1 00:00:00,160 --> 00:00:04,480 Hey guys. Today I want to talk about something that is really, really important 2 00:00:04,510 --> 00:00:07,150 and you're going to use many, many times in the future, 3 00:00:07,570 --> 00:00:10,570 and this is the concept of the Python list. 4 00:00:11,140 --> 00:00:15,520 And the list is what you would call a data structure. 5 00:00:15,880 --> 00:00:17,170 What does that mean? Well, 6 00:00:17,200 --> 00:00:21,910 it's just a way of organizing and storing data in Python. 7 00:00:22,540 --> 00:00:27,370 Now we've already seen ways of storing single pieces of data, 8 00:00:27,760 --> 00:00:32,759 and that was done through the simple variable where we just said a = 9 00:00:33,250 --> 00:00:35,680 3 or b = hello. 10 00:00:35,920 --> 00:00:40,900 But that's just storing one piece of data, right? Be it a number or a string. 11 00:00:41,650 --> 00:00:45,370 But sometimes you might want to store grouped pieces of data, 12 00:00:45,460 --> 00:00:49,690 data that has some sort of connection with each other. For example, 13 00:00:49,750 --> 00:00:54,750 if you wanted to store all of the names of the States in the US then it doesn't 14 00:00:55,240 --> 00:00:59,710 really make sense to store them all individually because they kind of belong 15 00:00:59,710 --> 00:01:02,140 together, right? They have a relationship to each other. 16 00:01:02,470 --> 00:01:06,970 So it would be nice if you had a variable that was called States_in_the_US and 17 00:01:06,970 --> 00:01:10,990 then you would be able to store all of the names of the States together in one 18 00:01:10,990 --> 00:01:13,900 variable. Now, in other cases, 19 00:01:13,960 --> 00:01:18,790 you might also want to have order in your data. So for example, 20 00:01:19,000 --> 00:01:22,930 if you were storing all of the people in a virtual queue, 21 00:01:23,230 --> 00:01:27,370 then you want to be able to keep hold of the order in which they join the queue. 22 00:01:27,760 --> 00:01:31,330 You don't wanna let the last person somehow skip the queue because you don't 23 00:01:31,330 --> 00:01:33,100 have a good data structure, right? 24 00:01:33,760 --> 00:01:38,760 This is why we need to learn about lists. And lists look pretty simple. 25 00:01:38,950 --> 00:01:43,480 It's just a set of square brackets with many items stored inside. 26 00:01:43,570 --> 00:01:47,820 And those items can be any data type. They can even have mixed data types like 27 00:01:47,820 --> 00:01:52,000 you could store strings together with numbers or a set of booleans. 28 00:01:52,330 --> 00:01:53,500 It doesn't really matter. 29 00:01:53,620 --> 00:01:58,620 But what does matter is the syntax. In Python lists always start with a open 30 00:01:59,980 --> 00:02:04,900 square bracket like this and a closing square bracket like this. 31 00:02:05,410 --> 00:02:09,009 And then in between you have your items separated by a comma. 32 00:02:09,280 --> 00:02:13,570 So pretty simple. In order to store it inside the variable, 33 00:02:13,750 --> 00:02:15,730 then its the same way as we've done before. 34 00:02:16,150 --> 00:02:19,360 The only difference is the right hand side of the equal sign. 35 00:02:19,780 --> 00:02:22,300 This is the list data structure. 36 00:02:23,170 --> 00:02:25,690 If we stored a bunch of fruits, for example, 37 00:02:25,750 --> 00:02:28,240 than it might look something like this: Cherry, 38 00:02:28,240 --> 00:02:32,410 Apple, Pear, separated by a comma inside a set of square brackets. 39 00:02:32,920 --> 00:02:35,170 Let's take a look at this using real code. 40 00:02:35,590 --> 00:02:40,300 Now I'm going to go ahead and comment out the code from our previous lesson on 41 00:02:40,330 --> 00:02:41,163 randomness 42 00:02:41,500 --> 00:02:45,790 and you can do the same if you wanna keep a note of the previous code and use it 43 00:02:45,790 --> 00:02:49,870 as sort of a live textbook where you can comment out of the code or comment it 44 00:02:49,870 --> 00:02:52,990 back in in order to see how it works or you can delete it. 45 00:02:53,140 --> 00:02:54,280 It's totally up to you. 46 00:02:55,030 --> 00:02:59,710 Now let's say that I wanted to store all of the names of the States of US. 47 00:03:00,160 --> 00:03:03,520 Previously, without knowing about this list data structure, 48 00:03:03,820 --> 00:03:07,450 we might've written state1 = Delaware, 49 00:03:08,530 --> 00:03:13,530 state2 = Pennsylvania and so on and so forth. 