WEBVTT 0 00:00.260 --> 00:07.130 The big topic that we're going to explore today is a concept known as a List Comprehension. 1 00:07.130 --> 00:12.020 And as I mentioned before, this is something that's really unique to the Python language. 2 00:12.020 --> 00:17.480 In many other programming languages, you don't actually have access to something like this. 3 00:17.480 --> 00:19.610 There's nothing that really compares to it. 4 00:19.610 --> 00:25.040 And it's something that Python developers really, really love because it cuts down on the amount of 5 00:25.040 --> 00:31.090 typing, and it just makes the code a lot shorter and in most cases a lot easier to read. 6 00:31.240 --> 00:34.570 So what exactly is a List Comprehension? 7 00:34.600 --> 00:38.950 Well, it's a case where you create a new list from a previous list. 8 00:38.980 --> 00:41.920 So far we've been doing that using for loops. 9 00:41.920 --> 00:48.940 So for example, if I have a list of numbers [1, 2, 3] and I want to create a new list where 10 00:48.940 --> 00:56.550 each number is increased by one, well then I would use a for loop, creating a new empty list and then 11 00:56.550 --> 01:02.910 looping through each of the numbers in that list in order to add one to each of those numbers. 12 01:02.910 --> 01:10.500 And then I would append each of those increased numbers to my new_list, and I would end up with a 13 01:10.500 --> 01:13.170 new_list that consists of three items. 14 01:13.170 --> 01:15.570 And it will be [2, 3, 4]. 15 01:15.720 --> 01:22.770 Now using list comprehension we can take these four lines of code and turn it into one. 16 01:22.770 --> 01:27.960 And the way that we create a list comprehension looks something like this. 17 01:27.990 --> 01:34.650 I like to use the keyword 'method', where you type out the list comprehension keywords, and then replace 18 01:34.650 --> 01:38.610 each of the words with the actual item in your code. 19 01:38.880 --> 01:45.400 The way that we would create any list comprehension is we would create the name of the new_list, and 20 01:45.400 --> 01:51.280 instead of creating an empty list and then having to append to it or populate it, we're actually going 21 01:51.280 --> 01:53.560 to create it straight in the same line. 22 01:53.560 --> 01:58.330 So we open up a set of square brackets to denote that we're creating a new list, 23 01:58.330 --> 02:04.510 and then we have this, "new_item for item in list" keyword pattern. 24 02:04.660 --> 02:06.010 Here's how it would work. 25 02:06.010 --> 02:11.520 Let's take that same thing that we did before, where we took a list of numbers, and we created a new 26 02:11.520 --> 02:14.340 list where each number is increased by one. 27 02:14.340 --> 02:17.430 And let's see how we would do it using list comprehension. 28 02:18.330 --> 02:22.080 So the first keyword I'm going to replace is the list. 29 02:22.080 --> 02:24.990 So this is the list that we're going to iterate through, 30 02:24.990 --> 02:28.320 and in our case it's this list of numbers. 31 02:28.320 --> 02:33.650 So let's replace that list keyword with the actual list, which is numbers. 32 02:33.740 --> 02:38.180 The next thing that is easy to understand is each item in the list. 33 02:38.180 --> 02:42.800 So we can call this anything we want, in the for loop here we called it n, 34 02:42.800 --> 02:46.430 so why don't we go ahead and call it n again. 35 02:47.600 --> 02:54.350 This n is going to represent each of the numbers in that list of numbers, just as it does in this for 36 02:54.350 --> 02:59.750 loop and the final keyword that we haven't yet replaced is the new_item. 37 02:59.750 --> 03:04.430 So what do we want each of the new items to be in this new list? 38 03:04.430 --> 03:11.210 Well, each of the new items is basically calculated by taking n and then adding 1 to it. 39 03:11.210 --> 03:19.610 So this line is the expression or the code that we need to execute in order to get a new_item. 40 03:19.610 --> 03:23.040 So this becomes our final list comprehension. 