1 00:00:05,580 --> 00:00:11,570 Alright so we left the last video with a challenge: to convert a nested for loop into a list comprehension. 2 00:00:11,740 --> 00:00:13,120 So let's go through the solution, 3 00:00:13,120 --> 00:00:14,640 one solution to that. 4 00:00:14,720 --> 00:00:17,370 So we're going to start on line 11, so times 5 00:00:17,650 --> 00:00:24,840 equals left square bracket, or left and right square brackets, in parentheses i comma space 6 00:00:24,850 --> 00:00:34,770 i times j, then a right parentheses, then we want for i in range, parentheses one comma space 11, closing 7 00:00:34,780 --> 00:00:41,750 parentheses, for j in range, parentheses 1 comma space 11. 8 00:00:42,040 --> 00:00:47,480 Then we've got our right closing, right parentheses and closing square bracket. Then on the next line 9 00:00:47,480 --> 00:00:51,510 we can print parentheses times. 10 00:00:51,640 --> 00:00:55,430 So once again that reads pretty much the same as the nested for loops. 11 00:00:55,430 --> 00:00:58,580 We run the program, 12 00:00:59,330 --> 00:01:05,269 and you can see that we're getting a list of tuples, very similar to the, or they should be identical to 13 00:01:05,269 --> 00:01:08,330 the first one you, left to right there. 14 00:01:10,530 --> 00:01:15,960 So obviously, here each tuple is containing the number from 1 to 10, with its product for values from 1 15 00:01:15,960 --> 00:01:16,890 to 10. 16 00:01:16,890 --> 00:01:22,350 Now if we wanted to print the same output as the nested for loops, just unpack the tuple when iterating over 17 00:01:22,350 --> 00:01:23,450 the list. 18 00:01:23,460 --> 00:01:26,210 So let's just do that, so we'll just close down the run window. 19 00:01:26,460 --> 00:01:30,470 We can just copy the output here and then we'll change it a bit. 20 00:01:30,480 --> 00:01:33,360 So I'm going to copy that, and we'll change it. 21 00:01:33,360 --> 00:01:42,500 So it's going to be for x comma space y in, then just a colon on the end. 22 00:01:43,890 --> 00:01:51,030 Then we can print x comma y. As it turns out I'll just delete that extra times there. Alright so lets just 23 00:01:51,030 --> 00:01:59,690 try running that, and you can see we got the same output now than from the original nested for loop example, 24 00:02:00,280 --> 00:02:01,370 the top of the file. 25 00:02:05,250 --> 00:02:07,410 OK, so that's the challenge completed. If you got the same results, 26 00:02:07,410 --> 00:02:08,910 congratulations. 27 00:02:08,910 --> 00:02:13,230 Now you may also have used a comprehension as the expression. 28 00:02:13,230 --> 00:02:21,360 So if I go back and just show you what I mean by that, you may have done it this way: then I'll paste that in line 17, then we'll say times 29 00:02:21,380 --> 00:02:28,890 2 equals, so you might have added two square brackets like we've used previously. 30 00:02:29,340 --> 00:02:32,040 Then add the right square bracket 31 00:02:32,040 --> 00:02:33,610 after the first range, 32 00:02:33,720 --> 00:02:40,620 so basically after here, after the right parentheses on the range, and print times two. 33 00:02:45,750 --> 00:02:50,720 A similar thing as you can see but, the structure's different though if you do it that way but of course the 34 00:02:51,140 --> 00:02:53,390 end result is still correct. 35 00:02:53,390 --> 00:02:55,430 We've got the output that we're, 36 00:02:55,610 --> 00:02:59,800 we've actually got it going through using nested comprehensions. 37 00:02:59,900 --> 00:03:04,690 Now if you want it to have been completely accurate, you should also have swapped i and j around. 38 00:03:04,820 --> 00:03:10,460 So the first loop is for j then the outer loop at the end is for i. Of course as they're both looping through 39 00:03:10,460 --> 00:03:13,520 exactly the same set of values it doesn't make any difference here. 40 00:03:14,000 --> 00:03:18,020 Alright so we've used a list comprehension here, which means it builds up a list containing all the 41 00:03:18,020 --> 00:03:19,100 values. 42 00:03:19,110 --> 00:03:23,340 Now that could use up a lot of memory, especially with very large ranges. 43 00:03:23,350 --> 00:03:28,270 Now if you intend to use the list again, then a list comprehension does make sense. 