WEBVTT 1 00:00:00.370 --> 00:00:02.290 Hi guys and welcome back, 2 00:00:02.290 --> 00:00:05.210 in this video we're going to learn about for loops. 3 00:00:05.210 --> 00:00:07.860 A for loop is used to repeat something 4 00:00:07.860 --> 00:00:10.170 a defined number of times. 5 00:00:10.170 --> 00:00:11.003 That is, 6 00:00:11.003 --> 00:00:13.180 when you know how many times you wanna repeat it. 7 00:00:14.650 --> 00:00:17.930 But a for loop in Python has a slightly different way 8 00:00:17.930 --> 00:00:20.140 about it that in other programming languages. 9 00:00:20.140 --> 00:00:22.440 So if you're coming from a different programming language, 10 00:00:22.440 --> 00:00:25.060 this is more like a foreach loop. 11 00:00:25.060 --> 00:00:26.640 And if you don't know what I'm talking about, 12 00:00:26.640 --> 00:00:29.000 don't worry let's learn about it. 13 00:00:29.000 --> 00:00:32.050 First of all I will start with a friends list, 14 00:00:32.050 --> 00:00:33.750 which has the names of my friends. 15 00:00:36.480 --> 00:00:38.710 And then we would create our for loop. 16 00:00:38.710 --> 00:00:41.640 What we want to do is we want to print the names 17 00:00:41.640 --> 00:00:45.650 of each friend separately into its own lines. 18 00:00:45.650 --> 00:00:48.596 So what we'll do is we'll do for, 19 00:00:48.596 --> 00:00:50.380 that is the key word that signals 20 00:00:50.380 --> 00:00:52.292 that we want to start a for loop. 21 00:00:52.292 --> 00:00:56.073 And then we will do friend in friends. 22 00:00:56.980 --> 00:00:58.240 Print friend. 23 00:00:58.240 --> 00:01:01.660 So a few familiar things going on in here. 24 00:01:01.660 --> 00:01:04.310 We've got the colon, 25 00:01:04.310 --> 00:01:08.210 we've got the indented body of the loop, 26 00:01:08.210 --> 00:01:12.690 but we do have something a bit weird going on in this line. 27 00:01:12.690 --> 00:01:14.740 Friend in friends, 28 00:01:14.740 --> 00:01:17.750 we learned that this was Boolean comparison, 29 00:01:17.750 --> 00:01:20.400 earlier on right? 30 00:01:20.400 --> 00:01:22.460 But what is friend? 31 00:01:22.460 --> 00:01:24.423 We've not defined this variable anyway. 32 00:01:25.470 --> 00:01:26.500 And here's the thing, 33 00:01:26.500 --> 00:01:29.270 this is not a Boolean comparison. 34 00:01:29.270 --> 00:01:33.200 What's going on here is you're telling Python to give you a 35 00:01:33.200 --> 00:01:38.200 new variable called friend once for every friend. 36 00:01:38.320 --> 00:01:39.520 So we're saying, 37 00:01:39.520 --> 00:01:44.140 friend will be the first element in friends. 38 00:01:44.140 --> 00:01:45.990 Then when the loop repeats again, 39 00:01:45.990 --> 00:01:49.490 friend will now be the second element in friends. 40 00:01:49.490 --> 00:01:53.460 Then friend will be the third element in friends and so on. 41 00:01:53.460 --> 00:01:57.030 This is why it uses also the in key word, 42 00:01:57.030 --> 00:01:58.470 but it means something different 43 00:01:58.470 --> 00:02:00.610 than if you use it anywhere else. 44 00:02:00.610 --> 00:02:05.610 In the for loop you've got for, new variable, in, list. 45 00:02:08.010 --> 00:02:11.240 And then you get one iteration of the loop, 46 00:02:11.240 --> 00:02:16.120 or one repeat of the loop for every value in that list. 47 00:02:16.120 --> 00:02:18.430 Note that instead of lists you can use tuples, 48 00:02:18.430 --> 00:02:20.570 you can use sets or dictionaries here, 49 00:02:20.570 --> 00:02:21.620 and that's totally fine. 50 00:02:21.620 --> 00:02:24.700 Those are all known as iterables, 51 00:02:24.700 --> 00:02:26.810 because you can iterate over them, 52 00:02:26.810 --> 00:02:29.223 you can use them for exactly this. 53 00:02:30.120 --> 00:02:32.880 So this is well and good when you've got a few values, 54 00:02:32.880 --> 00:02:35.840 and you wanna do something to each of them. 55 00:02:35.840 --> 00:02:39.460 But if you wanna repeat something say 10 times, 56 00:02:39.460 --> 00:02:43.590 you can do something like elements equal zero to 10. 