1 00:00:00,280 --> 00:00:04,300 The first topic I want to talk about today is the concept of loops, 2 00:00:04,660 --> 00:00:07,450 things that have to happen over and over and over again. 3 00:00:08,020 --> 00:00:11,560 The first type of loop I want to introduce you to is something called the For 4 00:00:11,560 --> 00:00:16,560 loop. And it can be used really easily in combination with something we learned 5 00:00:16,960 --> 00:00:21,520 in yesterday's lessons, which is lists. By using a 6 00:00:21,520 --> 00:00:22,600 for loop like this 7 00:00:22,660 --> 00:00:26,860 we can go through each item in a list and perform some action with each 8 00:00:26,890 --> 00:00:27,880 individual item. 9 00:00:28,600 --> 00:00:32,290 So let's try it out in practice. In order to follow along 10 00:00:32,590 --> 00:00:36,730 go ahead and fork the day 5 start rebel. 11 00:00:37,210 --> 00:00:41,200 And once I've explained the concept, then I want you to try it out yourself. 12 00:00:41,920 --> 00:00:46,920 But first, let's say we had a list called fruits and it contained three items, 13 00:00:48,430 --> 00:00:50,320 apple, peach, and pear. 14 00:00:51,370 --> 00:00:55,660 If we wanted to access each item in this list individually and print it out one 15 00:00:55,660 --> 00:00:58,390 by one, then we would use a for loop. 16 00:00:59,290 --> 00:01:01,270 So if we start out with the keyword for, 17 00:01:01,720 --> 00:01:05,620 and then we give a name to a single item. 18 00:01:06,250 --> 00:01:09,160 So in this case, we might call it fruit. 19 00:01:09,880 --> 00:01:12,130 And then we use the in keyword 20 00:01:12,430 --> 00:01:15,640 and finally the list that we want to loop through, 21 00:01:16,090 --> 00:01:17,860 which is our fruits here. 22 00:01:18,580 --> 00:01:23,580 Now we cap it off with a colon and we go on to the next line and notice how it's 23 00:01:24,730 --> 00:01:25,690 indented there. 24 00:01:26,770 --> 00:01:31,770 Now I'm going to go ahead and print the value of each of the fruit variables. 25 00:01:35,080 --> 00:01:36,520 And if I run this code, 26 00:01:36,580 --> 00:01:41,580 you'll see that it loops through my list of fruits and for each of the fruits 27 00:01:42,370 --> 00:01:46,540 inside the list, it prints it out into the console, 28 00:01:46,780 --> 00:01:50,260 apple, peach, pear, in that order. Now, 29 00:01:50,290 --> 00:01:54,670 the important thing to realize here is that basically you can imagine that 30 00:01:54,670 --> 00:01:58,170 behind the scenes, what this code is doing is 31 00:01:58,170 --> 00:02:00,670 it's taking this list of fruits 32 00:02:01,060 --> 00:02:05,140 and it's a signing a variable name, fruit, to each of them. 33 00:02:05,260 --> 00:02:09,039 So the first time this runs fruit is equal to apple, 34 00:02:09,190 --> 00:02:12,100 the second time this runs fruit is equal to peach, 35 00:02:12,610 --> 00:02:15,160 and we can see this even more clearly 36 00:02:15,220 --> 00:02:17,650 if you run it through the Thonny IDE, 37 00:02:17,680 --> 00:02:21,250 which I told you about at the beginning of the course. Now, 38 00:02:21,280 --> 00:02:25,870 if I go ahead and click on the debug icon and I step into each of the steps, 39 00:02:26,170 --> 00:02:31,060 you can see first it establishes that we have a list of three strings, 40 00:02:31,090 --> 00:02:34,750 apple, peach, pear, and then we go into the for loop. 41 00:02:35,170 --> 00:02:40,120 Now notice on the right here I've got all the variables being accounted for. 42 00:02:40,420 --> 00:02:43,300 And the first one it's noticed is the variable fruits, 43 00:02:43,540 --> 00:02:46,870 which holds a list of strings. Now, 44 00:02:46,930 --> 00:02:49,090 as I continue into the for loop, 45 00:02:49,510 --> 00:02:53,530 notice how it's going to look through this list of fruits 46 00:02:54,160 --> 00:02:59,160 and it's going to assign the variable name fruit to each of the items starting 47 00:03:00,460 --> 00:03:01,930 from the first one apple. 48 00:03:02,410 --> 00:03:05,950 So now by the time it's reached line 3, 49 00:03:06,130 --> 00:03:08,260 we've already got this variable called fruit 50 00:03:08,620 --> 00:03:12,790 that's been assigned to the value of apple. And so at this point, 51 00:03:12,820 --> 00:03:15,640 printing out the value of this fruit 52 00:03:15,880 --> 00:03:20,200 it's obviously going to print out apple and you'll see in the next step. 53 00:03:21,880 --> 00:03:25,000 There we go. Now, once I'm done here, 54 00:03:25,120 --> 00:03:28,510 then it's going to loop back to the start of the for loop 55 00:03:28,960 --> 00:03:33,960 and now this variable fruit is going to be assigned to the next value inside the 56 00:03:34,540 --> 00:03:35,410 list of fruits. 57 00:03:35,800 --> 00:03:40,180 So now notice how the variable fruit is attached to the value peach, 58 00:03:40,570 --> 00:03:45,570 and then it continues this and so on and so forth until it prints out each one 59 00:03:46,720 --> 00:03:47,553 of the fruits. 60 00:03:49,390 --> 00:03:53,020 And this really emphasizes the most important aspect of loops. 