50 00:03:13,720 --> 00:03:16,840 And we would create as many variables as we have States. 51 00:03:17,470 --> 00:03:19,720 But now that we know about lists, 52 00:03:20,080 --> 00:03:25,080 then we can just create a single variable and we call it states_of_america. 53 00:03:26,680 --> 00:03:31,680 And now we can create a list by creating a set of square brackets and inside 54 00:03:32,230 --> 00:03:35,560 those square brackets, we add our items. So again, 55 00:03:35,620 --> 00:03:38,710 the first item is the state of Delaware, 56 00:03:38,740 --> 00:03:42,880 which is going to be a string and then we've got Pennsylvania, 57 00:03:43,570 --> 00:03:47,710 et cetera, et cetera. And we can continue this list just by adding commas, 58 00:03:48,160 --> 00:03:52,450 adding a piece of data, adding another comma, adding piece of data. 59 00:03:52,900 --> 00:03:57,430 And this way we end up with a list data structure. 60 00:03:58,390 --> 00:03:58,630 Now, 61 00:03:58,630 --> 00:04:03,630 one of the interesting things about the United States is that the different 62 00:04:03,820 --> 00:04:08,530 States in the US actually joined the union at different times. 63 00:04:08,890 --> 00:04:13,630 You can actually head over to Wikipedia and watch this little animation and see 64 00:04:13,780 --> 00:04:18,490 each state join the union and at which time point they did 65 00:04:18,490 --> 00:04:23,490 so. The order of this data is now kind of pretty important because if we wanted 66 00:04:25,960 --> 00:04:30,960 a list of US States that are in the order that they joined the union, 67 00:04:31,930 --> 00:04:36,930 then the order in which they're stored in our data structure is now also 68 00:04:37,660 --> 00:04:39,010 immensely important. 69 00:04:39,520 --> 00:04:42,700 And this is another thing that you get with lists. 70 00:04:43,120 --> 00:04:47,590 You can use a list to store many pieces of related data, 71 00:04:48,010 --> 00:04:50,770 but they also have an order. 72 00:04:50,950 --> 00:04:54,760 And the order is determined by the order in the list. 73 00:04:55,120 --> 00:04:59,050 So this is the first piece of data, this is the second piece of data. 74 00:04:59,380 --> 00:05:01,270 And when you store it inside the variable, 75 00:05:01,480 --> 00:05:06,340 that order is not lost and you'll be able to use it later on when you need the 76 00:05:06,340 --> 00:05:07,173 list. 77 00:05:07,510 --> 00:05:12,510 Here's a list of States of America ordered by the date that they joined the 78 00:05:14,110 --> 00:05:17,680 union. And you can see that if later on, 79 00:05:17,680 --> 00:05:22,420 I decided that I wanted to know which was the state that joined first, 80 00:05:22,480 --> 00:05:25,420 then I can print this variable states_of_america. 81 00:05:25,810 --> 00:05:28,270 I can add a set of square brackets, 82 00:05:28,660 --> 00:05:32,590 and then I type zero as the index of the piece of data 83 00:05:32,620 --> 00:05:35,860 I want to pull out from my states_of_america list. 84 00:05:36,250 --> 00:05:40,660 So now if I go ahead and run this code, you can see it prints out Delaware. 85 00:05:40,930 --> 00:05:43,090 And if I keep increasing this number, 86 00:05:43,390 --> 00:05:48,190 you can see that it's going through my list in the order that it was saved. 87 00:05:48,880 --> 00:05:50,770 So you might be wondering, 88 00:05:51,010 --> 00:05:55,930 that's kind of weird that you typed zero and you got Delaware, 89 00:05:55,930 --> 00:06:00,200 right? Surely, Delaware should be the first item in the list. 90 00:06:00,830 --> 00:06:05,720 Well, this is a kind of peculiarity with computers and programming languages. 91 00:06:06,080 --> 00:06:10,520 You'll tend to find that programmers start counting from zero. 92 00:06:10,700 --> 00:06:14,240 So Delaware is at zero, Pennsylvania is at one and New Jersey is at two. 93 00:06:14,870 --> 00:06:17,570 And this idea, it might seem a little bit weird at first, 94 00:06:17,600 --> 00:06:21,170 why is the first item at position 0? 95 00:06:21,860 --> 00:06:26,030 But if you think about that index number, that 0, 1 or 2, 96 00:06:26,720 --> 00:06:28,910 instead of being the position, 97 00:06:29,330 --> 00:06:34,330 actually being an offset or a shift from the start of the list. 98 00:06:35,810 --> 00:06:40,520 Well then in this case, Cherry is right at the beginning of the list, 99 00:06:40,520 --> 00:06:44,030 so it has a offset or a shift of 0. 