41 03:23.040 --> 03:24.990 We have our list of numbers, 42 03:24.990 --> 03:31.830 and then we create a new_list where we loop through each of the numbers n inside this list of numbers, 43 03:31.830 --> 03:37.260 and then for each of the n, we add one to it in order to create the final list. 44 03:37.260 --> 03:43.500 So when you look at a list comprehension just like this, as in the real code, it can be a little bit 45 03:43.500 --> 03:44.610 confusing to see. 46 03:44.610 --> 03:45.680 How would you read it? 47 03:45.680 --> 03:47.390 Which order does it happen? 48 03:47.390 --> 03:53.300 But when you're using this keyword method that I often use, where you just type out what you need to 49 03:53.300 --> 03:56.750 have inside a list comprehension, you need a name for the new list. 50 03:56.750 --> 03:58.460 You need a set of square brackets. 51 03:58.460 --> 04:04.040 And then inside the square brackets, you have, "new_item for item in list," and then you go through it 52 04:04.040 --> 04:08.390 replacing each of the keywords list, item and new_item. 53 04:08.870 --> 04:12.500 So I want you to have a go at actually writing out this code. 54 04:12.500 --> 04:15.410 Go ahead and create a new PyCharm project. 55 04:15.410 --> 04:18.380 And I want you to go into the Python console. 56 04:18.380 --> 04:22.040 We're not actually going to be writing code inside our main.py, 57 04:22.040 --> 04:28.250 and the reason is because the console allows us to write code line by line, and then we can view our 58 04:28.250 --> 04:31.580 variables really easily on the right hand side pane here. 59 04:32.270 --> 04:38.440 For example, if I have my list of numbers [1, 2, 3] and then I have to write out my list 60 04:38.440 --> 04:42.790 comprehension where we create our new_list, which I'll call new_numbers. 61 04:42.790 --> 04:46.510 And then we have to create our list comprehension. 62 04:46.510 --> 04:52.360 So the way to remember it is, "new_item for item in list." 63 04:52.360 --> 05:00.330 So now go ahead and write this out and see if you can remember how we actually structure our list comprehension, 64 05:00.330 --> 05:04.260 and see if you can get it to work within your PyCharm console. 65 05:04.560 --> 05:05.670 So pause the video. 66 05:05.670 --> 05:06.570 Give that a go. 67 05:08.010 --> 05:08.490 All right. 68 05:08.490 --> 05:11.430 So our list is of course this list of numbers. 69 05:11.430 --> 05:13.320 So let's replace that first. 70 05:13.320 --> 05:18.390 And then for each of the items in that list of numbers, well we can call it anything we want. 71 05:18.390 --> 05:21.210 So I'm just going to call it n or num, 72 05:21.210 --> 05:22.140 it doesn't matter. 73 05:22.140 --> 05:28.970 But as long as you're consistent in using it when you create the new_item, because each new_item is 74 05:28.970 --> 05:34.460 going to be the n, which is each number in the list + 1. 75 05:34.460 --> 05:41.360 So now when I hit Enter, you can see I've got this new list called new_numbers created from the previous 76 05:41.360 --> 05:42.620 list numbers, 77 05:42.620 --> 05:48.340 and this now has the same number of items, but each item is now increased by one. 78 05:49.210 --> 05:55.060 Now it's important to remember that when we say list comprehension, it doesn't strictly mean that you 79 05:55.060 --> 05:56.710 can only work with lists, 80 05:56.710 --> 06:00.760 you can work with other sequences like strings, for example. 81 06:00.760 --> 06:04.810 So here I've got a variable called name which is just a string. 82 06:04.810 --> 06:06.400 It's the word, Angela. 