1 00:00:05,720 --> 00:00:10,560 Alright, so firstly, before we move on to conditional comprehensions, I just want to point out that I've created 2 00:00:10,560 --> 00:00:15,870 a new project, comprehensions, and I've just removed all the music files that were hanging around from 3 00:00:15,870 --> 00:00:22,460 a previous project, just to clear things up. So you can grab a download of that file from the last video. 4 00:00:22,560 --> 00:00:26,580 That's got everything that you're seeing at the start of this video. So moving on as 5 00:00:26,580 --> 00:00:29,940 I mentioned - conditional comprehensions. 6 00:00:30,000 --> 00:00:35,750 So it's actually very common to have an if clause inside a for loop, and we can do that with comprehensions 7 00:00:35,760 --> 00:00:36,760 as well. 8 00:00:36,760 --> 00:00:42,930 Now in one of the earlier videos about lists, we created a menu and then we printed out all the meals that 9 00:00:42,930 --> 00:00:44,610 didn't contain spam. 10 00:00:44,850 --> 00:00:47,230 So I'm going to put that code into a new file here. 11 00:00:47,550 --> 00:00:52,040 But also that file is available in the resources section of this video if you want to grab it and you 12 00:00:52,050 --> 00:00:53,590 don't care to type it in again. 13 00:00:54,060 --> 00:00:55,550 So the file I'm going to create here, 14 00:00:57,240 --> 00:00:59,100 I'm going to call it condcomp, 15 00:01:02,130 --> 00:01:03,900 conditional comprehensions. 16 00:01:04,379 --> 00:01:08,120 OK, I'm going to paste that code in, like so, 17 00:01:09,780 --> 00:01:12,620 and we'll just run the program to make sure that it works still. 18 00:01:13,820 --> 00:01:18,120 And you can see it's working correctly down the bottom. That's giving us all the meals without spam. 19 00:01:18,120 --> 00:01:23,750 Now we can do the same thing with a comprehension by adding an if clause to that comprehension. 20 00:01:23,880 --> 00:01:24,890 So let's go and do that, 21 00:01:24,890 --> 00:01:27,120 starting on line 17, and we'll type 22 00:01:27,120 --> 00:01:34,850 meals equals left square brackets meal for meal in menu. 23 00:01:35,460 --> 00:01:39,230 And then the if clause is, if double quotes spam double quotes, 24 00:01:39,640 --> 00:01:43,510 basically spam in double quotes, not in meal. 25 00:01:43,830 --> 00:01:51,900 Then we've got our closing right square bracket, and we can print out the result, meals. So when I to run that, we should 26 00:01:51,900 --> 00:01:57,100 get the correct meals in our lists, and you can see down here that that has actually happened. 27 00:01:58,540 --> 00:02:04,510 So our comprehension now specifies a filter, as you can see on screen, extending the incomplete definition 28 00:02:04,660 --> 00:02:05,910 that we saw earlier. 29 00:02:06,160 --> 00:02:12,010 Now a comprehension has an expression, just the value of meal in this example, but the expression can be 30 00:02:12,010 --> 00:02:13,310 much more complicated. 31 00:02:13,750 --> 00:02:19,330 Next is the iteration, and that can be followed by one or more filters. 32 00:02:19,330 --> 00:02:25,290 Alright so back in IntelliJ. So, I'm not making the comprehensions print out the values because I've showed 33 00:02:25,470 --> 00:02:26,770 you how to do that earlier. 34 00:02:27,010 --> 00:02:33,090 It's actually clearer here to see what the comprehension produces if we just print out the list itself. 35 00:02:33,100 --> 00:02:38,800 Now I could get exactly the same behavior by appending the meals to a list in the for loop, but that 36 00:02:38,800 --> 00:02:41,400 doesn't really add anything to the explanation. 37 00:02:41,410 --> 00:02:43,560 So just to be clear though, I'll quickly do that. 38 00:02:43,560 --> 00:02:50,170 So I'm going to add a line here, above the for, the for loops, so meals equals, creating an empty list, left and 39 00:02:50,170 --> 00:02:51,970 right square brackets. 