 00:07.280 already made it that far. 00:07.280 --> 00:13.100 And after this lesson and the next activity, you will be ready to create programs for Gpio, the camera, 00:13.130 --> 00:14.840 web servers, etc.. 00:14.840 --> 00:19.130 So in this lesson I'm going to introduce you to Python lists. 00:19.340 --> 00:21.650 First of all, why do we need lists? 00:21.650 --> 00:25.250 Well, for now you have worked with one value at a time. 00:25.250 --> 00:27.860 What if you need to process ten different values? 00:27.860 --> 00:31.880 Let's say to make a computation on ten different temperatures that you have measured? 00:31.880 --> 00:34.910 And what if it's not ten but 10,000 values? 00:34.910 --> 00:36.620 This can look like a nightmare. 00:36.620 --> 00:42.770 But don't worry, you won't have to create 10,000 variables to manipulate 10,000 values. 00:42.770 --> 00:48.830 You can put all of those values inside only one container, which is called a list. 00:48.860 --> 00:50.930 You may also have heard of arrays. 00:50.930 --> 00:55.220 Well, a list is simply the Python way of saying an array. 00:55.220 --> 00:56.300 So what is a list? 00:56.330 --> 00:58.880 A list is a collection of variables. 00:58.910 --> 01:03.720 Okay, it is very useful when you need to store multiple variables which are related. 01:03.810 --> 01:06.120 Let's create a simple list. 01:06.120 --> 01:10.770 So to create a list you will first to well define a name for that list. 01:10.770 --> 01:18.270 So let's say we want a list that contains numbers I'm going to write for example number underscore list 01:18.300 --> 01:18.720 okay. 01:18.750 --> 01:19.920 As simple as that. 01:19.950 --> 01:24.090 You can put what you store and then underscore list. 01:24.120 --> 01:27.540 Or you could also use array if you want to. 01:27.570 --> 01:29.730 But in Python we use list. 01:29.760 --> 01:32.700 You could also put just the word in plural okay. 01:32.730 --> 01:37.890 So you could have a variable named number and then a list named numbers. 01:38.100 --> 01:39.930 Okay here I'm going to follow this convention. 01:39.930 --> 01:41.520 So I call it number list. 01:41.520 --> 01:42.930 And then you put the equal sign. 01:42.960 --> 01:48.270 And then to create a list you just open and close some brackets like that. 01:48.300 --> 01:51.510 You can create a list with zero elements okay. 01:51.540 --> 01:53.730 Just like this to initialize an empty list. 01:53.730 --> 01:56.610 But here we're going to start with some values. 01:56.610 --> 02:00.360 So for example I'm going to add some integers let's say three. 02:00.360 --> 02:02.850 And then I put a comma to separate different values. 02:02.850 --> 02:06.100 So five I can put negative number. 02:06.130 --> 02:06.310 Okay. 02:06.310 --> 02:07.930 I can put any integers I want. 02:07.960 --> 02:10.990 -856 etc.. 02:10.990 --> 02:13.930 So here I only add four values in this list. 02:13.930 --> 02:18.490 And one important thing to note is that all those values should be related okay. 02:18.520 --> 02:22.600 And because they are related they should also be of the same data type. 02:22.630 --> 02:22.810 Okay. 02:22.810 --> 02:24.400 It's important that you follow this. 02:24.400 --> 02:28.870 Don't mix strings with float numbers with integers with booleans okay. 02:28.900 --> 02:34.390 Try to be consistent and use just one data type in the list for all the elements. 02:34.420 --> 02:37.780 Otherwise, you will surely get a lot of errors in the future. 02:37.780 --> 02:39.520 So we have our list. 02:39.700 --> 02:41.440 Just create a name. 02:41.440 --> 02:44.830 So just like a variable, we create a name and equal. 02:44.830 --> 02:48.040 And then we have those brackets with all the elements. 