83 06:06.400 --> 06:14.040 And I'm creating a new_list by using a list comprehension that has this kind of code. 84 06:14.070 --> 06:19.560 Now, I want you to pause for a second and think about what would be in this new_list. 85 06:19.560 --> 06:21.120 What would it look like? 86 06:21.120 --> 06:24.180 And then I want you to try it out inside PyCharm. 87 06:24.180 --> 06:25.680 So pause the video now. 88 06:27.870 --> 06:28.410 All right. 89 06:28.410 --> 06:30.420 So, name = "Angela". 90 06:30.450 --> 06:33.180 This is the variable that we're working with, 91 06:33.180 --> 06:39.530 and then I'm going to create a new_list which I'll call letters_list. 92 06:40.280 --> 06:44.720 This new list is going to be created using my list comprehension. 93 06:44.720 --> 06:50.660 So it's going to be, "letter for letter in name." 94 06:50.660 --> 06:56.510 And what this does is it takes this sequence, this string, 95 06:56.510 --> 06:59.900 it goes through each of the letters in that string, 96 06:59.900 --> 07:04.780 and then it adds each of the letters into this new_list. 97 07:04.780 --> 07:08.800 So when I hit Enter, you can see this is what letters_list looks like. 98 07:08.800 --> 07:16.090 It's just split off my string into individual letters, and it's added it into a brand new list. 99 07:17.080 --> 07:26.370 As I mentioned, these things in Python, like a list, or a string, or a range, or tuple, they're called 100 07:26.370 --> 07:29.790 sequences because they have a specific order, 101 07:29.790 --> 07:35.640 and when you perform a list comprehension, it's going to take that sequence and it's going to go through 102 07:35.670 --> 07:42.990 it in order, either be it the letters in the string, or the items in a list, and then it's going to 103 07:42.990 --> 07:48.060 take each of those items in that correct order and then do something with it. 104 07:48.060 --> 07:52.910 Either add 1 to it or in this case, do nothing and just add it to a new list. 105 07:53.450 --> 07:54.770 Here's a challenge for you. 106 07:54.800 --> 08:01.610 Can you take the range, which is also a sequence that we can iterate through, and then create a range 107 08:01.610 --> 08:03.590 between 1 and 5? 108 08:03.590 --> 08:09.200 And remember that the way that the range works is it's going to take 1, and then 2 and then 3 109 08:09.200 --> 08:12.770 and then 4, but it's not going to go up to 5. 110 08:12.860 --> 08:19.180 I want you to loop through this range and then create a list where each of the numbers in the range 111 08:19.180 --> 08:24.010 is doubled, and the final list should look like this, 112 08:24.040 --> 08:26.740 it'll be [2, 4, 6, 8]. 113 08:26.770 --> 08:29.920 Pause the video and see if you can complete this challenge. 114 08:32.230 --> 08:32.740 All right. 115 08:32.740 --> 08:37.120 So let's create our new list which I'm just going to call range_list, 116 08:37.120 --> 08:40.380 and we're going to use that keyword pattern to remind ourselves. 117 08:40.380 --> 08:44.730 So it's going to be, "new_item for item in list." 118 08:44.730 --> 08:48.360 And this list in this case, is actually not going to be a list, 119 08:48.360 --> 08:51.000 it's going to be a range between 1 and 5. 120 08:51.000 --> 08:55.380 And then each of the items we get to give it a name, which I'll just call num. 121 08:55.380 --> 09:00.030 And then in this new list, what is each new_item going to be? 122 09:00.030 --> 09:03.390 Well, it's going to be num * two. 123 09:03.390 --> 09:08.720 So now when I hit Enter, I get my range list, which is [2, 4, 6, 8]. 124 09:08.720 --> 09:13.970 So it's looped through each of the numbers in the range, and it's multiplied each of those numbers by 125 09:13.970 --> 09:14.450 2, 126 09:14.450 --> 09:18.860 and then it's created a new list using each of those numbers. 127 09:19.610 --> 09:25.760 Now the final thing I want to show you regarding list comprehensions is that there's also such a thing 128 09:25.760 --> 09:28.750 as a Conditional List Comprehension. 