1 00:00:05,550 --> 00:00:09,920 Now we had a problem in the last video, when we tried to use else in our comprehension. 2 00:00:10,130 --> 00:00:12,070 So let's start by reviewing the problem. 3 00:00:12,080 --> 00:00:18,050 Now what I've done is I've opened a new file here, condcomp1.py, or created it, and I've copied the code from 4 00:00:18,050 --> 00:00:25,020 our original condcomp.py that we used in the last video, and I've deleted all the code after the for loop 5 00:00:25,020 --> 00:00:26,640 and after we printed out the meals. 6 00:00:26,780 --> 00:00:28,850 So basically, starting from scratch there again. 7 00:00:29,240 --> 00:00:30,770 So if I run this file now, 8 00:00:34,040 --> 00:00:39,230 you can see that the for loop creates a list containing just the two meals that don't contain spam from 9 00:00:39,230 --> 00:00:41,990 the original list, and if a meal isn't included, 10 00:00:42,020 --> 00:00:45,370 we're getting the string 'a meal was skipped' instead. 11 00:00:45,470 --> 00:00:50,950 So the first part, getting a list of the meals that doesn't contain spam, is easy as a comprehension. 12 00:00:51,250 --> 00:00:53,330 So let's start that first, so on line 13 00:00:53,630 --> 00:01:01,440 twenty one we'll put meals equals, left and right square brackets, meal for meal in menu, 14 00:01:01,900 --> 00:01:07,180 if double quotes spam not in meal. 15 00:01:07,530 --> 00:01:12,250 We've got our right square bracket there, and we can print out meals. 16 00:01:12,530 --> 00:01:15,530 Let's just confirm that that works. 17 00:01:15,860 --> 00:01:17,650 And you can see that that works. 18 00:01:17,720 --> 00:01:21,380 So I'm getting the same two, basically the same two meals as the for loop, 19 00:01:21,500 --> 00:01:26,230 but of course we're not getting the 'meal was skipped' messages in that list. So that bit was easy, 20 00:01:26,310 --> 00:01:31,910 but at this point we're now stuck, because we've already seen in the previous video that we can't add an else 21 00:01:31,910 --> 00:01:33,200 clause to the filter. 22 00:01:33,500 --> 00:01:35,400 So how can we actually proceed here. 23 00:01:35,780 --> 00:01:41,180 Well the expression that we're using to build the list can be as complicated as we need. Now at the 24 00:01:41,180 --> 00:01:41,860 moment, 25 00:01:41,880 --> 00:01:44,420 we're just adding the individual items for menu - 26 00:01:44,420 --> 00:01:48,970 the expression part in the meal, in the variable meal, at the start of the comprehension. 27 00:01:49,190 --> 00:01:54,920 Now if we instead, replace the simple expressions with something a bit more complicated, we can then get our 28 00:01:54,920 --> 00:01:59,110 comprehension to produce exactly the same list as the for loop. 29 00:01:59,720 --> 00:02:03,920 So a conditional expression is pretty much what it sounds like. 30 00:02:03,920 --> 00:02:10,180 So it's a condition that we can assign to something or use anywhere else that an expression's needed. 31 00:02:10,190 --> 00:02:11,530 So let's see one in action. 32 00:02:11,570 --> 00:02:17,330 So we're going to add some code down here on line 24, x equals 12, 33 00:02:17,960 --> 00:02:24,170 and on the next line, expression equals double quotes, twelve with a capital T, although it doesn't really 34 00:02:24,170 --> 00:02:25,050 matter, 35 00:02:25,050 --> 00:02:32,920 space if x equals 12, two equals there. Now we've actually got an error on that line at the moment. 36 00:02:33,140 --> 00:02:36,230 So what we've typed is the first part of the expression with an error there. 