WEBVTT 1 00:00:01.570 --> 00:00:03.080 Hi guys and welcome back. 2 00:00:03.080 --> 00:00:04.610 In this video, we're going to talk about 3 00:00:04.610 --> 00:00:07.250 list comprehensions in Python. 4 00:00:07.250 --> 00:00:09.940 A list comprehension is a great feature of Python 5 00:00:09.940 --> 00:00:13.260 and allows us to create new lists on the fly 6 00:00:13.260 --> 00:00:14.950 from existing lists. 7 00:00:14.950 --> 00:00:17.840 In addition, it's very succinct and very powerful. 8 00:00:17.840 --> 00:00:19.100 Let's get started. 9 00:00:19.100 --> 00:00:21.450 Here we've got a list of numbers, 10 00:00:21.450 --> 00:00:24.090 and I want to end up with these numbers, 11 00:00:24.090 --> 00:00:25.920 but multiplied by two, 12 00:00:25.920 --> 00:00:29.560 so I'll say something like doubled equal empty list. 13 00:00:29.560 --> 00:00:32.290 How do we put each of these numbers, 14 00:00:32.290 --> 00:00:35.130 multiplied by two, into this new list? 15 00:00:35.130 --> 00:00:37.000 Traditionally and in most languages, 16 00:00:37.000 --> 00:00:40.740 you would end up doing something like for num in numbers, 17 00:00:40.740 --> 00:00:45.507 and then you'd do doubled.append num times two. 18 00:00:46.980 --> 00:00:48.640 This is quite a long piece of code 19 00:00:48.640 --> 00:00:51.820 just for creating a new list of doubled numbers, 20 00:00:51.820 --> 00:00:53.900 but this is how you do it normally. 21 00:00:53.900 --> 00:00:56.410 You would iterate over the numbers list, 22 00:00:56.410 --> 00:00:59.220 create a new variable for each of the numbers in it, 23 00:00:59.220 --> 00:01:01.450 and then run through it three times, 24 00:01:01.450 --> 00:01:03.620 each time putting the appropriate number, 25 00:01:03.620 --> 00:01:06.340 multiplied by two, into your new list. 26 00:01:06.340 --> 00:01:08.970 Python has a much more succinct way of doing this 27 00:01:08.970 --> 00:01:10.950 which is with a list comprehension. 28 00:01:10.950 --> 00:01:12.190 In a list comprehension, 29 00:01:12.190 --> 00:01:16.310 the order of these operations, first the iteration, 30 00:01:16.310 --> 00:01:19.810 and then the appending, seems to be reversed, 31 00:01:19.810 --> 00:01:21.570 but it's not really. 32 00:01:21.570 --> 00:01:23.920 So the first thing you do is you would put in 33 00:01:23.920 --> 00:01:26.970 what you want to add to your new list, 34 00:01:26.970 --> 00:01:28.673 which is num times two. 35 00:01:29.640 --> 00:01:32.800 But of course, num doesn't exist in this context. 36 00:01:32.800 --> 00:01:36.250 It only existed here because we've got our for loop. 37 00:01:36.250 --> 00:01:38.883 So the next thing you do is you put your for loop, 38 00:01:40.534 --> 00:01:44.200 and what this does in Python is it puts num times two 39 00:01:44.200 --> 00:01:49.200 into a new list for every num in the numbers list. 40 00:01:49.710 --> 00:01:52.970 So essentially you're iterating over the numbers list, 41 00:01:52.970 --> 00:01:56.216 and you're putting the variable multiplied by two 42 00:01:56.216 --> 00:02:00.600 every time that the loop runs into your new list. 43 00:02:00.600 --> 00:02:03.850 You end up with the doubled numbers. 44 00:02:03.850 --> 00:02:06.790 Because list comprehension are usually in a single line, 45 00:02:06.790 --> 00:02:09.740 you do want to keep them short if you can, 46 00:02:09.740 --> 00:02:12.210 so instead of num times two for example in this case, 47 00:02:12.210 --> 00:02:15.640 you could do x times two, and for x numbers. 48 00:02:15.640 --> 00:02:17.780 That just helps you keep it a little bit shorter 49 00:02:17.780 --> 00:02:19.280 and a little bit more readable. 