129 09:28.780 --> 09:35.200 This takes our keywords a little bit further, whereas previously we stopped right here, 130 09:35.830 --> 09:38.080 "new_item for item in list," 131 09:38.080 --> 09:42.580 we can also tag on two more keywords 'if' and a 'test'. 132 09:42.850 --> 09:51.430 What this is going to allow us to do is to only add this new item, and only to perform this code if 133 09:51.430 --> 09:53.880 the test actually passes. 134 09:54.720 --> 09:56.100 Here's an example. 135 09:56.100 --> 10:01.560 I've got a bunch of names here ["Alex", "Beth", "Caroline", "Dave", "Elanor", "Freddie"]. 136 10:01.590 --> 10:08.070 Now, one of the neat things that you can actually do with the Python console is not only can you view 137 10:08.070 --> 10:12.720 what all of the variables are equal to, but you can also edit them if you have an issue. 138 10:12.750 --> 10:17.870 The sharp-eyed amongst you will know that I've actually made a typo in this name. 139 10:17.870 --> 10:19.490 It's not Elanor, 140 10:19.520 --> 10:21.980 it's actually spelled a little bit differently. 141 10:21.980 --> 10:27.980 So what I can do on the right-hand pane here is I can right-click on that piece of data that I want 142 10:27.980 --> 10:33.860 to change, and then click Set Value, and then actually change the data here. 143 10:34.850 --> 10:41.300 Now that I've got that spelled out correctly, if I type names and hit Enter, you can see it's now been 144 10:41.300 --> 10:41.810 corrected. 145 10:41.830 --> 10:44.560 And this is my new list of names. 146 10:44.800 --> 10:50.110 So using this list of names, I'm going to try and create a new list of names, 147 10:50.110 --> 10:52.450 but I only want the short names. 148 10:52.450 --> 10:58.360 I only want the names which is made up of four letters or less, like Alex or Beth. 149 10:58.420 --> 11:05.080 The way I would do this is I would create a new list, which is called short_names, and then I would 150 11:05.080 --> 11:06.630 use my list comprehension. 151 11:06.630 --> 11:16.680 And remember in this case the keywords are, "new_item for item in list and then if and a test". 152 11:17.400 --> 11:19.200 So let's fill in the easy parts 153 11:19.200 --> 11:19.770 first. 154 11:19.770 --> 11:22.470 The list is going to be our list of names. 155 11:22.470 --> 11:29.400 The item I can call it anything I want, I'll just call it name in terms of the singular form of names. 156 11:29.400 --> 11:35.750 And then once I loop through each of the names inside my list of names, what is the new item going 157 11:35.750 --> 11:35.990 to be? 158 11:36.020 --> 11:38.030 Well, it's actually going to be unmodified, 159 11:38.030 --> 11:40.370 it's just going to be that same name, 160 11:40.370 --> 11:45.140 but in this case I'm only going to add that name to the list 161 11:45.140 --> 11:47.180 if it passes this test. 162 11:47.180 --> 11:53.060 And this test is going to look at the length of the name which I created here, 163 11:53.060 --> 11:58.910 so if I named this n, then I would be testing it against n and I would be adding n as well, 164 11:58.910 --> 12:01.010 but because I've called it name, 165 12:01.010 --> 12:04.490 so I'm going to use that name right here in the condition. 166 12:04.490 --> 12:13.400 So if the length of the name is less than 5, so it has 4 or less characters, then I'm going to 167 12:13.400 --> 12:15.260 add that name to my new_list. 168 12:15.950 --> 12:22.840 Now when I hit Enter you can see that I've got this new list of short_names and it only contains the 169 12:22.840 --> 12:25.180 names, Alex, Beth, and Dave. 170 12:25.330 --> 12:31.840 So this list comprehension is now a little bit more complex because the first thing it does is it 171 12:31.840 --> 12:35.860 goes through each of the names inside this list of names, 172 12:35.860 --> 12:39.280 it checks each of those names for its length, 173 12:39.280 --> 12:44.670 and if the length is 5, then it adds the name to this new list. 174 12:45.870 --> 12:47.790 Now here's a challenge for you. 175 12:47.820 --> 12:55.590 I want you to take all of the names from this list of names, which are made up of 5 letters or more, 176 12:55.590 --> 12:58.710 so basically, Caroline, Eleanor and Freddy, 177 12:58.830 --> 13:04.380 and I want you to turn each of these names to the uppercase version. 