1 00:00:04,529 --> 00:00:09,250 One of the reasons that computers are so useful is that they can repeat an 2 00:00:09,250 --> 00:00:13,480 operation multiple times, very quickly. That removes the need for a human to 3 00:00:13,480 --> 00:00:18,310 perform a repetitive task. Python provides several ways to repeat a block 4 00:00:18,310 --> 00:00:23,380 of code, things like for loops, while loops and list comprehensions and 5 00:00:23,380 --> 00:00:27,340 generator expressions. In the next few videos we'll look at for loops. 6 00:00:27,340 --> 00:00:31,570 We'll cover while loops after that but we'll be leaving list comprehensions and 7 00:00:31,570 --> 00:00:36,160 generator expressions for a later section. We've actually seen a for loop 8 00:00:36,160 --> 00:00:40,270 in earlier examples, and now it's time to look at them in more detail and explain 9 00:00:40,270 --> 00:00:45,130 what was happening in that code. A for loop works by iterating over some set of 10 00:00:45,130 --> 00:00:50,840 values. It assigns each of the values, one by one, to one or more variables. 11 00:00:50,840 --> 00:00:56,520 It then executes a block of code once for each value. The set of values comes from a 12 00:00:56,530 --> 00:01:01,960 sequence or some other iterable object. We've seen one sequence type, the string 13 00:01:01,960 --> 00:01:07,360 type. I'll use that for our first examples. An iterable type is anything that can 14 00:01:07,360 --> 00:01:12,670 be iterated over. So that means a sequence is also an iterable. In simple 15 00:01:12,670 --> 00:01:17,319 terms, if you can use it with a for loop, then its iterable. And I'll be 16 00:01:17,320 --> 00:01:22,920 introducing one iterable type - the range - in some of the following examples. 17 00:01:22,920 --> 00:01:25,008 Alright, so back to Intellij and what we're going 18 00:01:25,008 --> 00:01:34,900 to do here is create a new file, and we'll call this forloops.py. 19 00:01:34,920 --> 00:01:38,140 Alright, so we'll use the parrot string from an earlier video. 20 00:01:38,140 --> 00:01:45,860 So parrot is equal to "Norwegian Blue". 21 00:01:45,860 --> 00:01:52,480 So you saw how to use in to check if any particular letter is in the sequence. In also has another use 22 00:01:52,480 --> 00:01:56,540 when used with a for loop. So let's see how that works, so we're going to type for 23 00:01:56,540 --> 00:02:07,680 character in parrot: print (character) 24 00:02:07,680 --> 00:02:13,000 So a for loop starts with the keyword for. Next we provide the names of one or more 25 00:02:13,000 --> 00:02:16,819 variables. We'll stick to using a single variable at first because 26 00:02:16,819 --> 00:02:21,049 using more than one won't make much sense just yet. In this example, we've 27 00:02:21,049 --> 00:02:24,829 called the variable, character. You can call the variable anything you want, 28 00:02:24,829 --> 00:02:29,120 within the rules for variable names, of course. Variable names can't start with a 29 00:02:29,120 --> 00:02:34,489 digit, for example. Next we have the keyword, in. Our variable character will 30 00:02:34,489 --> 00:02:39,650 be bound to each of the items in the sequence, in turn. Finally we specify what 31 00:02:39,650 --> 00:02:43,819 the values are in. We're using the parrot variable here but we could use any 32 00:02:43,819 --> 00:02:49,159 iterable object. What should happen is that the variable character gets each of 33 00:02:49,159 --> 00:02:52,459 the characters in the string. On line 4 we're printing out the value of 34 00:02:52,460 --> 00:02:57,820 character, so let's run the program and see if it does what we expect it to do. 35 00:02:59,240 --> 00:03:03,019 And that looks right, so if I scroll the output there, you can see each letter of 36 00:03:03,019 --> 00:03:07,760 the string is output on a separate line, and I'm just going to increase this just 37 00:03:07,760 --> 00:03:11,000 so we can see a bit more of it. You can't quite see at all without actually 38 00:03:11,000 --> 00:03:13,639 scrolling all the source code off the screen but it's enough of it anyway. 