37 00:02:36,560 --> 00:02:39,510 And if you think about it, hopefully it's obvious why we've got that error - 38 00:02:39,700 --> 00:02:40,960 this is on this line. 39 00:02:41,290 --> 00:02:47,400 Now expression will be equal to the string 12 in double quotes if x equals 12, 40 00:02:47,660 --> 00:02:49,940 but what value will it have otherwise. 41 00:02:49,970 --> 00:02:53,600 So when using a conditional expression there must be an else 42 00:02:53,600 --> 00:02:54,240 part. 43 00:02:54,320 --> 00:02:57,910 So let's revise that, and add on the end of it else 44 00:02:58,040 --> 00:03:07,460 double quotes unknown, then on the next line we can do print expression. If we run this now, there shouldn't 45 00:03:07,460 --> 00:03:09,130 be any surprises. 46 00:03:09,290 --> 00:03:14,550 And you can see that we're getting the output 12, because you can clearly see that x is defined as 12, 47 00:03:14,780 --> 00:03:17,560 and the if clause there is basically true, 48 00:03:17,630 --> 00:03:24,350 so therefore we're getting the result 12. And just to really confirm that, lets change it to something else, to 15, run it 49 00:03:24,350 --> 00:03:27,160 again, and we've got unknown. 50 00:03:27,190 --> 00:03:29,470 So that's working like we think it should. 51 00:03:29,470 --> 00:03:31,730 So now that we know what conditional expressions are, 52 00:03:32,080 --> 00:03:35,060 let's see how one can help us with what we're trying to achieve here. 53 00:03:35,350 --> 00:03:40,810 So what we're trying to do, is we want an expression that evaluates to the meal if the meal doesn't 54 00:03:40,810 --> 00:03:44,400 contain spam, otherwise it evaluates to the string 55 00:03:44,410 --> 00:03:45,890 a meal was skipped. 56 00:03:46,030 --> 00:03:50,560 So how do we do that? Well we go back and, lets, we go back and close the run window, and we're going to modify line 57 00:03:50,580 --> 00:03:55,880 21, so it should be now meal equals, and we've got double quotes meal. 58 00:03:56,190 --> 00:04:00,420 And what I'm going to do is, just to make it easy, I'm going to type it, so you can see me type the whole thing in 59 00:04:00,420 --> 00:04:11,530 again. So, meal equals and we've got our double quotes meal, if double quotes spam not in meal, else a meal was skipped, 60 00:04:14,480 --> 00:04:19,800 for meal in menu, and we'll leave the print meals there. 61 00:04:20,029 --> 00:04:25,730 So lets actually run this, confirm that it works, and you can see that we're basically getting the identical 62 00:04:26,180 --> 00:04:31,270 list to what was outputted for us on the previous lines, with a meal skipped, 63 00:04:31,360 --> 00:04:36,740 eggs, sausages, bacon etc. So we're basically getting everything. We're getting the if and the else condition being 64 00:04:36,740 --> 00:04:37,840 evaluated. 65 00:04:37,930 --> 00:04:45,140 And just in case this isn't clear, what we've done is remove the filter and we've put the condition into 66 00:04:45,140 --> 00:04:48,870 a conditional expression in the comprehension. 67 00:04:49,010 --> 00:04:53,890 So in this slide you can see that this is just like our first list comprehension, 68 00:04:54,020 --> 00:04:57,940 but now the expression part's a bit more complex. Now 69 00:04:57,980 --> 00:05:00,280 the expression can be as complex as you need, 70 00:05:00,530 --> 00:05:08,300 although you can also use a function to avoid excessively complex code. Alright so back to IntelliJ again. 71 00:05:08,990 --> 00:05:12,770 So conditional expressions can be more complicated than this. 72 00:05:12,830 --> 00:05:18,890 Now whenever there's an else, it can be followed by another if, whenever there's an if part that must 73 00:05:18,890 --> 00:05:20,380 be followed by an else. 74 00:05:20,390 --> 00:05:25,460 So what I'm going to do is comment out all the code other than the definition. I'm just going to just close the run 75 00:05:25,460 --> 00:05:26,510 window down. 76 00:05:26,780 --> 00:05:33,620 So basically, comment out all this code from line 13, right down to and including line 26. 