50 00:02:19.280 --> 00:02:20.870 After all, the most important thing here 51 00:02:20.870 --> 00:02:25.050 is that you're putting x times two into your new list 52 00:02:25.050 --> 00:02:27.510 while you iterate over the previous list. 53 00:02:27.510 --> 00:02:31.000 So again to recap, how you build a list comprehension 54 00:02:31.000 --> 00:02:33.900 is what you wanna put into your new list 55 00:02:33.900 --> 00:02:38.583 for the variable that you're using in numbers. 56 00:02:40.150 --> 00:02:42.740 Now let's say you've got a friend's list, 57 00:02:42.740 --> 00:02:44.730 and you wanna create a new list 58 00:02:44.730 --> 00:02:47.370 which contains the names of those friends 59 00:02:47.370 --> 00:02:49.500 that start with an s. 60 00:02:49.500 --> 00:02:51.820 So Sam, Samantha, and Saurabh. 61 00:02:51.820 --> 00:02:54.140 But it seems that a list comprehension 62 00:02:54.140 --> 00:02:56.070 may not be the tool for this job. 63 00:02:56.070 --> 00:03:00.030 After all, you do need to only get some of these out. 64 00:03:00.030 --> 00:03:01.600 Let's start with a for loop, 65 00:03:01.600 --> 00:03:03.580 and then I'll show you how the list comprehension 66 00:03:03.580 --> 00:03:04.920 can deal with it. 67 00:03:04.920 --> 00:03:08.160 So you would do for friend in friends. 68 00:03:08.160 --> 00:03:10.660 Then you would add an if statement to check 69 00:03:10.660 --> 00:03:14.030 that the friend name in question starts with an s. 70 00:03:14.030 --> 00:03:18.293 You could say if friend.startswith capital s, 71 00:03:19.340 --> 00:03:24.010 then you're gonna do starts_s.append friend, 72 00:03:24.010 --> 00:03:28.660 and then at the end, we're gonna print starts_s like that. 73 00:03:28.660 --> 00:03:30.900 Let me run this and show you what happens. 74 00:03:30.900 --> 00:03:33.870 Down here, you have Sam, Samantha, and Saurabh, 75 00:03:33.870 --> 00:03:35.370 and I think I'm not pronouncing that right, 76 00:03:35.370 --> 00:03:38.120 so my apologies if that's the case. 77 00:03:38.120 --> 00:03:40.580 So what we've got is a for loop, 78 00:03:40.580 --> 00:03:44.310 and then an if statement, and then an append. 79 00:03:44.310 --> 00:03:47.310 This structure here can also be reproduced 80 00:03:47.310 --> 00:03:50.780 with a list comprehension although it does start to get 81 00:03:50.780 --> 00:03:52.363 a little bit more confusing. 82 00:03:53.500 --> 00:03:57.140 Again, the process is more or less the same. 83 00:03:57.140 --> 00:04:00.480 First of all, you put what you wanna add to your new list, 84 00:04:00.480 --> 00:04:01.873 so that's your friend name. 85 00:04:02.780 --> 00:04:05.480 Then you put your iteration or your for loop, 86 00:04:05.480 --> 00:04:08.020 for friend in friends. 87 00:04:08.020 --> 00:04:10.760 And now if you wanna add an if statement, 88 00:04:10.760 --> 00:04:12.040 you do it at the end, 89 00:04:12.040 --> 00:04:13.970 so you're gonna add your friend 90 00:04:13.970 --> 00:04:18.970 for the friend in your list if the friend starts with s, 91 00:04:21.830 --> 00:04:23.340 and that does exactly the same thing, 92 00:04:23.340 --> 00:04:26.020 and you no longer need that loop there, 93 00:04:26.020 --> 00:04:27.570 so you can save that and run it again, 94 00:04:27.570 --> 00:04:30.230 and you see that the same thing comes out. 95 00:04:30.230 --> 00:04:33.870 So this is how you add a conditional, an if statement, 96 00:04:33.870 --> 00:04:36.023 into a list comprehension. 97 00:04:37.170 --> 00:04:38.650 Very important to nice that 98 00:04:38.650 --> 00:04:40.770 when you use a list comprehension, 99 00:04:40.770 --> 00:04:43.430 a new list is created, 100 00:04:43.430 --> 00:04:47.930 so even though these two lists may be the same 101 00:04:47.930 --> 00:04:49.483 if we take these names out, 102 00:04:52.350 --> 00:04:53.860 so now these two lists will be the same 103 00:04:53.860 --> 00:04:55.210 if we print them both out, 104 00:04:55.210 --> 00:04:57.210 you'll see that they are both identical. 