40 00:02:51,970 --> 00:03:01,370 Then after the 'if spam not in meal' we can put meals dot append meal. 41 00:03:02,250 --> 00:03:06,990 And what I could do then is indent this one back to print meal, to make that then print 42 00:03:07,060 --> 00:03:13,570 meals. Run it and basically we've got the same result, but that's just taking up more time and really doesn't 43 00:03:13,570 --> 00:03:15,460 add anything to the explanation. 44 00:03:15,460 --> 00:03:15,690 Alright, 45 00:03:15,700 --> 00:03:22,120 so what we've done there in the comprehension, well all I've done is add an if clause to the end, and I'm talking 46 00:03:22,120 --> 00:03:23,650 about this code on line 19. 47 00:03:24,000 --> 00:03:27,910 In fact the comprehension reads almost the same as the for loop. 48 00:03:28,000 --> 00:03:33,650 Now the values added to the list will be meal for each meal in menu, and we're adding the condition 49 00:03:33,820 --> 00:03:37,180 if spam and double quotes not in meal. 50 00:03:37,190 --> 00:03:42,570 Now that's probably straightforward but it may not be obvious exactly what's happening here. 51 00:03:42,700 --> 00:03:48,460 Now looking at it at face value it looks like a normal condition using if, but in fact it's more like 52 00:03:48,460 --> 00:03:49,600 a filter. 53 00:03:49,780 --> 00:03:54,960 So the meal here is only included in the list if the condition is true, 54 00:03:55,300 --> 00:03:57,130 otherwise it's skipped. 55 00:03:57,340 --> 00:03:58,830 So does the difference matter? 56 00:03:59,080 --> 00:04:04,540 Well the answer to that is, it can do. So I modified the loop earlier, so lets just add the message to the list 57 00:04:04,540 --> 00:04:05,960 instead of printing it. 58 00:04:06,010 --> 00:04:08,320 So back up here, we've got meal dot append, 59 00:04:08,330 --> 00:04:13,930 if not spam, but we can put an else in here, 60 00:04:14,150 --> 00:04:19,360 and I'll put meals dot append. Because we're not printing it out here, 61 00:04:19,510 --> 00:04:25,870 we'll just put an entry to make it obvious a meal was skipped. Alright 62 00:04:25,870 --> 00:04:26,070 so run it, 63 00:04:28,990 --> 00:04:35,420 and you can see that that seems to behave as we'd expect it to behave, showing the entries that have been skipped, 64 00:04:35,420 --> 00:04:39,620 and also showing the entries that were included because they weren't spam. 65 00:04:39,620 --> 00:04:42,620 So that's a very common thing to see in a for loop. 66 00:04:42,890 --> 00:04:46,390 So how do we get our comprehensions to do the same thing? 67 00:04:46,430 --> 00:04:52,360 So at first attempt you might come back to that comprehension which is now line 21, and add the else 68 00:04:52,370 --> 00:04:57,850 clause here. So you might put else and double quotes, a meal. 69 00:05:01,590 --> 00:05:02,560 I can't type! 70 00:05:02,860 --> 00:05:06,130 But obviously, that's giving us an error so there's no point me continuing on to that. 71 00:05:06,130 --> 00:05:08,120 So how do we actually fix this? 72 00:05:08,170 --> 00:05:11,100 Well the answer is we can't. 73 00:05:11,230 --> 00:05:18,220 The if in our comprehension is being used to filter the meals, and you can't have an else clause in there. 74 00:05:18,220 --> 00:05:20,910 So the key here to keep in mind is the word filter. 75 00:05:21,250 --> 00:05:27,070 So we're not making up new values, we're just iterating over the sequence and allowing through certain values 76 00:05:27,400 --> 00:05:29,090 based on that condition. 77 00:05:29,100 --> 00:05:31,470 Now if a value meets the condition, it's allowed in, 78 00:05:31,570 --> 00:05:36,150 otherwise it's not. So filter either lets things through or it doesn't. 79 00:05:36,340 --> 00:05:41,330 So in other words it can't be used to produce anything else, which is why we can't use an else clause. 80 00:05:41,360 --> 00:05:42,150 We're getting an error. 81 00:05:42,400 --> 00:05:47,580 But don't despair because there's something else we can do in this case. 82 00:05:47,590 --> 00:05:52,180 Now I don't want to digress too much at the moment, so I'll cover that a bit later. For now though, 83 00:05:52,210 --> 00:05:54,810 I'm just going to undo that change and we'll look at that 84 00:05:54,820 --> 00:05:57,660 if filter in a bit more detail. 