77 00:05:33,740 --> 00:05:41,210 I'm going to start again on line 28, and let's try this; for meal in menu, 78 00:05:41,600 --> 00:05:44,570 colon, then on the indented line, on the next line, 79 00:05:44,660 --> 00:05:54,100 print parentheses meal comma space double quotes, contains chicken, double quote, 80 00:05:54,180 --> 00:06:08,110 if double quote chicken double quote in meal, else contains bacon, if double quotes bacon in meal, 81 00:06:09,120 --> 00:06:13,970 else contains egg. 82 00:06:14,230 --> 00:06:21,080 Then we've got our right parentheses. So lets actually try running that to get an indication of what each meal 83 00:06:21,080 --> 00:06:26,250 contains. 84 00:06:26,480 --> 00:06:30,580 So note here that the order is important when creating the conditional expression. 85 00:06:30,810 --> 00:06:34,550 So the first two meals don't have a message that they contain egg, 86 00:06:34,850 --> 00:06:38,140 and the reason for that is in the expression which we can't quite see 87 00:06:38,140 --> 00:06:39,320 now - that's better. 88 00:06:39,460 --> 00:06:43,330 We're actually checking for bacon before checking for egg. Now 89 00:06:43,360 --> 00:06:45,550 only if there's no chicken and no bacon 90 00:06:45,620 --> 00:06:48,180 do we get that 'contains egg' message. 91 00:06:48,300 --> 00:06:53,280 Now that example was to show how to include several conditions in an expression. 92 00:06:53,310 --> 00:06:57,350 Now you really wouldn't perform the test that way and it's also wrong. 93 00:06:57,380 --> 00:07:03,350 Now we can see that, by changing chicken to sausage. So I'll go through to our expression 94 00:07:03,370 --> 00:07:04,380 and change that to sausage. 95 00:07:10,960 --> 00:07:12,100 OK and we'll run that, 96 00:07:15,090 --> 00:07:21,550 and you can see that it's actually showing our chicken and chips meal down the bottom, as containing egg. 97 00:07:21,640 --> 00:07:27,170 Now containing egg is the default and it's going to be printed if none of the other conditions match. 98 00:07:27,170 --> 00:07:31,150 Now as I said, you probably wouldn't perform this particular test in that way. 99 00:07:31,420 --> 00:07:33,490 Now exactly how you would write the code 100 00:07:33,490 --> 00:07:37,530 depends on what you're trying to do, and the requirements here are a bit vague. 101 00:07:37,600 --> 00:07:41,080 It was just really an example of a more complex conditional expression. 102 00:07:41,700 --> 00:07:45,180 So you'd probably want to test for membership in a list or a set instead. 103 00:07:45,430 --> 00:07:50,710 So first we can create a set of all the items that our meals can contain, and I'll actually print a blank line 104 00:07:50,710 --> 00:07:55,250 to separate this output from the preceeding results, so lets go ahead and do that. Close down the run window 105 00:07:55,290 --> 00:07:56,420 for now. 106 00:07:57,300 --> 00:07:59,780 So I'll just do a print first, an empty line. 107 00:08:00,200 --> 00:08:07,930 So we'll start with items equals set parentheses, then for meal in menu on the next line, colon, for 108 00:08:07,930 --> 00:08:14,950 item in meal colon, items dot add parentheses item. 109 00:08:15,200 --> 00:08:24,400 Then we're going to go back and print items print, just to make another space there, so print parentheses. 110 00:08:25,290 --> 00:08:26,560 So I'll run the program. 111 00:08:26,980 --> 00:08:29,750 And I'll just put a space there to keep Python happy, 112 00:08:29,770 --> 00:08:31,660 run. 113 00:08:31,690 --> 00:08:35,570 So we've now got a set of all items that could appear in the meals. 