44 00:03:28,670 --> 00:03:34,470 And in our original solution, we could use the times list as often as necessary in the code. 45 00:03:34,630 --> 00:03:39,020 Now if you just want to iterate over the list as we're doing here, then a generator expression is probably 46 00:03:39,020 --> 00:03:41,780 more suitable than a list comprehension. 47 00:03:41,860 --> 00:03:44,740 Now generator expressions are very similar to write. 48 00:03:45,020 --> 00:03:48,470 We just replace the square brackets with parentheses. 49 00:03:48,500 --> 00:03:57,840 So if I go back and copy lines 14 and 15 again. I'm going to paste them in, see I'll put that up there to be consistent. 50 00:03:58,550 --> 00:04:01,320 So we've got here, the square bracket, 51 00:04:01,330 --> 00:04:04,280 I'm going to change that to a parentheses, 52 00:04:04,460 --> 00:04:11,990 then the second square bracket. Come down to the end here, and replace that with parentheses, and just 53 00:04:11,990 --> 00:04:18,589 run it to confirm it is actually working. 54 00:04:18,690 --> 00:04:20,860 And you can see we've got the same output there. 55 00:04:22,370 --> 00:04:28,400 But the difference here with this code is that it's a generator expression, and it isn't creating the entire 56 00:04:28,400 --> 00:04:29,630 list in memory. 57 00:04:29,630 --> 00:04:32,930 Now just like generators, and we saw those a few videos ago, 58 00:04:33,050 --> 00:04:39,080 generator expressions calculate the values as they're requested. The only value that exists is the one that's 59 00:04:39,080 --> 00:04:40,700 just been returned. 60 00:04:40,730 --> 00:04:46,470 Now bear that in mind and consider using a generator expression rather than a list comprehension, 61 00:04:46,520 --> 00:04:49,460 if you're not going to be using the list more than once. 62 00:04:49,550 --> 00:04:54,380 Now one thing that's currently unclear, is whether performance suffers when you use generator expressions 63 00:04:54,620 --> 00:04:56,510 rather than list comprehensions. 64 00:04:56,540 --> 00:05:02,970 Now Guido van Rossum said that they should both be as fast as each other in the blog post that I've linked to previously. 65 00:05:03,370 --> 00:05:06,860 However that was written quite a while ago and things may well have changed. 66 00:05:06,860 --> 00:05:10,880 And also he was only talking about the CPython implementations of Python. 67 00:05:10,880 --> 00:05:14,910 If you're using a different implementation, things may well be different. 68 00:05:15,350 --> 00:05:17,430 So that says, does performance suffer. 69 00:05:17,750 --> 00:05:23,100 Well I'm not going to tell you. Instead though, what we're going to do is look at how you can find out for yourself. 70 00:05:23,180 --> 00:05:29,210 Now Python has a timeit module in the standard library, and we can use that to compare the performance 71 00:05:29,480 --> 00:05:33,640 of different code snippets, and we'll start working on that in the next video. 72 00:05:33,920 --> 00:05:39,950 But before that, I promised that we'd use a nested comprehension to produce the list of destinations 73 00:05:39,950 --> 00:05:41,410 from an earlier challenge, 74 00:05:41,420 --> 00:05:42,810 that's Challenge 2. 75 00:05:42,860 --> 00:05:51,000 Now what I'm going to do, is go back to that Challenge 2 solution, that was compchallenge2. Alright so lets 76 00:05:51,020 --> 00:05:56,780 go ahead and do that, so actually what I'm going to do is do a file new, Python file, 77 00:05:56,790 --> 00:06:07,840 and we'll call this Challenge 2b, I'll call it compchallenge2b. I'm going to paste in the first part of the code, which 78 00:06:07,840 --> 00:06:13,930 we were aware of before, the locations and exits. Right so lets type in some code here now, so I'm going to type, 79 00:06:14,340 --> 00:06:17,620 so print parentheses nested for loops, 80 00:06:21,040 --> 00:06:23,780 print, let's just do a bit of a header there, 81 00:06:26,670 --> 00:06:40,080 for loc in sorted, parentheses locations colon, exits to destination underscore one, equals left and right square brackets. 