1 1 00:00:01,570 --> 00:00:03,080 Hi guys and welcome back. 2 2 00:00:03,080 --> 00:00:04,610 In this video, we're going to talk about 3 3 00:00:04,610 --> 00:00:07,250 list comprehensions in Python. 4 4 00:00:07,250 --> 00:00:09,940 A list comprehension is a great feature of Python 5 5 00:00:09,940 --> 00:00:13,260 and allows us to create new lists on the fly 6 6 00:00:13,260 --> 00:00:14,950 from existing lists. 7 7 00:00:14,950 --> 00:00:17,840 In addition, it's very succinct and very powerful. 8 8 00:00:17,840 --> 00:00:19,100 Let's get started. 9 9 00:00:19,100 --> 00:00:21,450 Here we've got a list of numbers, 10 10 00:00:21,450 --> 00:00:24,090 and I want to end up with these numbers, 11 11 00:00:24,090 --> 00:00:25,920 but multiplied by two, 12 12 00:00:25,920 --> 00:00:29,560 so I'll say something like doubled equal empty list. 13 13 00:00:29,560 --> 00:00:32,290 How do we put each of these numbers, 14 14 00:00:32,290 --> 00:00:35,130 multiplied by two, into this new list? 15 15 00:00:35,130 --> 00:00:37,000 Traditionally and in most languages, 16 16 00:00:37,000 --> 00:00:40,740 you would end up doing something like for num in numbers, 17 17 00:00:40,740 --> 00:00:45,507 and then you'd do doubled.append num times two. 18 18 00:00:46,980 --> 00:00:48,640 This is quite a long piece of code 19 19 00:00:48,640 --> 00:00:51,820 just for creating a new list of doubled numbers, 20 20 00:00:51,820 --> 00:00:53,900 but this is how you do it normally. 21 21 00:00:53,900 --> 00:00:56,410 You would iterate over the numbers list, 22 22 00:00:56,410 --> 00:00:59,220 create a new variable for each of the numbers in it, 23 23 00:00:59,220 --> 00:01:01,450 and then run through it three times, 24 24 00:01:01,450 --> 00:01:03,620 each time putting the appropriate number, 25 25 00:01:03,620 --> 00:01:06,340 multiplied by two, into your new list. 26 26 00:01:06,340 --> 00:01:08,970 Python has a much more succinct way of doing this 27 27 00:01:08,970 --> 00:01:10,950 which is with a list comprehension. 28 28 00:01:10,950 --> 00:01:12,190 In a list comprehension, 29 29 00:01:12,190 --> 00:01:16,310 the order of these operations, first the iteration, 30 30 00:01:16,310 --> 00:01:19,810 and then the appending, seems to be reversed, 31 31 00:01:19,810 --> 00:01:21,570 but it's not really. 32 32 00:01:21,570 --> 00:01:23,920 So the first thing you do is you would put in 33 33 00:01:23,920 --> 00:01:26,970 what you want to add to your new list, 34 34 00:01:26,970 --> 00:01:28,673 which is num times two. 35 35 00:01:29,640 --> 00:01:32,800 But of course, num doesn't exist in this context. 36 36 00:01:32,800 --> 00:01:36,250 It only existed here because we've got our for loop. 37 37 00:01:36,250 --> 00:01:38,883 So the next thing you do is you put your for loop, 38 38 00:01:40,534 --> 00:01:44,200 and what this does in Python is it puts num times two 39 39 00:01:44,200 --> 00:01:49,200 into a new list for every num in the numbers list. 40 40 00:01:49,710 --> 00:01:52,970 So essentially you're iterating over the numbers list, 41 41 00:01:52,970 --> 00:01:56,216 and you're putting the variable multiplied by two 42 42 00:01:56,216 --> 00:02:00,600 every time that the loop runs into your new list. 43 43 00:02:00,600 --> 00:02:03,850 You end up with the doubled numbers. 44 44 00:02:03,850 --> 00:02:06,790 Because list comprehension are usually in a single line, 45 45 00:02:06,790 --> 00:02:09,740 you do want to keep them short if you can, 46 46 00:02:09,740 --> 00:02:12,210 so instead of num times two for example in this case, 47 47 00:02:12,210 --> 00:02:15,640 you could do x times two, and for x numbers. 48 48 00:02:15,640 --> 00:02:17,780 That just helps you keep it a little bit shorter 49 49 00:02:17,780 --> 00:02:19,280 and a little bit more readable. 50 50 00:02:19,280 --> 00:02:20,870 After all, the most important thing here 51 51 00:02:20,870 --> 00:02:25,050 is that you're putting x times two into your new list 52 52 00:02:25,050 --> 00:02:27,510 while you iterate over the previous list. 53 53 00:02:27,510 --> 00:02:31,000 So again to recap, how you build a list comprehension 54 54 00:02:31,000 --> 00:02:33,900 is what you wanna put into your new list 55 55 00:02:33,900 --> 00:02:38,583 for the variable that you're using in numbers. 