105 00:04:58.490 --> 00:05:01.500 When you print friends is starts_s, 106 00:05:01.500 --> 00:05:03.713 you'll see that false comes out. 107 00:05:05.430 --> 00:05:07.220 That's because these two lists, 108 00:05:07.220 --> 00:05:09.470 although they have the same content, 109 00:05:09.470 --> 00:05:11.370 are not the same list. 110 00:05:11.370 --> 00:05:13.880 They are two different lists, two different places 111 00:05:13.880 --> 00:05:17.803 in your computer's memory with the same values. 112 00:05:18.840 --> 00:05:22.460 For context, friends zero is starts_s a zero, 113 00:05:22.460 --> 00:05:27.460 so Sam is Sam will give you true, see that? 114 00:05:27.640 --> 00:05:30.380 But the lists themselves are not the same, 115 00:05:30.380 --> 00:05:31.960 so the elements in them are the same, 116 00:05:31.960 --> 00:05:34.270 but the lists themselves are not the same, 117 00:05:34.270 --> 00:05:38.860 and by using the is keyword, then you can check that, 118 00:05:38.860 --> 00:05:41.840 but there's also something else you can do, 119 00:05:41.840 --> 00:05:44.870 which is check the IDs of these lists, 120 00:05:44.870 --> 00:05:47.833 so you can do something like friends, 121 00:05:49.160 --> 00:05:51.620 then print out the ID of friends, 122 00:05:51.620 --> 00:05:56.620 and then starts_s and print the ID of starts-s, 123 00:05:57.310 --> 00:05:58.460 and what I've done here by the way 124 00:05:58.460 --> 00:06:01.470 is I have passed multiple different things 125 00:06:01.470 --> 00:06:03.937 to the print function, and you can do that in print, 126 00:06:03.937 --> 00:06:05.610 and it just separates them with a space, 127 00:06:05.610 --> 00:06:08.780 so it's gonna separate these different parts with a space, 128 00:06:08.780 --> 00:06:10.030 the different arguments there. 129 00:06:10.030 --> 00:06:15.030 So by running this, you get that friends has an ID 130 00:06:16.780 --> 00:06:21.530 of this, ends in 2096, and starts_s has an ID 131 00:06:21.530 --> 00:06:24.120 that ends in 5280. 132 00:06:24.120 --> 00:06:27.560 So these two have different IDs, and in Cpython, 133 00:06:27.560 --> 00:06:29.040 which is what most of you will be using, 134 00:06:29.040 --> 00:06:30.330 certainly what I'm using, 135 00:06:30.330 --> 00:06:32.997 the ID is related to the memory address 136 00:06:32.997 --> 00:06:35.360 in which the list is stored, 137 00:06:35.360 --> 00:06:37.330 so you can see that these two things 138 00:06:37.330 --> 00:06:38.710 are in different places in memory. 139 00:06:38.710 --> 00:06:40.960 They're different objects entirely. 140 00:06:40.960 --> 00:06:43.650 They just happen to have the same contents. 141 00:06:43.650 --> 00:06:46.510 So this is something that will be very important 142 00:06:46.510 --> 00:06:48.340 as we proceed with the course, 143 00:06:48.340 --> 00:06:51.060 so this is why I wanted to show it to you at this point. 144 00:06:51.060 --> 00:06:54.140 Creating new lists gives you a different thing 145 00:06:54.140 --> 00:06:55.680 entirely in Python. 146 00:06:55.680 --> 00:06:57.560 It's not the same as the thing before 147 00:06:57.560 --> 00:06:59.410 even if it has the same contents, 148 00:06:59.410 --> 00:07:00.350 but as I mentioned earlier, 149 00:07:00.350 --> 00:07:03.890 if you do want them to be the same exact list, 150 00:07:03.890 --> 00:07:05.760 then you can do this. 151 00:07:05.760 --> 00:07:08.610 Starts_s equal friends, and now when you press play, 152 00:07:08.610 --> 00:07:09.730 you'll see that you get true, 153 00:07:09.730 --> 00:07:11.760 and these IDs are identical. 154 00:07:11.760 --> 00:07:15.620 So that is how you make two lists the same thing. 155 00:07:15.620 --> 00:07:18.500 Otherwise, you do end up with two different lists. 156 00:07:18.500 --> 00:07:19.730 All right, that's enough of that. 157 00:07:19.730 --> 00:07:21.020 Thank you for joining me in this video. 158 00:07:21.020 --> 00:07:22.140 I hope it's been useful, 159 00:07:22.140 --> 00:07:23.790 and I'll see you in the next one.