82 00:06:40,960 --> 00:06:42,810 Then we want for exit, 83 00:06:42,810 --> 00:06:52,230 xit in exits colon, if loc in exits, left and right square brackets, and xit in those square brackets, 84 00:06:52,580 --> 00:06:58,200 dot values. You've seen a lot of this code before but it's helpful to type this in again, so we can see the list comprehension 85 00:06:58,200 --> 00:07:02,700 shortly, colon then exits 86 00:07:03,140 --> 00:07:10,930 underscore to underscore destination underscore one dot append, then two parentheses, 87 00:07:11,170 --> 00:07:18,270 xit comma space locations, left and right square brackets, xit within that, closing square bracket. Then 88 00:07:18,360 --> 00:07:26,310 we've got two right parentheses, and I've added three there to the append, that should only have been two. Lets fix that up, fixes 89 00:07:26,310 --> 00:07:35,400 that line up, and on the next line then, we go back to our for loop, to the same code level, and we'll print parentheses 90 00:07:35,420 --> 00:07:46,610 locations leading to, left and right curly braces, double quotes dot format, parentheses loc comma, then end 91 00:07:46,620 --> 00:07:55,800 equals, in single quotes a tab character, so that's backslash t parentheses. Then we'll do a print exits to destination 92 00:07:55,790 --> 00:07:56,400 underscore one. 93 00:07:59,530 --> 00:08:02,220 Right, then we'll just do an empty print there. 94 00:08:02,770 --> 00:08:04,690 Alright, so that's the nested for loop. 95 00:08:04,690 --> 00:08:08,690 Now let's look at creating the list comprehension inside a for loop. 96 00:08:08,970 --> 00:08:19,760 So we're going to do a print. print parentheses double quotes list comprehension inside a for loop, 97 00:08:26,080 --> 00:08:26,990 bit of a header there 98 00:08:31,060 --> 00:08:39,490 again, for loc in sorted locations colon, then exits to destination. 99 00:08:39,500 --> 00:08:47,840 This time it's going to be 2, equals left and right square brackets, parentheses xit comma locations, left and right 100 00:08:48,290 --> 00:08:55,810 square brackets, then xit, then our closing square bracket, closing parentheses, then for exit, 101 00:08:56,330 --> 00:08:57,610 xit in exits, 102 00:08:57,640 --> 00:09:06,530 e x i t s, if loc in exits, left and right square brackets, xit dot values parentheses. 103 00:09:06,650 --> 00:09:11,240 Then we've got our closing right square bracket, and then we just do print 104 00:09:14,200 --> 00:09:24,870 locations leading to left and right curly braces, double quotes dot format, then we've got loc comma, then the same end 105 00:09:24,870 --> 00:09:33,820 equals, single quotes and our tab character, backslash t again. Then we're just going to print out 'exits to destination 106 00:09:33,870 --> 00:09:40,090 2', and we'll just do an empty print on line 34. 107 00:09:40,090 --> 00:09:46,780 Right so that's two examples of producing lists of all the locations leading to each of the destinations. 108 00:09:46,790 --> 00:09:47,690 So if we run the program, 109 00:09:51,360 --> 00:09:56,110 I'll just make this a little bit bigger so we can see both examples there, and left to right you can see the output. 110 00:09:59,780 --> 00:10:04,440 So for each destination we get a list of the locations that lead directly to it. 111 00:10:04,510 --> 00:10:09,470 So we're getting six separate lists, which means we're going to need a listed list comprehension if you 112 00:10:09,470 --> 00:10:11,320 want to keep the same sort of structure. 113 00:10:11,830 --> 00:10:19,380 And obviously you can see here when we run this that we've got the same output for both methods. 114 00:10:19,470 --> 00:10:25,000 So again the first one was a nested for loop, the second was a list comprehension inside a for loop. 115 00:10:25,330 --> 00:10:29,280 And importantly remember that we need to put the outer loop at the end of the comprehension. 116 00:10:29,500 --> 00:10:31,310 So our nested comprehension then, 117 00:10:31,330 --> 00:10:34,900 let's just add that, it's going to be as follows: 118 00:10:34,900 --> 00:10:37,820 so I'm going to do that on line 36, we'll start, 119 00:10:38,740 --> 00:10:40,840 that's going to be print, lets do our nested, 120 00:10:47,620 --> 00:10:50,320 print, then a bit of a heading there. 121 00:10:57,390 --> 00:11:04,470 Alright, and so exits to destination 3 equals, lets add that. 122 00:11:04,510 --> 00:11:06,330 So we've done our two examples 123 00:11:06,430 --> 00:11:14,140 above. Lets do the third one, so it's going to be two square brackets, parentheses xit comma then locations, 124 00:11:14,720 --> 00:11:21,030 left and right square brackets, and xit within that, then closing parentheses, for xit in exits, 125 00:11:21,730 --> 00:11:22,610 in e x i t s that is. 126 00:11:22,630 --> 00:11:24,600 The second one, 127 00:11:25,120 --> 00:11:28,320 if loc in exits, 128 00:11:28,620 --> 00:11:35,440 so left and right square brackets, xit dot values parentheses, 129 00:11:35,520 --> 00:11:43,150 oops parentheses is already added, then for loc in sorted locations. 