WEBVTT 0 00:00.110 --> 00:04.400 The first topic I want to talk about today is the concept of Loops. 1 00:04.400 --> 00:07.670 Things that have to happen over and over and over again. 2 00:08.000 --> 00:13.550 The first type of loop I want to introduce you to is something called the for loop, and it can be used 3 00:13.550 --> 00:20.330 really easily in combination with something we learned in yesterday's lessons, which is Lists. 4 00:20.540 --> 00:26.810 By using a for loop like this, we can go through each item in a list and perform some action with each 5 00:26.840 --> 00:28.220 individual item. 6 00:28.250 --> 00:30.770 So let's try it out in practice. 7 00:30.800 --> 00:39.380 First, let's say we had a list called fruits and it contained three items apple, peach, and pear. 8 00:39.830 --> 00:46.040 If we wanted to access each item in this list individually and print it out one by one, then we would 9 00:46.040 --> 00:47.510 use a for loop. 10 00:47.780 --> 00:54.650 So we start out with the keyword "for" and then we give a name to a single item, 11 00:54.950 --> 00:58.220 so in this case we might call it, fruit. 12 00:58.370 --> 01:01.010 And then we use the "in" keyword. 13 01:01.010 --> 01:07.010 And finally, the list that we want to loop through, which is our fruits here. 14 01:07.070 --> 01:14.810 Now we cap it off with a colon, and we go on to the next line and notice how it's indented there. 15 01:15.320 --> 01:23.000 Now I'm going to go ahead and print the value of each of the fruit variables. 16 01:23.510 --> 01:28.790 And if I run this code you'll see that it loops through my list of fruits, 17 01:28.790 --> 01:35.450 and for each of the fruits inside the list, it prints it out into the console. 18 01:35.480 --> 01:35.960 Apple, 19 01:35.960 --> 01:36.290 Peach, 20 01:36.290 --> 01:38.330 Pear in that order. 21 01:38.360 --> 01:44.930 Now, the important thing to realize here is that basically, you can imagine that behind the scenes, 22 01:44.930 --> 01:52.850 what this code is doing is it's taking this list of fruits and it's assigning a variable name fruit 23 01:52.850 --> 01:53.960 to each of them. 24 01:53.960 --> 01:57.890 So the first time this runs fruit is equal to Apple. 25 01:57.920 --> 02:00.980 The second time this runs fruit is equal to Peach. 26 02:01.010 --> 02:07.560 And we can see this even more clearly if you run it through the Thonny IDE which I told you about 27 02:07.560 --> 02:09.450 at the beginning of the course. 28 02:09.480 --> 02:15.450 Now, if I go ahead and click on the debug icon and I step into each of the steps, you can see, 29 02:15.480 --> 02:21.060 first, it establishes that we have a list of three strings Apple, Peach, Pear. 30 02:21.060 --> 02:23.670 And then we go into the for loop. 31 02:23.700 --> 02:29.010 Now notice on the right here I've got all the variables being accounted for. 32 02:29.010 --> 02:34.950 And the first one it's noticed is the variable fruits which holds a list of strings. 33 02:34.980 --> 02:42.570 Now as I continue into the for loop, notice how it's going to look through this list of fruits. 34 02:42.570 --> 02:50.910 And it's going to assign the variable name fruit to each of the items starting from the first one, Apple. 35 02:50.940 --> 02:57.900 So now by the time it's reached line three, we've already got this variable called fruit that's been 36 02:57.900 --> 03:00.060 assigned to the value of Apple. 37 03:00.090 --> 03:07.200 And so at this point printing out the value of this fruit, it's obviously going to print out Apple 38 03:07.230 --> 03:09.390 as you'll see in the next step. 39 03:10.530 --> 03:11.490 There we go. 40 03:11.520 --> 03:17.490 Now, once I'm done here, then it's going to loop back to the start of the for loop, 41 03:17.490 --> 03:24.300 and now this variable fruit is going to be assigned to the next value inside the list of fruits. 42 03:24.330 --> 03:29.040 So now notice how the variable fruit is attached to the value Peach. 43 03:29.070 --> 03:36.540 And then it continues this and so on and so forth until it prints out each one of the fruits. 44 03:37.800 --> 03:41.970 And this really emphasizes the most important aspect of loops. 45 03:41.970 --> 03:46.980 The loop allows us to execute the same line of code multiple times. 