100 00:06:44,540 --> 00:06:49,540 But Apple is shifted from the beginning by 1, Pear shifted from the beginning 101 00:06:49,790 --> 00:06:52,100 by 2 and so on and so forth. 102 00:06:52,400 --> 00:06:56,240 Then it kind of makes more sense that the first item in the list is at the 103 00:06:56,240 --> 00:06:59,480 beginning of the list. So it has no offset. So it's 0. 104 00:07:00,770 --> 00:07:03,410 And you'll find that in many, many programming languages, 105 00:07:03,680 --> 00:07:05,960 there are similar data structures to lists 106 00:07:06,230 --> 00:07:10,760 and this is how they're usually ordered starting from 0 and then adding by 107 00:07:10,760 --> 00:07:11,593 1. 108 00:07:11,900 --> 00:07:16,400 Now you can see that when you want to get hold of a particular piece of data 109 00:07:16,670 --> 00:07:18,830 stored inside a list, 110 00:07:19,190 --> 00:07:24,190 what you do is you get the name of the list and then you add another set of 111 00:07:25,340 --> 00:07:28,250 square brackets. So whenever you see square brackets, 112 00:07:28,520 --> 00:07:30,200 you should be thinking to yourself, oh, 113 00:07:30,230 --> 00:07:34,850 this might be related to a list because when you create the list, 114 00:07:34,880 --> 00:07:39,260 you use square brackets. And when you try to get items out of the list, 115 00:07:39,320 --> 00:07:40,820 you also use square brackets. 116 00:07:41,420 --> 00:07:45,860 And then inside of the square brackets is where you put the index or the offset 117 00:07:45,950 --> 00:07:49,490 of the item that you want. So if we wanted New Jersey, 118 00:07:49,490 --> 00:07:52,340 it's offset from the beginning by one two. 119 00:07:52,790 --> 00:07:56,990 So now this part of the code is equal to New Jersey. 120 00:07:57,530 --> 00:07:58,280 And we could, 121 00:07:58,280 --> 00:08:03,280 if we wanted to save it into another variable or we could simply print it as we 122 00:08:04,400 --> 00:08:06,320 did before. Now, 123 00:08:06,350 --> 00:08:09,560 in addition to using the positive index, 124 00:08:09,770 --> 00:08:12,350 so say 0, 1, 2, 3, 125 00:08:12,620 --> 00:08:14,960 you can also use a negative index. 126 00:08:15,320 --> 00:08:20,320 So if I wrote -1 or -2, then it actually starts counting from the 127 00:08:22,460 --> 00:08:26,330 end of the list. So if I wrote states_of_america 128 00:08:26,540 --> 00:08:31,010 [-1] as the index, then I get Hawaii. 129 00:08:31,430 --> 00:08:36,429 -1 is the last item in the list because you can't really have minus zero. 130 00:08:36,679 --> 00:08:40,400 That's not actually a real thing in math. Now, 131 00:08:40,460 --> 00:08:42,799 as I continue and I go to -2, 132 00:08:42,799 --> 00:08:47,780 then that's Alaska, -3 will be Arizona and so on and so forth. 133 00:08:47,840 --> 00:08:50,870 So you can have positive indices and negative indices. 134 00:08:51,440 --> 00:08:56,370 But so far we've only been pulling things out of our list by using our square 135 00:08:56,370 --> 00:08:57,840 brackets and the index. 136 00:08:58,260 --> 00:09:03,090 But you can also change the items in the list using very similar code. 137 00:09:03,480 --> 00:09:03,990 For example, 138 00:09:03,990 --> 00:09:08,990 if I decided that Pennsylvania is actually not spelled Pennsylvania and I wanted 139 00:09:10,020 --> 00:09:13,440 to change it to Pencilvania, 140 00:09:14,280 --> 00:09:18,270 then I can simply write my code like this. 141 00:09:18,810 --> 00:09:22,530 I get hold of my list. And then using the square brackets, 142 00:09:22,590 --> 00:09:25,890 I get hold of the item at index 1, which is this one. 143 00:09:26,790 --> 00:09:30,570 And then I set it equal to a new piece of data. 144 00:09:31,140 --> 00:09:36,140 So now if I go ahead and print my states_of_america list, 145 00:09:37,440 --> 00:09:40,050 you'll see that the list looks a little bit different now. 146 00:09:40,200 --> 00:09:44,250 Instead of Pennsylvania, it's now Pencilvania. 147 00:09:45,150 --> 00:09:48,090 So you can alter any item inside the list 148 00:09:48,120 --> 00:09:51,150 pretty easily using this kind of syntax. 149 00:09:51,540 --> 00:09:55,650 You could also add to the list if you wanted too. So for example, 150 00:09:55,650 --> 00:09:58,620 if you wanted to add an item at the end of the list, 151 00:09:58,650 --> 00:10:01,170 which is what happens most commonly, right? 