57 00:02:43.590 --> 00:02:45.870 And then you can do for index, 58 00:02:45.870 --> 00:02:46.930 which is a new variable, 59 00:02:46.930 --> 00:02:48.970 you can call this whatever you want by the way. 60 00:02:48.970 --> 00:02:52.820 In elements and we will print index. 61 00:02:52.820 --> 00:02:55.100 What this is doing is now we're going to 62 00:02:55.100 --> 00:02:59.260 repeat this loop 10 times because there are 10 numbers here. 63 00:02:59.260 --> 00:03:01.440 So by running this we will see 64 00:03:01.440 --> 00:03:04.563 that we get the numbers zero to nine printed out. 65 00:03:06.050 --> 00:03:09.830 So we are getting the numbers zero to nine, 66 00:03:09.830 --> 00:03:12.530 but we don't have to print them out. 67 00:03:12.530 --> 00:03:15.993 We can print something else like hello world. 68 00:03:16.860 --> 00:03:17.693 And we know, 69 00:03:17.693 --> 00:03:20.240 that because there are 10 elements here, 70 00:03:20.240 --> 00:03:22.930 this loop will run 10 times and therefore, 71 00:03:22.930 --> 00:03:25.190 we will get 10 hello worlds. 72 00:03:25.190 --> 00:03:28.350 So what I'm trying to say here is that just because 73 00:03:28.350 --> 00:03:32.240 you are getting a variable for each element, 74 00:03:32.240 --> 00:03:34.730 you don't have to use it for anything. 75 00:03:34.730 --> 00:03:37.970 And usually when you do something like this where you 76 00:03:37.970 --> 00:03:40.910 create a variable but you don't use it inside a loop, 77 00:03:40.910 --> 00:03:44.940 we often in Python give it the name of underscore. 78 00:03:44.940 --> 00:03:46.470 So we just call it underscore, 79 00:03:46.470 --> 00:03:49.040 and that usually signals to other Python programmers 80 00:03:49.040 --> 00:03:52.370 that you don't intend on using this variable for anything. 81 00:03:52.370 --> 00:03:53.800 You could if you wanted to, 82 00:03:53.800 --> 00:03:55.920 because that's a valid variable name, 83 00:03:55.920 --> 00:03:58.810 but this signals that you don't want to. 84 00:03:58.810 --> 00:04:00.600 Of course you can give it any other name you want, 85 00:04:00.600 --> 00:04:02.260 and that's fine. 86 00:04:02.260 --> 00:04:05.800 I will go back to for index in elements just because 87 00:04:05.800 --> 00:04:08.710 I think it's a little bit more readable for you guys. 88 00:04:08.710 --> 00:04:12.360 But instead of creating our own list of 10 elements, 89 00:04:12.360 --> 00:04:14.993 you can actually just do range 10. 90 00:04:16.250 --> 00:04:18.040 And this is pretty much the same thing 91 00:04:18.040 --> 00:04:20.493 in that it will also run 10 times. 92 00:04:22.090 --> 00:04:23.830 What range is doing behind the scenes, 93 00:04:23.830 --> 00:04:28.390 it is creating something similar to that 10 element list. 94 00:04:28.390 --> 00:04:31.370 But it comes with a few more powerful things, 95 00:04:31.370 --> 00:04:35.040 such as you can do five comma 10 inside range, 96 00:04:35.040 --> 00:04:39.660 and this will give you the numbers five to 10. 97 00:04:39.660 --> 00:04:41.370 So five six seven eight and nine. 98 00:04:41.370 --> 00:04:46.040 Or you can do something like two twenty and three. 99 00:04:46.040 --> 00:04:46.873 And what this does, 100 00:04:46.873 --> 00:04:49.410 is it gives you the numbers two to 19, 101 00:04:49.410 --> 00:04:51.860 but only every three numbers, 102 00:04:51.860 --> 00:04:55.113 so two five eight 11 14 17. 103 00:04:56.120 --> 00:04:58.510 Now these are just some of the things you can do with range 104 00:04:58.510 --> 00:05:02.050 and I don't expect you to get much use out of them just now. 105 00:05:02.050 --> 00:05:04.421 But it is one of the reasons to use range over 106 00:05:04.421 --> 00:05:07.140 creating your own list of 10 elements. 