178 13:04.620 --> 13:13.610 What you should end up with is a list where you have Caroline in all caps, Eleanor in all caps, and 179 13:13.610 --> 13:15.440 Freddie in all caps. 180 13:15.440 --> 13:17.390 But of course, you're not going to just type this out, 181 13:17.390 --> 13:19.580 you're going to use list comprehension. 182 13:19.790 --> 13:23.030 Pause the video and see if you can complete this challenge. 183 13:24.830 --> 13:30.320 All right I'm going to call this new list long_names because I haven't eaten for a while and I lack 184 13:30.320 --> 13:31.430 imagination. 185 13:31.610 --> 13:37.540 Now in order to create this list of long_names, I'm going to use that new keyword pattern that we learned, 186 13:37.540 --> 13:43.060 which is, "new_item for item in list if test". 187 13:43.330 --> 13:46.510 The list is again going to be our list of names. 188 13:46.510 --> 13:51.430 Each of the items we can call it anything we want, but I'm going to call it name. 189 13:51.430 --> 13:55.840 And if you find that this is too similar to this, you can call it anything you want, 190 13:55.870 --> 13:56.440 nom, 191 13:56.440 --> 14:01.590 you can use French, you can just write n or you can do anything you want. 192 14:02.550 --> 14:09.030 This name that we're looping through is going to be tested in order to see if we're actually going to 193 14:09.030 --> 14:10.290 create a new_item. 194 14:10.620 --> 14:17.430 The test in this case, is we're checking to see if the length of the name is now greater than 5, 195 14:18.720 --> 14:25.850 and if this name that we're currently looping through passes this test, then we get to specify what 196 14:25.850 --> 14:27.590 the new_item should be. 197 14:27.590 --> 14:33.320 So the new_item is going to be that particular name that passed the test, but then it's going to be 198 14:33.320 --> 14:34.100 uppercased. 199 14:34.100 --> 14:40.220 So we're going to call name.upper() in order to create the new_item that's going to be added to the 200 14:40.220 --> 14:40.820 list. 201 14:40.820 --> 14:45.980 So now once I hit Enter, you can see the new list that I've created. 202 14:45.980 --> 14:51.200 Long names has Caroline, Eleanor, and Freddie all in uppercase. 203 14:51.470 --> 14:54.770 Essentially, we looped through each of the names in the list, 204 14:54.770 --> 14:58.550 we took each of those names and checked that they're longer than 5, 205 14:58.550 --> 15:00.860 if they were in fact longer than 5, 206 15:00.860 --> 15:05.990 then we looked at this part of the code to see what we should do with each of those names, and in our 207 15:05.990 --> 15:08.120 case, we've turned it into uppercase. 208 15:08.300 --> 15:12.060 So there's quite a lot of theory covered in today's lesson. 209 15:12.090 --> 15:17.220 Now, in order to be able to fully understand what's going on, you can't just watch me talk about it, 210 15:17.220 --> 15:18.840 you have to practice. 211 15:18.870 --> 15:25.050 So in the coming lessons, I've got a whole bunch of exercises for you to practice using and creating 212 15:25.050 --> 15:26.850 list comprehensions for yourself. 213 15:26.850 --> 15:31.410 And that way, once you've done the drills, you've done the code press-ups, then you'll be able to 214 15:31.410 --> 15:35.550 show off your list comprehension muscles. For all of that and more, 215 15:35.550 --> 15:36.900 I'll see you on the next lesson.