46 03:46.980 --> 03:50.520 In this case we're executing the print statement three times. 47 03:50.520 --> 03:54.570 But our for loop isn't limited to just executing a single statement. 48 03:54.600 --> 03:57.840 We don't just have to print out the name of an item in the list. 49 03:57.870 --> 04:03.780 We can execute a whole block of statements multiple times, and we can do many things inside this for 50 04:03.780 --> 04:04.200 loop, 51 04:04.200 --> 04:06.840 and by inside I mean indented. 52 04:07.530 --> 04:13.440 So let's say that in addition to printing out the fruit, I'm going to write another print statement, 53 04:13.440 --> 04:20.970 and I'm going to not only print out the name of the fruit, but I'm also going to say fruit + space 54 04:20.970 --> 04:22.350 plus pie. 55 04:22.380 --> 04:26.040 So when I run this code, what do you think will happen? 56 04:28.290 --> 04:33.720 Well, it prints out Apple, and then it prints out Apple Plus pie. 57 04:33.720 --> 04:38.640 And then it goes back to the start and it assigns the variable fruit to the next item. 58 04:38.670 --> 04:39.210 Peach. 59 04:39.240 --> 04:39.960 Peach pie. 60 04:39.990 --> 04:40.410 Pear. 61 04:40.440 --> 04:41.190 Pear pie. 62 04:41.610 --> 04:49.530 So this is how we can implement a simple for loop that loops through a list and assigns a variable name 63 04:49.530 --> 04:55.410 to each of the items in the list in order, and then inside the for loop after the colon, 64 04:55.410 --> 05:02.760 after some indentation, we can do something with that temporary variable for each of the items. 65 05:02.970 --> 05:10.950 Now, I've been talking a lot about the concept of being inside a for loop versus being outside a for 66 05:10.950 --> 05:17.880 loop, and this is really, really important. Whenever you see a colon, say in our if statements that 67 05:17.880 --> 05:25.140 we saw previously, or the for loop that we're using here, the indentation is really, really important 68 05:25.140 --> 05:30.210 because if it's indented then it means that it's inside the for loop 69 05:30.210 --> 05:36.390 and these instructions will get carried out for as many times as the for loop will need to repeat. 70 05:36.420 --> 05:43.050 Now, if I decided that I wanted to say print my fruits, 71 05:43.050 --> 05:50.610 so my list of fruits up here, and I put it inside my for loop, so indented after the for loop. 72 05:50.640 --> 05:56.760 Then, as you can imagine, it's going to print that for as many times as the loop runs, which is going 73 05:56.760 --> 06:00.390 to be three because there's three items in our list. 74 06:00.420 --> 06:08.520 Now, if I had indented that back to the beginning, so now it's no longer inside the for loop, then 75 06:08.520 --> 06:14.850 it's only going to print once and it's going to print it after the for loop is done. 76 06:14.850 --> 06:21.520 So notice how it's doing the whole Apple Apple pie, etc. and then once the loop is finished, then 77 06:21.520 --> 06:25.810 it jumps to the next line and carries out this instruction. 78 06:25.810 --> 06:28.270 So the indentation is really, really important, 79 06:28.270 --> 06:30.400 and you have to be really careful with this. 80 06:31.300 --> 06:33.460 Have a play around with this code. 81 06:33.460 --> 06:39.190 And if you're still struggling to see how it works, then move the code over to Thonny and step through 82 06:39.190 --> 06:42.130 each step, one at a time and see how it works. 83 06:42.130 --> 06:47.680 But I hope you're starting to see how loops are really handy at executing an instruction over and over 84 06:47.680 --> 06:51.520 and over again, getting the computer to save us time and energy. 85 06:51.880 --> 06:58.210 A really good example of when loops would come in really handy is, you know, how Bart Simpson gets 86 06:58.240 --> 07:02.590 punished and has to write out a sentence over and over again on the blackboard? 87 07:02.590 --> 07:08.740 Well, if only Bart was a programmer, then he would be able to use loops to do this and he would be 88 07:08.740 --> 07:10.150 able to chill in the corner. 89 07:10.510 --> 07:16.360 Now, once you're happy with this type of for loop, then head over to the next lesson where I've got 90 07:16.360 --> 07:19.630 a coding exercise that's going to put your knowledge to the test. 91 07:19.660 --> 07:21.220 All right, so I'll see you there.