56 56 00:02:40,150 --> 00:02:42,740 Now let's say you've got a friend's list, 57 57 00:02:42,740 --> 00:02:44,730 and you wanna create a new list 58 58 00:02:44,730 --> 00:02:47,370 which contains the names of those friends 59 59 00:02:47,370 --> 00:02:49,500 that start with an s. 60 60 00:02:49,500 --> 00:02:51,820 So Sam, Samantha, and Saurabh. 61 61 00:02:51,820 --> 00:02:54,140 But it seems that a list comprehension 62 62 00:02:54,140 --> 00:02:56,070 may not be the tool for this job. 63 63 00:02:56,070 --> 00:03:00,030 After all, you do need to only get some of these out. 64 64 00:03:00,030 --> 00:03:01,600 Let's start with a for loop, 65 65 00:03:01,600 --> 00:03:03,580 and then I'll show you how the list comprehension 66 66 00:03:03,580 --> 00:03:04,920 can deal with it. 67 67 00:03:04,920 --> 00:03:08,160 So you would do for friend in friends. 68 68 00:03:08,160 --> 00:03:10,660 Then you would add an if statement to check 69 69 00:03:10,660 --> 00:03:14,030 that the friend name in question starts with an s. 70 70 00:03:14,030 --> 00:03:18,293 You could say if friend.startswith capital s, 71 71 00:03:19,340 --> 00:03:24,010 then you're gonna do starts_s.append friend, 72 72 00:03:24,010 --> 00:03:28,660 and then at the end, we're gonna print starts_s like that. 73 73 00:03:28,660 --> 00:03:30,900 Let me run this and show you what happens. 74 74 00:03:30,900 --> 00:03:33,870 Down here, you have Sam, Samantha, and Saurabh, 75 75 00:03:33,870 --> 00:03:35,370 and I think I'm not pronouncing that right, 76 76 00:03:35,370 --> 00:03:38,120 so my apologies if that's the case. 77 77 00:03:38,120 --> 00:03:40,580 So what we've got is a for loop, 78 78 00:03:40,580 --> 00:03:44,310 and then an if statement, and then an append. 79 79 00:03:44,310 --> 00:03:47,310 This structure here can also be reproduced 80 80 00:03:47,310 --> 00:03:50,780 with a list comprehension although it does start to get 81 81 00:03:50,780 --> 00:03:52,363 a little bit more confusing. 82 82 00:03:53,500 --> 00:03:57,140 Again, the process is more or less the same. 83 83 00:03:57,140 --> 00:04:00,480 First of all, you put what you wanna add to your new list, 84 84 00:04:00,480 --> 00:04:01,873 so that's your friend name. 85 85 00:04:02,780 --> 00:04:05,480 Then you put your iteration or your for loop, 86 86 00:04:05,480 --> 00:04:08,020 for friend in friends. 87 87 00:04:08,020 --> 00:04:10,760 And now if you wanna add an if statement, 88 88 00:04:10,760 --> 00:04:12,040 you do it at the end, 89 89 00:04:12,040 --> 00:04:13,970 so you're gonna add your friend 90 90 00:04:13,970 --> 00:04:18,970 for the friend in your list if the friend starts with s, 91 91 00:04:21,830 --> 00:04:23,340 and that does exactly the same thing, 92 92 00:04:23,340 --> 00:04:26,020 and you no longer need that loop there, 93 93 00:04:26,020 --> 00:04:27,570 so you can save that and run it again, 94 94 00:04:27,570 --> 00:04:30,230 and you see that the same thing comes out. 95 95 00:04:30,230 --> 00:04:33,870 So this is how you add a conditional, an if statement, 96 96 00:04:33,870 --> 00:04:36,023 into a list comprehension. 97 97 00:04:37,170 --> 00:04:38,650 Very important to nice that 98 98 00:04:38,650 --> 00:04:40,770 when you use a list comprehension, 99 99 00:04:40,770 --> 00:04:43,430 a new list is created, 100 100 00:04:43,430 --> 00:04:47,930 so even though these two lists may be the same 101 101 00:04:47,930 --> 00:04:49,483 if we take these names out, 102 102 00:04:52,350 --> 00:04:53,860 so now these two lists will be the same 103 103 00:04:53,860 --> 00:04:55,210 if we print them both out, 104 104 00:04:55,210 --> 00:04:57,210 you'll see that they are both identical. 