107 00:05:07.140 --> 00:05:07.973 It's a lot shorter, 108 00:05:07.973 --> 00:05:10.253 it also more convenient and more powerful. 109 00:05:11.620 --> 00:05:13.920 Imagine you've got a list of dictionaries, 110 00:05:13.920 --> 00:05:16.030 where each dictionary contains 111 00:05:16.030 --> 00:05:18.453 the name and the grade of a student. 112 00:05:19.750 --> 00:05:23.280 You could do for student in students, 113 00:05:23.280 --> 00:05:24.940 and remember what we've got here is 114 00:05:24.940 --> 00:05:27.450 the for key word the student, 115 00:05:27.450 --> 00:05:31.530 the new variable we are creating every iteration, 116 00:05:31.530 --> 00:05:36.530 and in students which is our list or our iterable. 117 00:05:36.590 --> 00:05:38.880 We can then get the name of each 118 00:05:38.880 --> 00:05:41.815 student by doing student name. 119 00:05:41.815 --> 00:05:46.230 Now remember student is each of the values in the list, 120 00:05:46.230 --> 00:05:48.410 so that's the first time the loop runs, 121 00:05:48.410 --> 00:05:51.270 student will be this dictionary here. 122 00:05:51.270 --> 00:05:52.460 The second time it runs, 123 00:05:52.460 --> 00:05:54.650 student will be this dictionary here, 124 00:05:54.650 --> 00:05:56.200 and then it will be this one, 125 00:05:56.200 --> 00:05:57.810 and then this one. 126 00:05:57.810 --> 00:05:59.760 So by doing student name, 127 00:05:59.760 --> 00:06:03.690 what you're going to do is you're going to get the name key 128 00:06:03.690 --> 00:06:06.290 of this dictionary the first time the loop runs, 129 00:06:06.290 --> 00:06:09.170 and you're going to put that inside a variable name. 130 00:06:09.170 --> 00:06:10.310 The second time the loop runs, 131 00:06:10.310 --> 00:06:13.980 you're gonna get the name property of this dictionary, 132 00:06:13.980 --> 00:06:16.260 and you're gonna put that inside this variable. 133 00:06:16.260 --> 00:06:17.400 And then you're gonna repeat that for 134 00:06:17.400 --> 00:06:19.213 this dictionary and this one too. 135 00:06:20.330 --> 00:06:22.113 Lets do the same with grade. 136 00:06:25.170 --> 00:06:27.170 And finally lets use them for something. 137 00:06:29.680 --> 00:06:31.790 Lets run this and you'll see that we can print this 138 00:06:31.790 --> 00:06:34.160 information much more nicely rather than 139 00:06:34.160 --> 00:06:35.640 just printing out a dictionary, 140 00:06:35.640 --> 00:06:37.210 by using this for loop and saying 141 00:06:37.210 --> 00:06:40.653 Rolf has a grade of 90, 78, 100, and 80. 142 00:06:42.160 --> 00:06:44.810 So when do you want to use a for loop? 143 00:06:44.810 --> 00:06:47.590 When you want to repeat something a certain number of times 144 00:06:47.590 --> 00:06:49.600 and you know how many that is, 145 00:06:49.600 --> 00:06:53.660 or when you want to use each of the values of an iterable, 146 00:06:53.660 --> 00:06:56.806 such as a list, tuple, or set, or dictionary, 147 00:06:56.806 --> 00:07:00.583 and you want to do something once for each of the values. 148 00:07:02.200 --> 00:07:03.810 I hope you've learned something in this video. 149 00:07:03.810 --> 00:07:04.990 I know that we're looking at a 150 00:07:04.990 --> 00:07:07.010 lot of different bits of Python, 151 00:07:07.010 --> 00:07:08.720 and maybe right now you don't quite 152 00:07:08.720 --> 00:07:11.490 understand why they're all important. 153 00:07:11.490 --> 00:07:13.530 But very soon everything's going to come together 154 00:07:13.530 --> 00:07:17.030 and we're going to be able to develop very useful programmes 155 00:07:17.030 --> 00:07:19.320 just with the knowledge we've got so far. 156 00:07:19.320 --> 00:07:20.820 So thanks for sticking with me, 157 00:07:20.820 --> 00:07:21.780 hope you've enjoyed it, 158 00:07:21.780 --> 00:07:23.530 and I'll see you in the next video.