152 00:10:01,170 --> 00:10:05,340 If you had a list of people who are cuing in your shop, 153 00:10:05,550 --> 00:10:08,790 then every subsequent person usually gets added to the end. 154 00:10:09,090 --> 00:10:12,210 If you have a new state that joined America, 155 00:10:12,480 --> 00:10:17,040 then it's probably going to be added after Hawaii. How do we do that? 156 00:10:17,370 --> 00:10:19,830 Well, we can write the name of the list 157 00:10:20,340 --> 00:10:25,340 and then we use a function called append and append 158 00:10:25,410 --> 00:10:29,130 will add a single item to the end of the list. 159 00:10:29,580 --> 00:10:34,580 So let's say that Angelaland is joining the United States of America. 160 00:10:36,600 --> 00:10:41,600 So now once I've appended Angelaland to the end of my states_of_america list, 161 00:10:42,870 --> 00:10:47,730 and I print the states_of_america, you can see, there it is added at the end. 162 00:10:48,870 --> 00:10:52,080 Now there's actually a whole load of other functions that you can use in 163 00:10:52,080 --> 00:10:56,610 addition to append. And you'll find this on the documentation for Python. 164 00:10:57,300 --> 00:11:02,250 In addition to the append function that we saw just now where we add an item at 165 00:11:02,250 --> 00:11:03,083 the end of the list, 166 00:11:03,390 --> 00:11:07,110 there's a whole load of all the functions that you can use with lists. 167 00:11:07,470 --> 00:11:09,750 For example, you can use the extend, 168 00:11:09,990 --> 00:11:14,880 which adds a whole bunch of items at the end of the list. And in this case, 169 00:11:14,910 --> 00:11:18,540 what you're actually adding is going to be a list. 170 00:11:18,900 --> 00:11:23,850 So I would be creating the list using square brackets and then adding my items 171 00:11:23,880 --> 00:11:24,713 in here. 172 00:11:25,620 --> 00:11:30,620 And now what's happening is I'm extending this states_of_america list with this 173 00:11:31,500 --> 00:11:35,640 additional list. So now if we print the states_of_america, 174 00:11:35,910 --> 00:11:40,910 you can see that these two items that used to be inside of the list have now 175 00:11:41,370 --> 00:11:44,580 been added to the previous list 176 00:11:44,910 --> 00:11:47,640 and it's now extended it by two more items. 177 00:11:48,630 --> 00:11:52,890 But the important thing is you don't have to memorize these functions. 178 00:11:52,930 --> 00:11:57,790 That's the whole point of documentation and why we have Google because there's 179 00:11:57,790 --> 00:12:00,820 too much information in the world for you to memorize. 180 00:12:01,240 --> 00:12:04,330 And it's a very inefficient way of learning. 181 00:12:04,900 --> 00:12:09,220 And if you tried to memorize every single method, it's not impossible, 182 00:12:09,220 --> 00:12:12,850 but it means that you don't have space in your brain for the important stuff, 183 00:12:12,850 --> 00:12:14,350 which is how things work. 184 00:12:14,380 --> 00:12:17,650 How do you actually use it to do what you want it to do? 185 00:12:18,070 --> 00:12:22,300 So what I recommend when you come across a new thing, such as, 186 00:12:22,600 --> 00:12:27,370 um, the list data structure is to just have a look through the documentation, 187 00:12:27,790 --> 00:12:31,690 read through it and see what are the possible things you can do. 188 00:12:32,140 --> 00:12:34,870 And once you've got the idea of this is possible, 189 00:12:35,020 --> 00:12:39,040 then the next time when you need to use it inside your code, you'll know, ah, 190 00:12:39,070 --> 00:12:41,050 I remember this is possible. 191 00:12:41,290 --> 00:12:44,290 And all you have to do is just be able to use Google, 192 00:12:44,530 --> 00:12:47,500 to find the exact bit of the documentation 193 00:12:47,770 --> 00:12:52,770 and then implement it. Programming is kind of like an open book exam. 194 00:12:53,140 --> 00:12:55,660 You shouldn't need to memorize anything. 195 00:12:55,870 --> 00:12:59,440 You should spend your time trying things out and try to get things to work 196 00:12:59,440 --> 00:13:04,330 instead. Now that I've introduced you to this new data structure, 197 00:13:04,420 --> 00:13:05,410 the mighty list, 198 00:13:05,740 --> 00:13:09,880 it's time for a code exercise to see if you can use it in practice. 199 00:13:10,210 --> 00:13:13,600 So head over to the next lesson and give the challenge that go.