105 105 00:04:58,490 --> 00:05:01,500 When you print friends is starts_s, 106 106 00:05:01,500 --> 00:05:03,713 you'll see that false comes out. 107 107 00:05:05,430 --> 00:05:07,220 That's because these two lists, 108 108 00:05:07,220 --> 00:05:09,470 although they have the same content, 109 109 00:05:09,470 --> 00:05:11,370 are not the same list. 110 110 00:05:11,370 --> 00:05:13,880 They are two different lists, two different places 111 111 00:05:13,880 --> 00:05:17,803 in your computer's memory with the same values. 112 112 00:05:18,840 --> 00:05:22,460 For context, friends zero is starts_s a zero, 113 113 00:05:22,460 --> 00:05:27,460 so Sam is Sam will give you true, see that? 114 114 00:05:27,640 --> 00:05:30,380 But the lists themselves are not the same, 115 115 00:05:30,380 --> 00:05:31,960 so the elements in them are the same, 116 116 00:05:31,960 --> 00:05:34,270 but the lists themselves are not the same, 117 117 00:05:34,270 --> 00:05:38,860 and by using the is keyword, then you can check that, 118 118 00:05:38,860 --> 00:05:41,840 but there's also something else you can do, 119 119 00:05:41,840 --> 00:05:44,870 which is check the IDs of these lists, 120 120 00:05:44,870 --> 00:05:47,833 so you can do something like friends, 121 121 00:05:49,160 --> 00:05:51,620 then print out the ID of friends, 122 122 00:05:51,620 --> 00:05:56,620 and then starts_s and print the ID of starts-s, 123 123 00:05:57,310 --> 00:05:58,460 and what I've done here by the way 124 124 00:05:58,460 --> 00:06:01,470 is I have passed multiple different things 125 125 00:06:01,470 --> 00:06:03,937 to the print function, and you can do that in print, 126 126 00:06:03,937 --> 00:06:05,610 and it just separates them with a space, 127 127 00:06:05,610 --> 00:06:08,780 so it's gonna separate these different parts with a space, 128 128 00:06:08,780 --> 00:06:10,030 the different arguments there. 129 129 00:06:10,030 --> 00:06:15,030 So by running this, you get that friends has an ID 130 130 00:06:16,780 --> 00:06:21,530 of this, ends in 2096, and starts_s has an ID 131 131 00:06:21,530 --> 00:06:24,120 that ends in 5280. 132 132 00:06:24,120 --> 00:06:27,560 So these two have different IDs, and in Cpython, 133 133 00:06:27,560 --> 00:06:29,040 which is what most of you will be using, 134 134 00:06:29,040 --> 00:06:30,330 certainly what I'm using, 135 135 00:06:30,330 --> 00:06:32,997 the ID is related to the memory address 136 136 00:06:32,997 --> 00:06:35,360 in which the list is stored, 137 137 00:06:35,360 --> 00:06:37,330 so you can see that these two things 138 138 00:06:37,330 --> 00:06:38,710 are in different places in memory. 139 139 00:06:38,710 --> 00:06:40,960 They're different objects entirely. 140 140 00:06:40,960 --> 00:06:43,650 They just happen to have the same contents. 141 141 00:06:43,650 --> 00:06:46,510 So this is something that will be very important 142 142 00:06:46,510 --> 00:06:48,340 as we proceed with the course, 143 143 00:06:48,340 --> 00:06:51,060 so this is why I wanted to show it to you at this point. 144 144 00:06:51,060 --> 00:06:54,140 Creating new lists gives you a different thing 145 145 00:06:54,140 --> 00:06:55,680 entirely in Python. 146 146 00:06:55,680 --> 00:06:57,560 It's not the same as the thing before 147 147 00:06:57,560 --> 00:06:59,410 even if it has the same contents, 148 148 00:06:59,410 --> 00:07:00,350 but as I mentioned earlier, 149 149 00:07:00,350 --> 00:07:03,890 if you do want them to be the same exact list, 150 150 00:07:03,890 --> 00:07:05,760 then you can do this. 151 151 00:07:05,760 --> 00:07:08,610 Starts_s equal friends, and now when you press play, 152 152 00:07:08,610 --> 00:07:09,730 you'll see that you get true, 153 153 00:07:09,730 --> 00:07:11,760 and these IDs are identical. 154 154 00:07:11,760 --> 00:07:15,620 So that is how you make two lists the same thing. 155 155 00:07:15,620 --> 00:07:18,500 Otherwise, you do end up with two different lists. 156 156 00:07:18,500 --> 00:07:19,730 All right, that's enough of that. 157 157 00:07:19,730 --> 00:07:21,020 Thank you for joining me in this video. 158 158 00:07:21,020 --> 00:07:22,140 I hope it's been useful, 159 159 00:07:22,140 --> 00:07:23,790 and I'll see you in the next one.