1 00:00:04,660 --> 00:00:10,060 So there's one other thing we can do with loops, and that's to include an else clause. 2 00:00:10,060 --> 00:00:14,080 Experienced programmers often avoid using else in this way, and that's 3 00:00:14,080 --> 00:00:18,200 because less experienced programmers can struggle to understand their code. 4 00:00:18,200 --> 00:00:22,120 But it's something you might come across, so I want to cover it here. We've seen else 5 00:00:22,130 --> 00:00:26,179 when we looked at testing conditions using if, but it can also be used at the 6 00:00:26,179 --> 00:00:30,650 end of loops. It's possibly an unfortunate word to use for what it does 7 00:00:30,650 --> 00:00:35,900 with loops though, because it's use doesn't imply "otherwise" in any sense. 8 00:00:35,900 --> 00:00:40,730 What it does, is cause a block of code to be executed if the loop was allowed to 9 00:00:40,730 --> 00:00:45,140 continue to the end, in other words, if the loop wasn't broken out of. With what 10 00:00:45,140 --> 00:00:49,070 we've covered so far, any example we use will either be contrived, or could be 11 00:00:49,070 --> 00:00:52,399 written better some other way. I don't like showing you code that could be 12 00:00:52,399 --> 00:00:57,230 written better, so the first example will be a bit contrived. So let's assume we've 13 00:00:57,230 --> 00:01:02,000 noticed that lottery numbers divisible by 8 are less likely to appear. Now I 14 00:01:02,000 --> 00:01:06,860 know that's not realistic - I did say this example was contrived. We want to write 15 00:01:06,860 --> 00:01:11,210 some code that will check a list of numbers, and let us know if the list 16 00:01:11,210 --> 00:01:18,060 contains any values that are divisible by 8. So let's start with a new file. 17 00:01:18,060 --> 00:01:32,140 We'll call this one contrived.py, and we'll start with a new list as well: 18 00:01:32,140 --> 00:01:38,719 numbers is equal to left and right square brackets 1 comma 45 comma 32 19 00:01:38,719 --> 00:01:46,969 comma 12 and 60. Now we're going to look at lists in the next section. All we need 20 00:01:46,969 --> 00:01:52,020 to know here, is that they're enclosed in square brackets and are a sequence type. 21 00:01:52,020 --> 00:01:54,979 So we know that we can iterate over sequences - we did that a lot with the 22 00:01:54,980 --> 00:02:02,760 string type. Let's try out some code. So I'm going to type for number in numbers: 23 00:02:02,760 --> 00:02:12,080 if number percent, which is the remainder operator, 8 is equal to 0 : and I'll 24 00:02:12,080 --> 00:02:15,700 put a comment here - I'm going to put reject the list. 25 00:02:17,180 --> 00:02:22,519 Then I'll print out the message, print parentheses double quotes The numbers 26 00:02:22,519 --> 00:02:32,819 are unacceptable, and we'll also break. So here what we're doing, for each number in 27 00:02:32,819 --> 00:02:36,930 the list, we're checking if it's exactly divisible by 8, using a modulo or 28 00:02:36,930 --> 00:02:41,010 remainder operator. If it is, we print a message rejecting the list and break 29 00:02:41,010 --> 00:02:45,060 out of the loop. Now these numbers should be rejected because we've got the value 30 00:02:45,060 --> 00:02:52,160 32 in the list. So we run the program and the numbers are rejected, so that works. 31 00:02:52,160 --> 00:02:56,099 But let's see what happens with the list of numbers that aren't divisible by 8. 32 00:02:56,099 --> 00:03:05,010 So I'm going to change the 32 to a 31. Run the code again, and you see this time, we 33 00:03:05,010 --> 00:03:08,220 don't get the message that they're unacceptable, which we're expecting, having 34 00:03:08,220 --> 00:03:13,460 changed 32 back to 31. So the code works but it's not very friendly. 35 00:03:13,460 --> 00:03:17,700 It hasn't given us any output. So what would be a lot friendlier, is if we've 36 00:03:17,700 --> 00:03:21,419 got a message saying that these numbers were okay. So this is where Pythons else 37 00:03:21,419 --> 00:03:25,829 can be useful, when used in a loop. When our loop finds a value that's not 38 00:03:25,829 --> 00:03:29,400 acceptable, our code breaks out of the loop, as we saw the first time we ran the 39 00:03:29,400 --> 00:03:31,680 program. That means the loop will only terminate 40 00:03:31,680 --> 00:03:36,209 normally if all the values are acceptable. So what we want to do, is run 41 00:03:36,209 --> 00:03:40,019 some code when the loop terminates normally. We don't want that code to 42 00:03:40,019 --> 00:03:43,829 execute if we break out of the loop, only when it goes all the way around, and 43 00:03:43,829 --> 00:03:50,220 that's exactly what else does. So let's see that working. So I'm going to put an 44 00:03:50,220 --> 00:03:56,940 else here on line 8, colon and print parentheses double quotes All those 45 00:03:56,940 --> 00:04:04,060 numbers are fine. So if I run the program, 46 00:04:04,060 --> 00:04:08,040 this time we get the message, All those numbers are fine. We didn't break out of the 47 00:04:08,040 --> 00:04:11,760 loop, in other words, and the code in the else block is executed. Then when I 48 00:04:11,760 --> 00:04:17,760 change to this, to include an unacceptable number, we'll change 12 to 16, 49 00:04:17,760 --> 00:04:24,000 run that again. This time we get the output, The numbers are unacceptable. 50 00:04:24,000 --> 00:04:26,670 In other words, we've broken out of the loop, and because the loop didn't 51 00:04:26,670 --> 00:04:30,930 terminate normally, the code in the else block - this is the code on line 9 - didn't 52 00:04:30,930 --> 00:04:35,940 get executed. Now once again, take careful notice of the indentation. This is 53 00:04:35,940 --> 00:04:42,400 particularly tricky, here as an else can follow both the if or the for statements. 54 00:04:42,400 --> 00:04:47,360 It's only the indentation level that ties the else to the for. Here, the else 55 00:04:47,370 --> 00:04:51,480 on line 8, is at the same level as the for, on line 3. 56 00:04:51,480 --> 00:04:56,820 That means else is associated with the for loop and not with the if statement. 57 00:04:56,820 --> 00:05:00,320 Alright so that was a contrived example to show how else works when it's used 58 00:05:00,330 --> 00:05:04,650 with a loop. You may want to step through the code in the debugger, to see exactly 59 00:05:04,650 --> 00:05:09,480 what's happening. If you do, remember to change 16 back to 12 and debug the code 60 00:05:09,480 --> 00:05:13,410 again, and that'll let you see the else block executing, when the loop terminates 61 00:05:13,410 --> 00:05:16,830 normally. Alright so moving on, in the next video 62 00:05:16,830 --> 00:05:22,790 we'll see a more realistic use of else. See you in the next video.