02:48.070 --> 02:52.540 Now if I want to print this list I can just do print number list okay. 02:52.570 --> 02:55.750 Just like I can print a variable I can print a list. 02:55.810 --> 02:57.820 Let's play this. 02:57.910 --> 02:59.920 And you can see I have my list here. 03:00.160 --> 03:00.370 Okay. 03:00.400 --> 03:02.260 So I can just print the entire list. 03:02.290 --> 03:02.530 Okay. 03:02.560 --> 03:02.980 That's great. 03:02.980 --> 03:05.920 But now what if I want to get access. 03:05.920 --> 03:11.070 So by getting access, I mean read one element of the array. 03:11.100 --> 03:17.040 Well, to do that, after you have created the list, of course you can just write the name of the list 03:17.040 --> 03:23.040 followed by some brackets like this and the index of the element. 03:23.070 --> 03:23.280 Okay. 03:23.310 --> 03:26.730 So each element in the list will have an index. 03:26.760 --> 03:29.460 And let's just start with one to see what we have. 03:29.460 --> 03:32.370 So we want to print the index one of the list. 03:32.400 --> 03:34.170 What value will we get. 03:34.200 --> 03:35.190 So you can guess. 03:35.190 --> 03:37.110 And now I'm going to click on play. 03:37.110 --> 03:38.460 And we get the value five. 03:38.460 --> 03:42.210 And the value five is actually the second element of the list. 03:42.210 --> 03:43.410 And why is that. 03:43.440 --> 03:48.450 When we ask for the index one because well in programming and you may be already know that. 03:48.450 --> 03:54.060 And that's what we did also with for loops in programming we start to count at zero okay. 03:54.090 --> 03:54.960 Not one. 03:54.960 --> 04:00.270 So if I ask you to count for like four values you will not do one, two, three, four. 04:00.300 --> 04:02.670 You will do zero, one, two and three. 04:02.670 --> 04:08.620 So if I put zero here zero corresponds to the first element of the list. 04:08.650 --> 04:10.030 Okay that's three. 04:10.030 --> 04:12.070 And then one was this one. 04:12.070 --> 04:13.960 Then I can do two. 04:14.650 --> 04:14.920 Okay. 04:14.950 --> 04:16.570 It's going to be minus eight. 04:16.600 --> 04:18.070 Then I can do three. 04:18.100 --> 04:20.200 It's going to be 156. 04:20.230 --> 04:22.120 And let's try with four. 04:22.150 --> 04:23.830 What's going to happen with four. 04:23.950 --> 04:25.600 You see we get an error. 04:25.600 --> 04:27.370 And this error is a list. 04:27.370 --> 04:28.510 So index error. 04:28.540 --> 04:31.810 List index is out of range okay. 04:31.840 --> 04:37.150 So if you try to access a value here that doesn't exist you will get an error. 04:37.150 --> 04:42.790 So make sure that you always use indexes that are actually corresponding to existing elements in the 04:42.790 --> 04:43.240 list. 04:43.240 --> 04:50.500 So here for example I go back to zero and I print I print the first element. 04:50.500 --> 04:54.250 So that's how you can read an element how you get an element. 04:54.280 --> 04:56.470 Now if you want to set an element. 04:56.470 --> 05:02.290 So if you want to write into the list and change one value, you can do this. 05:02.290 --> 05:05.530 So you just put the list name. 05:05.530 --> 05:06.400 So number list. 05:06.400 --> 05:11.660 And you use the same syntax here with brackets to select an element. 05:11.660 --> 05:13.880 So for example still we want to modify the first one. 05:13.880 --> 05:15.260 So I'm going to put zero. 05:15.260 --> 05:16.910 And then you put an equal sign. 05:16.910 --> 05:20.270 And let's say now I want to write ten okay. 05:20.300 --> 05:24.350 So I want to change the first element instead of three is going to become ten. 05:24.350 --> 05:31.250 And just to make sure let's just print this element again after we modify it okay. 05:31.280 --> 05:32.540 So let's run. 05:32.570 --> 05:39.230 You can see okay I have this error but it's not a problem I have three and then I have ten okay. 05:39.230 --> 05:40.910 So three was the first value. 