130 00:11:45,430 --> 00:11:50,210 Then we've got our closing parentheses and closing right bracket, and after the values it should've been a square 131 00:11:50,210 --> 00:11:51,030 bracket as well. 132 00:11:51,110 --> 00:12:03,430 Put square bracket, then let's print exits to destination underscore 3, and lets just make that so it fits better. 133 00:12:03,600 --> 00:12:05,480 I'm going to put that here, put the line break so that it goes 134 00:12:05,540 --> 00:12:08,910 to the next line. That's a little bit nicer to see now. 135 00:12:09,320 --> 00:12:16,940 So basically the output when we run it, contains each of the six lists of the previous two code snippets 136 00:12:16,940 --> 00:12:17,930 created. 137 00:12:18,080 --> 00:12:23,200 Now being a nested list, a list containing other lists is not formatting as nicely. 138 00:12:23,360 --> 00:12:31,940 So we can see the six locations, from 0 to 5 in the first list. I'll just move that over, 2, 3, 4, 5, but 139 00:12:31,940 --> 00:12:33,450 then a new list starts, and 140 00:12:33,490 --> 00:12:40,310 you see that it contains location 3, 4 and then 5 and so on, and it goes on to another one. 141 00:12:40,310 --> 00:12:47,910 Basically the one after that is 1, 4 and 5 and the next 2, just scrolling over, only contained location 1 because 142 00:12:47,910 --> 00:12:56,130 the last list has locations 1 and 2, and everything agrees with the output produced using the previous two methods. 143 00:12:56,140 --> 00:13:00,810 Now that said, if you wanted to print it out as prettily as the first two bis of code, 144 00:13:00,910 --> 00:13:07,370 what we can do is use the enumerate function to get the index of each list in the other list. 145 00:13:07,480 --> 00:13:08,790 So lets go about doing that. 146 00:13:11,740 --> 00:13:13,970 So we'll have a print there, 147 00:13:14,980 --> 00:13:21,010 then for index comma space loc in enumerate 148 00:13:21,510 --> 00:13:31,450 then it's going to be parentheses exits to destination 3 colon, and we'll do a print, parentheses locations leading 149 00:13:31,450 --> 00:13:41,500 to, left and right curly braces, dot format parentheses and index within that, comma, and end equals, in single 150 00:13:41,500 --> 00:13:43,970 quotes our tab character, backslash t, 151 00:13:44,370 --> 00:13:52,130 then you could print loc. So we can run that, and you can see we've now got our output 152 00:13:55,100 --> 00:14:00,400 in the correct format. So Python contains a load of useful functions such as 153 00:14:00,410 --> 00:14:05,390 enumerate, and if you weren't aware that it existed you might mess around incrementing an index variable 154 00:14:05,630 --> 00:14:06,830 inside the for loop, 155 00:14:07,040 --> 00:14:09,890 but enumerate takes care of all of that for us. 156 00:14:09,970 --> 00:14:15,230 And if you haven't already, I'd suggest it's worth spending time to become familiar with the builtin functions, 157 00:14:15,780 --> 00:14:19,540 and you can find the documentation list, or find the documentation I should say, 158 00:14:22,510 --> 00:14:26,880 here, and I'll put a link in the resources section of the video. 159 00:14:26,990 --> 00:14:28,670 That's a good place to start. 160 00:14:28,790 --> 00:14:31,940 Now, I'm not suggesting you memorise all these functions. 161 00:14:32,030 --> 00:14:37,160 Just be aware that they exist so that you can refer back here when you need to know how to use them. 162 00:14:37,160 --> 00:14:41,690 There's a lot of very useful functions in the standard library, and we're going to be looking at a few 163 00:14:41,690 --> 00:14:44,720 that are related to comprehensions after the next video. 164 00:14:44,990 --> 00:14:48,890 But for now I'm going to stop this video because it's got quite long, and in the next one we're going 165 00:14:48,890 --> 00:14:54,080 to look at that timeit module that I mentioned previously, to see how these various approaches compare 166 00:14:54,410 --> 00:14:56,230 in terms of performance. 167 00:14:56,240 --> 00:14:57,670 So see you in the next video.