WEBVTT 0 00:00.140 --> 00:04.430 Hey guys, today I want to talk about something that is really, really important, 1 00:04.430 --> 00:07.310 and you're going to use many, many times in the future, 2 00:07.310 --> 00:10.790 and this is the concept of the Python List. 3 00:10.790 --> 00:15.620 And the list is what you would call a Data Structure. 4 00:15.620 --> 00:16.730 What does that mean? 5 00:16.730 --> 00:22.190 Well, it's just a way of organizing and storing data in Python. 6 00:22.190 --> 00:27.560 Now, we've already seen ways of storing single pieces of data, 7 00:27.560 --> 00:35.120 and that was done through the simple variable where we just said a = 3, or 8 00:35.120 --> 00:35.690 b = hello, 9 00:35.690 --> 00:38.720 but that's just storing one piece of data, right? 10 00:38.720 --> 00:41.090 Be it a number or a string. 11 00:41.510 --> 00:47.510 But sometimes you might want to store grouped pieces of data, data that has some sort of connection 12 00:47.510 --> 00:48.530 with each other. 13 00:48.530 --> 00:55.680 For example, if you wanted to store all of the names of the states in the US, then it doesn't really 14 00:55.680 --> 01:00.390 make sense to store them all individually because they kind of belong together, right? 15 01:00.390 --> 01:02.250 They have a relationship to each other. 16 01:02.250 --> 01:07.560 So it would be nice if you had a variable that was called states in the US, and then you would be able 17 01:07.560 --> 01:11.820 to store all of the names of the states together in one variable. 18 01:12.060 --> 01:17.460 Now, in other cases, you might also want to have order in your data. 19 01:17.460 --> 01:24.480 So for example, if you were storing all of the people in a virtual queue, then you want to be able 20 01:24.480 --> 01:27.480 to keep hold of the order in which they join the queue. 21 01:27.510 --> 01:32.730 You don't want to let the last person somehow skip the queue because you don't have a good data structure, 22 01:32.730 --> 01:33.390 right? 23 01:33.420 --> 01:36.150 This is why we need to learn about lists. 24 01:36.150 --> 01:38.940 And lists look pretty simple, 25 01:38.940 --> 01:46.080 it's just a set of square brackets with many items stored inside, and those items can be any data type. 26 01:46.080 --> 01:52.140 They can even have mixed data types like you could store strings together with numbers or a set of booleans. 27 01:52.140 --> 01:59.610 It doesn't really matter, but what does matter is the syntax. In Python, lists always start with an 28 01:59.610 --> 02:05.100 open square bracket like this - [, and a closing square bracket like this - ]. 29 02:05.100 --> 02:09.090 And then in between you have your items separated by a comma. 30 02:09.090 --> 02:10.590 So pretty simple. 31 02:10.890 --> 02:15.840 In order to store it inside a variable, then it's the same way as we've done before. 32 02:15.870 --> 02:19.500 The only difference is the right hand side of the equal sign. 33 02:19.500 --> 02:22.530 This is the list data structure. 34 02:22.890 --> 02:27.720 If we stored a bunch of fruits, for example, then it might look something like this, 35 02:27.720 --> 02:32.560 ["Cherry", "Apple", "Pear"], separated by a comma inside a set of square brackets. 36 02:32.830 --> 02:35.260 Let's take a look at this using real code. 37 02:35.290 --> 02:42.130 Now let's say that I wanted to store all of the names of the states of the US. Previously, without knowing 38 02:42.130 --> 02:50.980 about this list data structure, we might have written state1 = "Delaware", state2 == "Pennsylvania", 39 02:50.980 --> 02:54.190 and so on and so forth. 40 02:54.190 --> 02:57.610 And we would create as many variables as we have states. 41 02:57.730 --> 03:05.590 But now that we know about lists, then we can just create a single variable and we call it states_of_america 42 03:05.590 --> 03:06.490 . 43 03:06.910 --> 03:11.380 And now we can create a list by creating a set of square brackets. 44 03:11.380 --> 03:15.190 And inside those square brackets we add our items. 45 03:15.190 --> 03:21.100 So again the first item is the state of Delaware which is going to be a string. 46 03:21.580 --> 03:28.450 And then we've got Pennsylvania, et cetera, et cetera, and we can continue this list just by adding commas, 47 03:28.450 --> 03:33.160 adding a piece of data, adding another comma, adding a piece of data. 48 03:33.160 --> 03:38.110 And this way we end up with a list data structure. 49 03:38.800 --> 03:46.690 Now one of the interesting things about the United States is that the different states in the US actually 50 03:46.690 --> 03:49.240 joined the Union at different times. 51 03:49.240 --> 03:55.900 You can actually head over to Wikipedia and watch this little animation and see each state join the 52 03:55.900 --> 03:59.590 union, and at which time point they did so. 53 03:59.650 --> 04:08.320 The order of this data is now kind of pretty important, because if we wanted a list of US states that 54 04:08.320 --> 04:16.610 are in the order that they joined the Union, then the order in which they're stored in our data structure 55 04:16.610 --> 04:19.760 is now also immensely important. 