05:40.940 --> 05:44.060 Then I modify it and then I can print it again. 05:44.060 --> 05:45.530 And it's a different value okay. 05:45.560 --> 05:46.670 And of course same thing. 05:46.670 --> 05:52.340 When you modify an element make sure that you use an index that is inside the list. 05:52.370 --> 05:52.760 Great. 05:52.760 --> 05:53.930 So you can get an element. 05:53.930 --> 05:55.220 You can set an element. 05:55.220 --> 05:58.310 But also in Python lists are dynamic. 05:58.310 --> 06:01.100 So you can also add elements to a list. 06:01.100 --> 06:02.630 You can add a new element. 06:02.630 --> 06:05.480 So here we have the index 0123. 06:05.510 --> 06:07.010 As you can see index four. 06:07.040 --> 06:08.810 We had an error because it doesn't exist. 06:08.810 --> 06:10.310 But you can add a new element. 06:10.310 --> 06:14.430 And then you will have something on the index for how to do this. 06:14.430 --> 06:17.490 Well, you can use the append function. 06:17.490 --> 06:20.370 So you will put the name of the list number list. 06:20.400 --> 06:24.120 And then you will do dot append like this. 06:24.120 --> 06:25.740 You open and close parentheses. 06:25.740 --> 06:27.600 So here make sure to use parentheses okay. 06:27.630 --> 06:28.710 This is a function. 06:28.710 --> 06:32.760 And let's put a new value for example 12 okay. 06:32.790 --> 06:37.230 So what I'm doing here is I'm adding a new element at the end of the list. 06:37.260 --> 06:40.830 Now let's just print the list number list okay. 06:40.860 --> 06:41.970 Let's print that. 06:42.180 --> 06:44.370 You can see that when we print the list here. 06:44.370 --> 06:45.990 So the first element is still ten. 06:46.020 --> 06:48.510 Then we have five minus eight 156. 06:48.510 --> 06:50.760 And we have 12 at the end okay. 06:50.790 --> 06:56.190 So we can add new elements to a list with this function right here okay. 06:56.220 --> 06:59.880 So with this you will be able to do lots of things with lists. 06:59.910 --> 07:05.670 And to finish here I want to show you a great combination you can do with lists and for loops. 07:05.700 --> 07:05.880 Okay. 07:05.910 --> 07:08.040 So now that you have seen for loops we can do that. 07:08.040 --> 07:14.590 So let's say that you want to get access to each element of Off the list. 07:14.590 --> 07:17.560 So I thought to read it or to modify it or to do something. 07:17.590 --> 07:17.830 Else. 07:17.830 --> 07:21.130 But you want to get access to each element one by one. 07:21.160 --> 07:22.030 How to do this? 07:22.060 --> 07:27.820 Well, you're not just going to do number list zero and then number one, number two etc. okay. 07:27.850 --> 07:30.040 You're going to use a for loop. 07:30.220 --> 07:34.570 So using for loops with lists is a very powerful combo in programming. 07:34.570 --> 07:35.920 So let's see how to do that. 07:35.950 --> 07:37.960 We have our for keyword. 07:37.960 --> 07:43.420 And then because we are going through a number list instead of just writing I am going to write number. 07:43.420 --> 07:45.850 And then I have the in keyword. 07:45.880 --> 07:46.060 Okay. 07:46.090 --> 07:50.860 So for variable in and then I don't have a range here I'm not going to provide a range I'm going to 07:50.860 --> 07:53.590 provide the list name. 07:54.010 --> 07:54.250 Okay. 07:54.250 --> 07:59.500 So for number in number list I add a colon I go back to a new line. 07:59.500 --> 08:04.420 And then I know that with this well, the block of code that is indented here is going to be repeated 08:04.420 --> 08:06.820 for each number in the number list. 08:06.850 --> 08:07.030 Okay. 08:07.030 --> 08:10.480 So for each item in the number list for now we have. 08:10.480 --> 08:14.470 So you can see we have 12345 elements. 08:14.470 --> 08:20.570 So this list is going to run five times, and each time the number variable is going to contain the 08:20.570 --> 08:21.230 element. 