114 00:08:35,710 --> 00:08:40,990 So now that we've got that, what we can do instead is loop through the meals and print the first matching 115 00:08:40,990 --> 00:08:43,740 item that each meal contains. So lets do that. 116 00:08:47,900 --> 00:08:53,130 So on line 44, meal in menu colon, for item in menu, 117 00:08:53,320 --> 00:08:56,900 Sorry, for item in items I should say. 118 00:08:56,910 --> 00:09:08,620 If item in meal colon print parentheses left and right curly braces in double quotes, contains, and 119 00:09:08,650 --> 00:09:09,780 another left and right curly 120 00:09:09,820 --> 00:09:14,730 braces double quote dot format, meal 121 00:09:14,830 --> 00:09:21,880 comma space item, two right parentheses and then we'll do a break. Alright so now we can loop through 122 00:09:21,880 --> 00:09:28,890 the meals and print the first matching item that each meal contains. So lets try running that, and you 123 00:09:28,890 --> 00:09:34,410 can see we've got a much better result this time. So conditional expressions can be very useful but don't 124 00:09:34,410 --> 00:09:36,320 try to use one just because you can. 125 00:09:36,360 --> 00:09:39,750 They really should only be used to solve specific problems. 126 00:09:39,930 --> 00:09:41,340 So as another example, 127 00:09:41,340 --> 00:09:43,810 let's get the computer to play fizzbuzz. 128 00:09:43,830 --> 00:09:45,310 Now that's a very simple game. 129 00:09:45,480 --> 00:09:51,010 If a number is divisible by three you say fizz instead of the number, if it's divisible by five you say 130 00:09:51,030 --> 00:09:56,100 buzz, and if it's divisible by both 3 and 5 you say fizzbuzz - 131 00:09:56,130 --> 00:09:59,580 so if it's divisible by 15, in other words. Otherwise you say the number. 132 00:09:59,850 --> 00:10:05,940 So let's have a look how to code that expression, and we'll put that into a loop to get a range of results, 133 00:10:06,010 --> 00:10:07,440 or so we get a range of results. 134 00:10:07,530 --> 00:10:16,790 So I'm going to start typing here, for x in range, parentheses, we go from 1 to 31 colon. So 135 00:10:16,850 --> 00:10:28,470 fizzbuzz equals fizz buzz in double quotes, space if x modulus, so the percent sign, 15 is 136 00:10:28,470 --> 00:10:30,160 equal to zero, two equals, 137 00:10:30,390 --> 00:10:44,730 else double quotes fizz if x modulus again percent three equals zero, else double quotes buzz if x 138 00:10:45,480 --> 00:10:50,500 modulus five - that's the percent sign again - equals zero, else, 139 00:10:50,600 --> 00:11:01,780 str x. Then on the next line we're going to print fizzbuzz. So remember that x modulus 15 will be 140 00:11:01,860 --> 00:11:09,110 0 if X is divisible by 15 and the same for x modulus 3 and x modulus 5. 141 00:11:09,130 --> 00:11:13,030 Now the order's important here. We have to check 15 fist, 142 00:11:13,210 --> 00:11:15,880 otherwise one of the other conditions will be true. 143 00:11:16,220 --> 00:11:17,070 Now let's run the program 144 00:11:20,050 --> 00:11:21,580 and confirm that it actually works. 145 00:11:26,640 --> 00:11:32,040 So you can see we get the number or buzz word printed out for values from 1 to 30 inclusive, 146 00:11:33,420 --> 00:11:34,470 and that looks pretty good to me. 147 00:11:36,940 --> 00:11:40,110 Alright, so I'm going to finish the video here. In the next one, 148 00:11:40,160 --> 00:11:44,840 we've got some challenges to practice what we've done so far, and you can probably guess what the first one's 149 00:11:44,860 --> 00:11:45,690 going to be. 150 00:11:45,690 --> 00:11:47,050 So I'll see you in the next video.