85 00:05:58,440 --> 00:06:01,530 OK so what if we didn't want chicken in the meal either? 86 00:06:01,870 --> 00:06:04,720 Now this syntax is going to look a bit strange but we can actually do this. 87 00:06:04,720 --> 00:06:16,100 We can put, we've got our 'if spam not in meal', we can come along on the end here and put 'if chicken not in meal', 88 00:06:16,660 --> 00:06:18,670 remembering that we had chicken in the output before. 89 00:06:18,850 --> 00:06:23,270 Run it this time and you can see that the chicken has been successfully removed. 90 00:06:23,650 --> 00:06:28,610 Now you may see filters written that way, and it's actually certainly perfectly valid. 91 00:06:28,630 --> 00:06:34,180 In fact it can be a good way to make complicated conditions more readable, but a more usual way to write 92 00:06:34,180 --> 00:06:35,380 that would be to use 93 00:06:35,380 --> 00:06:44,920 and. So lets duplicate this line and we'll comment out the first one, and we're going to change this line. And 94 00:06:44,920 --> 00:06:52,990 the obvious change here would be 'if spam not in meal', and instead of 'if' there, we use the word 'and', 'and chicken not 95 00:06:52,990 --> 00:06:53,920 in meal'. 96 00:06:54,060 --> 00:06:59,340 And if we run that just to confirm that it works, you can see we get the same result there - 97 00:06:59,360 --> 00:07:01,440 we've got no spam but also no chicken. 98 00:07:01,780 --> 00:07:07,240 So we've presented many alternative ways of doing the same thing throughout the course, and a common question 99 00:07:07,240 --> 00:07:10,090 we get asked is "which should I use"? 100 00:07:10,180 --> 00:07:11,990 Now sometimes that's easy to answer 101 00:07:12,160 --> 00:07:14,850 and generally, I'll mention that in the videos. 102 00:07:15,070 --> 00:07:17,150 But other times it really doesn't make a difference, 103 00:07:17,320 --> 00:07:21,760 and this is one of those other times. Use whichever form here makes the most sense. 104 00:07:21,940 --> 00:07:27,550 You can even mix them, which is what I meant when I said it can make complicated conditions more readable. 105 00:07:27,550 --> 00:07:29,240 Now to see what I mean by that, 106 00:07:29,560 --> 00:07:32,480 let's say we get a fussy customer to our restaurant. 107 00:07:32,500 --> 00:07:36,340 Now they love spam and they love eggs. They also like bacon and sausage, 108 00:07:36,340 --> 00:07:39,890 but for some reason they can't stand bacon with sausage. 109 00:07:39,940 --> 00:07:45,460 Now there's quite a few ways to phrase a condition like that, but it comes down to; choose meals with 110 00:07:45,460 --> 00:07:46,850 spam or eggs, 111 00:07:46,860 --> 00:07:50,810 meals can contain bacon or sausage, as long as they're not together. 112 00:07:50,980 --> 00:07:55,540 Now reading that as two separate filters makes the code a bit more readable. 113 00:07:55,540 --> 00:08:00,810 So let's see the code which I'm going to add to the end of this program. Close down the run window for now. 114 00:08:00,810 --> 00:08:02,590 So we'll start on line 25 here, 115 00:08:02,920 --> 00:08:10,290 fussy meals equals left and right square bracket, or brackets, meal for meal in menu. 116 00:08:10,590 --> 00:08:28,030 And if spam in double quotes, or eggs in double quote, quotes, in meal, if not, parentheses double quote bacon double quote 117 00:08:28,170 --> 00:08:37,520 in meal and sausage in meal. 118 00:08:38,409 --> 00:08:45,690 And then we want a right parentheses there to close off and then our right square bracket - and I made a typo there, 119 00:08:45,720 --> 00:08:49,160 that should be in meal, if spam in meal. 120 00:08:49,180 --> 00:08:50,880 So there's the code, fussy meals equals, 121 00:08:54,400 --> 00:08:55,180 and that's a bit better. 