61 00:03:53,410 --> 00:03:57,940 The loop allows us to execute the same line of code multiple times. 62 00:03:58,390 --> 00:04:01,600 In this case, we're executing the print statement three times, 63 00:04:01,960 --> 00:04:05,590 but our for loop isn't limited to just executing a single statement. 64 00:04:06,010 --> 00:04:08,950 We don't just have to print out the name of an item in the list. 65 00:04:09,310 --> 00:04:12,460 We can execute a whole block of statements multiple times, 66 00:04:12,910 --> 00:04:17,769 and we can do many things inside this for loop and by inside, I mean indented. 67 00:04:18,790 --> 00:04:21,820 So let's say that in addition to printing out the fruit, 68 00:04:22,240 --> 00:04:26,980 I'm going to write another print statement and I'm going to not only print out 69 00:04:26,980 --> 00:04:27,940 the name of the fruit, 70 00:04:28,240 --> 00:04:33,240 but I'm also going to say fruit + + "Pie." 71 00:04:33,820 --> 00:04:36,940 So when I run this code, what do you think will happen? 72 00:04:39,730 --> 00:04:44,680 Well, it prints out apple, and then it prints out Apple Pie 73 00:04:45,220 --> 00:04:49,270 And then it goes back to the start and it assigns the variable fruit to the next 74 00:04:49,270 --> 00:04:52,360 item, peach peach pie, pear pear pie. 75 00:04:53,080 --> 00:04:58,080 So this is how we can implement a simple for loop that loops through a list 76 00:04:58,930 --> 00:05:03,370 and assigns a variable name to each of the items in the list in order, 77 00:05:03,760 --> 00:05:07,870 and then inside the for loop after the colon, after some indentation, 78 00:05:08,140 --> 00:05:13,140 we can do something with that temporary variable for each of the items. 79 00:05:14,470 --> 00:05:19,470 Now I've been talking a lot about the concept of being inside a for loop versus 80 00:05:20,530 --> 00:05:25,210 being outside a for loop. And this is really, really important. 81 00:05:25,510 --> 00:05:30,130 Whenever you see a colon say in our if statement that we saw previously, 82 00:05:30,190 --> 00:05:32,350 or the for loop that we're using here, 83 00:05:32,860 --> 00:05:35,380 the indentation is really, 84 00:05:35,380 --> 00:05:38,470 really important because if it's indented, 85 00:05:38,770 --> 00:05:42,760 then it means that it's inside the for loop and these instructions 86 00:05:42,820 --> 00:05:47,320 all get carried out for as many times as the for-loop will need to repeat. 87 00:05:47,920 --> 00:05:52,540 Now, if I decided that I wanted to say print my, 88 00:05:52,600 --> 00:05:54,250 uh, fruits, 89 00:05:54,430 --> 00:05:59,430 so my list of fruits up here, and I put it inside my for loop, 90 00:05:59,510 --> 00:06:03,410 so indented after the for loop, then as you can imagine, 91 00:06:03,410 --> 00:06:07,160 it's going to print that for as many times as the loop runs, 92 00:06:07,430 --> 00:06:12,290 which is going to be three because there's three items in our list. Now, 93 00:06:12,320 --> 00:06:15,800 if I had indented that back to the beginning, 94 00:06:16,160 --> 00:06:18,860 so now it's no longer inside the for loop, 95 00:06:19,340 --> 00:06:24,340 then it's only going to print once and it's going to print it after the for 96 00:06:24,800 --> 00:06:29,480 loop is done. So notice how it's doing the whole Apple, Apple pie, 97 00:06:29,480 --> 00:06:32,090 et cetera. And then once the loop is finished, 98 00:06:32,150 --> 00:06:36,770 then it jumps to the next line and carries out this instruction. 99 00:06:37,280 --> 00:06:39,500 So the indentation is really, really important, 100 00:06:39,500 --> 00:06:41,450 and you have to be really careful with this. 101 00:06:42,620 --> 00:06:44,630 Have a play around with this code 102 00:06:44,870 --> 00:06:47,300 and if you're still struggling to see how it works 103 00:06:47,300 --> 00:06:52,160 then move the code over to Thonny and step through each step one at a time and 104 00:06:52,160 --> 00:06:53,120 see how it works. 105 00:06:53,600 --> 00:06:57,260 But I hope you're starting to see how loops are really handy at executing an 106 00:06:57,260 --> 00:06:59,600 instruction over and over and over again, 107 00:06:59,990 --> 00:07:02,510 getting the computer to save us time and energy. 108 00:07:03,110 --> 00:07:07,970 A really good example of when loops would come in really handy is, 109 00:07:08,000 --> 00:07:09,920 you know, how Bart Simpson gets punished 110 00:07:09,920 --> 00:07:14,450 and it has to write out a sentence over and over again on the Blackboard. Well, 111 00:07:14,600 --> 00:07:19,070 if only Bart was a programmer, then he would be able to use loops to do this 112 00:07:19,310 --> 00:07:22,520 and he would be able to chill in the corner. Now, 113 00:07:22,760 --> 00:07:24,890 once you're happy with this type of for-loop, 114 00:07:25,190 --> 00:07:28,730 then head over to the next lesson where I've got a coding exercise 115 00:07:28,940 --> 00:07:31,370 that's going to put your knowledge to the test. All right. 116 00:07:31,370 --> 00:07:32,150 So I'll see you there.