56 04:19.760 --> 04:23.390 And this is another thing that you get with lists. 57 04:23.390 --> 04:31.280 You can use lists to store many pieces of related data, but they also have an order. 58 04:31.280 --> 04:35.390 And the order is determined by the order in the list. 59 04:35.390 --> 04:37.670 So this is the first piece of data, 60 04:37.700 --> 04:39.650 this is the second piece of data, 61 04:39.650 --> 04:45.560 and when you store it inside the variable, that order is not lost and you'll be able to use it later 62 04:45.560 --> 04:47.360 on when you need the list. 63 04:47.960 --> 04:55.340 Here's a list of states of America ordered by the date that they joined the union. 64 04:55.340 --> 05:02.910 And you can see that if later on I decided that I wanted to know which was the state that joined first, 65 05:02.910 --> 05:09.810 then I can print this variable states_of_america, I can add a set of square brackets, and then I type 66 05:09.810 --> 05:16.530 0 as the index of the piece of data I want to pull out from my states_of_america list. 67 05:16.530 --> 05:21.240 So now if I go ahead and run this code, you can see it prints out Delaware. 68 05:21.240 --> 05:27.950 And if I keep increasing this number, you can see that it's going through my list in the order that 69 05:27.950 --> 05:28.940 it was saved. 70 05:29.090 --> 05:36.890 So you might be wondering, that's kind of weird that you typed zero and you got Delaware, right? 71 05:36.890 --> 05:41.030 Surely Delaware should be the first item in the list. 72 05:41.030 --> 05:46.400 Well, this is a kind of peculiarity with computers and programming languages. 73 05:46.400 --> 05:51.120 You'll tend to find that programmers start counting from 0. 74 05:51.120 --> 05:54.990 So Delaware is at 0, Pennsylvania is at 1, and New Jersey is at 2. 75 05:55.020 --> 05:58.080 And this idea might seem a little bit weird at first, 76 05:58.080 --> 06:01.980 why is the first item at position 0? 77 06:02.130 --> 06:10.200 But if you think about that index number, that 0, 1, or 2, instead of being the position, actually 78 06:10.200 --> 06:15.990 being an offset or a shift from the start of the list, 79 06:16.080 --> 06:23.760 well, then, in this case, Cherry is right at the beginning of the list, so it has an offset or a shift 80 06:23.760 --> 06:24.750 of 0, 81 06:24.750 --> 06:31.710 but Apple is shifted from the beginning by 1, Pear is shifted from the beginning by 2, and so 82 06:31.710 --> 06:32.730 on and so forth. 83 06:32.730 --> 06:38.010 Then it kind of makes more sense that the first item in the list is at the beginning of the list, so 84 06:38.010 --> 06:38.880 it has no offset. 85 06:38.880 --> 06:40.140 So it's 0. 86 06:40.980 --> 06:46.530 And you'll find that in many, many programming languages there are similar data structures to lists 87 06:46.530 --> 06:51.900 and this is how they're usually ordered starting from 0 and then adding by 1. 88 06:52.170 --> 06:59.460 Now you can see that when you want to get hold of a particular piece of data stored inside a list, 89 06:59.460 --> 07:06.720 what you do is you get the name of the list and then you add another set of square brackets. 90 07:06.720 --> 07:11.850 So whenever you see square brackets, you should be thinking to yourself, oh, this might be related 91 07:11.850 --> 07:16.800 to a list, because when you create the list, you use square brackets, 92 07:16.800 --> 07:21.570 and when you try to get items out of the list, you also use square brackets. 93 07:21.570 --> 07:27.790 And then inside the square brackets is where you put the index or the offset of the item that you want. 94 07:27.820 --> 07:33.010 So if we wanted New Jersey, it's offset from the beginning by 1, 2. 95 07:33.040 --> 07:37.660 So now this part of the code is equal to New Jersey. 96 07:37.690 --> 07:46.900 Now in addition to using the positive index so say 0123 you can also use a negative index. 97 07:46.900 --> 07:55.600 So if I wrote -1 or -2, then it actually starts counting from the end of the list. 98 07:55.600 --> 08:03.040 So if I wrote states_of_america minus one as the index, then I get Hawaii. 99 08:03.070 --> 08:08.500 Minus one is the last item in the list, because you can't really have -0. 100 08:08.500 --> 08:10.930 That's not actually a real thing in maths. 101 08:11.650 --> 08:18.970 Now as I continue and I go to [-2], then that's Alaska, [-3] will be Arizona and so on 102 08:18.970 --> 08:19.660 and so forth. 103 08:19.660 --> 08:22.930 So you can have positive indices and negative indices. 