122 00:08:55,280 --> 00:08:55,620 OK, 123 00:08:55,650 --> 00:08:59,520 so there's our code, fussy meals equals meal for meal in menu. 124 00:08:59,730 --> 00:09:07,380 If spam in double quotes in meal, or eggs in double quote in meal, and another condition, if not, then we've got 125 00:09:07,750 --> 00:09:15,240 parentheses there, bacon in double quotes in meal and sausage in meal, then right parentheses and a square bracket. 126 00:09:15,710 --> 00:09:17,970 OK now let's print it out. 127 00:09:20,050 --> 00:09:22,140 OK, we'll run it. 128 00:09:22,970 --> 00:09:27,750 So they should all contain spam and eggs, as you can see here, but you can't see any, I can't see any, 129 00:09:28,070 --> 00:09:31,410 that have actually got both bacon and sausage. 130 00:09:31,470 --> 00:09:40,440 So basically you can see with that line, that the first one lets through any meals, that's the first part, 131 00:09:40,600 --> 00:09:47,480 so lets through any meals that contain spam or eggs. We've used or instead of and there, so either of those. Then 132 00:09:47,490 --> 00:09:55,510 the second one, the second filter here, filters out any meal that contains both bacon and sausages in a meal. 133 00:09:55,570 --> 00:10:01,560 So it's not a trivial condition, and specifying it as two separate filters makes it easier to see what's 134 00:10:01,560 --> 00:10:02,770 going on. 135 00:10:02,850 --> 00:10:08,110 So what I'll do though, just to show you to be consistent here, and I might just move this over a bit so we've got a bit 136 00:10:08,110 --> 00:10:08,970 more space, 137 00:10:11,110 --> 00:10:16,020 I'm going to actually paste in the equivalent code using a single filter, 138 00:10:16,530 --> 00:10:19,420 and again, you can get this pasted in code from the resources section of this video. 139 00:10:24,450 --> 00:10:27,580 Firstly we'll just run it to make sure we've got the equivalent results, 140 00:10:27,670 --> 00:10:29,650 and you can see the results are identical. 141 00:10:29,790 --> 00:10:31,310 And in terms of the actual code, 142 00:10:34,500 --> 00:10:37,220 we have got a warning here about the line being too long. 143 00:10:37,740 --> 00:10:40,730 And what we can do and the quickest way of solving that; Code, Reformat. 144 00:10:43,780 --> 00:10:44,660 So we'll get IntelliJ to 145 00:10:44,660 --> 00:10:45,230 help us, 146 00:10:45,240 --> 00:10:48,160 and you can see that's nicely formatted things for us. 147 00:10:48,560 --> 00:10:51,080 And let's do the same here, just to make it a little bit easier to read. 148 00:10:58,580 --> 00:10:59,650 I'll just delete that comment. 149 00:10:59,840 --> 00:11:00,090 Alright, 150 00:11:00,090 --> 00:11:01,400 so what have we got here. 151 00:11:01,980 --> 00:11:04,750 Well this is the code that I've pasted in on line 29. 152 00:11:05,360 --> 00:11:10,620 That's now using a single filter instead of the double, or the two filters we used on the code, with the 153 00:11:10,620 --> 00:11:12,190 code on line 25. 154 00:11:12,440 --> 00:11:16,620 And again we ran the code, you can see down the bottom - it performs the same result each time. 155 00:11:16,920 --> 00:11:22,260 So the second form, using a single complex condition, which you can see that I've used again on line 29, 156 00:11:22,500 --> 00:11:28,230 that's harder to understand, and it was even harder to write correctly first time. The parentheses around 157 00:11:28,230 --> 00:11:30,450 the first condition are essential. 158 00:11:30,450 --> 00:11:35,630 Now of course, if you find the second method more readable, then by all means write your code that way. 159 00:11:35,950 --> 00:11:39,650 In terms of performance though, there's nothing to choose between either one. 160 00:11:39,660 --> 00:11:44,580 Alright, so I'm going to stop the video here. In the next one, we're going to review using if as an expression, 161 00:11:44,910 --> 00:11:50,340 and then see how we can solve that earlier problem, using else in the comprehension to add something that 162 00:11:50,340 --> 00:11:52,540 doesn't appear in the original sequence. 163 00:11:52,560 --> 00:11:53,970 So I'll see you in the next video.