08:21.260 --> 08:21.530 Okay. 08:21.560 --> 08:23.720 So the first one, the second one, the third one, etc.. 08:23.720 --> 08:26.900 So let's just print the number actually to see what we have. 08:28.940 --> 08:29.930 Like this. 08:30.770 --> 08:38.180 I run this and you can see that we print each element one by one. 08:38.210 --> 08:41.630 Okay so what happened is that first this variable. 08:41.630 --> 08:46.820 So this is the local variable inside the for loop is going to be created with the first element of number 08:46.820 --> 08:47.060 list. 08:47.060 --> 08:50.060 So the index zero which is not three anymore. 08:50.060 --> 08:52.460 It's ten because we have modified it here. 08:52.490 --> 08:58.400 Then it goes to the second element the third etc. until it has gone through all elements of the list. 08:58.400 --> 09:00.770 And now let's do something a bit more interesting. 09:00.770 --> 09:06.650 Let's say we want to create a new list that contains the same element as that list, but multiplied 09:06.650 --> 09:07.610 by two. 09:07.640 --> 09:13.940 So for this I will create here new list and I'm going to initialize it to. 09:14.060 --> 09:15.950 Actually I'm just going to write this. 09:15.950 --> 09:18.940 So open and close brackets with nothing inside. 09:18.970 --> 09:20.770 All right, so I create an empty list. 09:20.770 --> 09:23.230 This is a quite common practice to do in Python. 09:23.230 --> 09:27.730 You create an empty list and then you will fill some values in it with a for loop. 09:27.730 --> 09:33.790 So here instead of print I will just do new list dot append. 09:33.790 --> 09:35.410 And what do I add? 09:35.440 --> 09:39.820 I will just add the number times two. 09:39.850 --> 09:44.590 Okay, so I go through the number list here with a for loop. 09:44.590 --> 09:51.070 And for each item of the number list, I add a new element in the new list that corresponds to the number 09:51.070 --> 09:52.450 multiplied by two. 09:52.480 --> 09:55.330 And now if I want to print the new list. 09:55.330 --> 09:59.320 So make sure that you go back you see to the previous indentation here. 09:59.350 --> 10:02.230 Print a new list. 10:02.500 --> 10:11.260 That list should contain five values and you can see those five values are the double of the five values 10:11.260 --> 10:13.060 that we have in the number list. 10:13.060 --> 10:16.600 And we did that with just two lines of code here with this for loop. 10:16.630 --> 10:20.030 And just one thing to note here is related to the scope. 10:20.030 --> 10:26.960 You can see that we create the new list list here in that scope, and then we use it in the for loop. 10:26.960 --> 10:30.380 And we can do that because this is a more nested scope. 10:30.380 --> 10:32.300 You can see we add one indentation. 10:32.300 --> 10:33.770 This is a more nested scope. 10:33.770 --> 10:37.310 So we can use that variable that we've created here. 10:37.310 --> 10:40.040 So let's make a very quick recap. 10:40.070 --> 10:46.160 A list in Python will allow you to store a collection of values inside only one container. 10:46.160 --> 10:51.770 You can put as many values as you want in a list, and to create a list you first need to give it a 10:51.770 --> 10:52.850 meaningful name. 10:52.850 --> 11:00.080 Then the equal sign and you initialize values inside brackets, each value being separated by a comma. 11:00.110 --> 11:06.890 You can also use the append function to add a new element to an existing list, and when you access 11:06.890 --> 11:12.980 an element of a list, make sure you use the correct index and don't go out of the range. 11:12.980 --> 11:19.220 Finally, going through lists with a for loop is something that's very common and that you will use 11:19.220 --> 11:21.410 very often in your programs.