104 08:22.930 --> 08:29.800 But so far we've only been pulling things out of our list by using our square brackets and the index, 105 08:29.800 --> 08:35.110 but you can also change the items in the list using very similar code. 106 08:35.110 --> 08:42.610 For example, if I decided that Pennsylvania is actually not spelled Pennsylvania and I wanted to change 107 08:42.610 --> 08:53.440 it to Pencilvania, then I can simply write my code like this I get hold of my list, and then using 108 08:53.440 --> 09:00.730 the square brackets, I get hold of the item at index 1, which is this one, and then I set it equal 109 09:00.730 --> 09:02.710 to a new piece of data. 110 09:02.710 --> 09:11.180 So now if I go ahead and print my states_of_america list, you'll see that the list looks a little bit 111 09:11.180 --> 09:11.510 different 112 09:11.540 --> 09:11.960 now. 113 09:11.960 --> 09:16.310 Instead of Pennsylvania, it's now Pencilvania. 114 09:16.760 --> 09:23.180 So you can alter any item inside the list pretty easily using this kind of syntax. 115 09:23.180 --> 09:26.300 You can also add to the list if you wanted to. 116 09:26.330 --> 09:32.720 So for example, if you wanted to add an item at the end of the list, which is what happens most commonly, 117 09:32.720 --> 09:32.990 right? 118 09:32.990 --> 09:39.770 If you had a list of people who are queuing in your shop, then every subsequent person usually gets 119 09:39.770 --> 09:40.730 added to the end. 120 09:40.730 --> 09:47.390 If you have a new state that joined America, then it's probably going to be added after Hawaii. 121 09:47.720 --> 09:49.040 How do we do that? 122 09:49.040 --> 09:55.860 Well, we can write the name of the list and then we use a function called append(). 123 09:55.860 --> 10:01.170 And append() will add a single item to the end of the list. 124 10:01.170 --> 10:07.980 So let's say that Angelaland is joining the United States of America. 125 10:08.190 --> 10:16.320 So now once I've appended Angelaland to the end of my states_of_america list, and I print the 126 10:16.320 --> 10:19.730 states_of_america, you can see there it is added right at the end. 127 10:20.510 --> 10:25.310 Now, there's actually a whole load of other functions that you can use in addition to append(), 128 10:25.310 --> 10:28.730 and you'll find this on the documentation for Python. 129 10:28.790 --> 10:35.000 In addition to the append() function that we saw just now, where we add an item to the end of the list, 130 10:35.000 --> 10:39.080 there's a whole load of other functions that you can use with lists. 131 10:39.080 --> 10:45.040 For example, you can use the extend() which adds a whole bunch of items to the end of the list. 132 10:45.610 --> 10:50.560 And in this case, what you're actually adding is going to be a list. 133 10:50.560 --> 10:56.560 So I would be creating the list using square brackets and then adding my items in here. 134 10:57.370 --> 11:04.810 And now what's happening is I'm extending this states_of_America list with this additional list. 135 11:04.810 --> 11:12.090 So now if we print the states_of_america, you can see that these two items that used to be inside a 136 11:12.090 --> 11:19.620 list have now been added to the previous list, and it's now extended it by two more items. 137 11:20.250 --> 11:24.720 But the important thing is you don't have to memorize these functions, 138 11:24.720 --> 11:30.810 that's the whole point of documentation and why we have Google, because there's too much information 139 11:30.810 --> 11:36.420 in the world for you to memorize, and it's a very inefficient way of learning. 140 11:36.420 --> 11:41.040 And if you tried to memorize every single method, it's not impossible, 141 11:41.040 --> 11:46.260 but it means that you don't have space in your brain for the important stuff, which is how things work. 142 11:46.260 --> 11:49.650 How do you actually use it to do what you want it to do? 143 11:49.650 --> 11:57.360 So what I recommend when you come across a new thing such as the list data structure, is to just 144 11:57.360 --> 12:03.210 have a look through the documentation, read through it and see what are the possible things you can 145 12:03.210 --> 12:03.720 do. 146 12:03.720 --> 12:09.270 And once you've got the idea of this is possible, then the next time when you need to use it inside 147 12:09.270 --> 12:15.390 your code, you'll know, "Uh, I remember this is possible," and all you have to do is just be able to 148 12:15.390 --> 12:20.860 use Google to find the exact bit of the documentation and then implement it. 149 12:21.730 --> 12:24.850 Programming is kind of like an open book exam, 150 12:24.850 --> 12:27.580 you shouldn't need to memorize anything. 151 12:27.580 --> 12:32.050 You should spend your time trying things out and try to get things to work instead. 152 12:32.200 --> 12:39.700 Now that I've introduced you to this new data structure, the mighty List, it's time for a code exercise 153 12:39.700 --> 12:41.860 to see if you can use it in practice. 154 12:41.860 --> 12:45.640 So head over to the next lesson and give the challenge a go.