39 00:03:13,639 --> 00:03:17,750 So that output's probably not very useful but it does demonstrate what our for 40 00:03:17,750 --> 00:03:22,940 loop is actually doing. Students often find this confusing, especially if 41 00:03:22,940 --> 00:03:26,959 they've never programmed before. Even students who've used other languages can 42 00:03:26,959 --> 00:03:31,280 be a bit confused because the for loop in Python behaves differently to C or 43 00:03:31,280 --> 00:03:35,690 Java. So let's set a breakpoint on line 3 and run the program in the 44 00:03:35,690 --> 00:03:38,320 debugger to see what's happening. 45 00:03:41,780 --> 00:03:44,280 So you can see now that I've chosen debug for loops, 46 00:03:44,280 --> 00:03:47,620 the debugger stopped on line 3, where we set the breakpoint. In the 47 00:03:47,620 --> 00:03:51,280 variables pane we can see the value of parrot. It's bound to the string 48 00:03:51,280 --> 00:03:56,380 Norwegian Blue. If we use step over, 49 00:03:56,380 --> 00:03:58,080 you can see having done that, 50 00:03:58,080 --> 00:04:00,760 the variables pane's now showing the value of character. 51 00:04:00,760 --> 00:04:06,460 It's been given the first character in Norwegian Blue, so it's bound to the capital letter N. When we 52 00:04:06,460 --> 00:04:11,920 step over again, execution has going back to line 3, as you can see there, 53 00:04:11,920 --> 00:04:17,000 and the print on line 4 has been executed. If I switch to the console pane, you can see 54 00:04:17,000 --> 00:04:22,920 we've got the first letter there, N. Alright, so if I step over again now, 55 00:04:22,920 --> 00:04:27,040 we can see that line 3 is executed again. Now we haven't yet printed again so 56 00:04:27,040 --> 00:04:31,860 there's no change to the console pane, but if I switch back to the debugger pane now, 57 00:04:31,860 --> 00:04:35,210 you can see that character now has the value o, the next character from 58 00:04:35,210 --> 00:04:39,260 Norwegian Blue. So a for loop executes all the code in its code block 59 00:04:39,260 --> 00:04:43,220 repeatedly. It loops around until there's no more values, in other words. 60 00:04:43,220 --> 00:04:48,500 We're ready to execute the print line again so I'm going to step over, and I won't keep 61 00:04:48,500 --> 00:04:50,990 switching between the debugger and console panes. That I'll just get 62 00:04:50,990 --> 00:04:55,220 confusing, but if you do you'll see that the letter o has now been printed and we're 63 00:04:55,220 --> 00:05:00,800 now back on line 3. Step over again, which I did to execute the line once 64 00:05:00,800 --> 00:05:05,390 more and now character has the value r. So each time we get back to line 3 65 00:05:05,390 --> 00:05:09,950 and execute it, character gets the next letter from the parrot string, and that's 66 00:05:09,950 --> 00:05:14,030 why I called the variable character, by the way. It's been assigned each of the 67 00:05:14,030 --> 00:05:17,750 characters in the string, one after the other. There's nothing special about that 68 00:05:17,750 --> 00:05:22,310 name - it just reflects the values that our code will assign to it. Alright, so 69 00:05:22,310 --> 00:05:24,860 I'm going to continue stepping over the code and you'll see each letter being 70 00:05:24,860 --> 00:05:29,620 assigned to our character variable, each time around the loop. 71 00:05:31,259 --> 00:05:35,620 And you can see finally now, we've hit the last letter e. If we step over once 72 00:05:35,620 --> 00:05:41,080 more now. It's now executed the print. If we have a look on the console, we can see 73 00:05:41,080 --> 00:05:46,160 we've got the entire line there, Norwegian Blue, and back to the debugger. 74 00:05:46,160 --> 00:05:50,580 One more time, I'll press step over. This time you can see what's happened. It's 75 00:05:50,580 --> 00:05:54,520 caused the for loop to terminate because there's nothing left for it to do, 76 00:05:54,520 --> 00:05:58,360 and again, you just saw that when I tapped over to the console pane; all letters had 77 00:05:58,360 --> 00:06:02,529 been printed out. Alright, so I'll stop the video here. Run the code a few more 78 00:06:02,529 --> 00:06:05,440 times in the debugger to make sure you understand what's happening here. 79 00:06:05,440 --> 00:06:10,440 When you're happy with this basic for loop, I'll see you in the next video.