WEBVTT 1 00:00:00.520 --> 00:00:02.190 Hi guys, and welcome back. 2 00:00:02.190 --> 00:00:04.940 In this video, we're going to learn about advanced set 3 00:00:04.940 --> 00:00:09.350 line:15% operations, that is, why sets are useful. 4 00:00:09.350 --> 00:00:13.420 So let's say we've got some friends, Bob, Rolf, and Anne, 5 00:00:13.420 --> 00:00:16.763 and then we have two friends that are abroad, Bob and Anne. 6 00:00:17.690 --> 00:00:20.500 If only Bob and Anne are abroad, that means that Rolf 7 00:00:20.500 --> 00:00:25.450 must not be abroad, so the local friends are actually 8 00:00:25.450 --> 00:00:28.140 a set with a single element, Rolf. 9 00:00:28.140 --> 00:00:30.480 This is how you define a set with a single element, 10 00:00:30.480 --> 00:00:32.670 exactly the same as with multiple elements, 11 00:00:32.670 --> 00:00:35.150 but you don't have a comma with more elements after it. 12 00:00:35.150 --> 00:00:37.160 So you've got just your single friend 13 00:00:37.160 --> 00:00:38.563 inside the curly braces. 14 00:00:39.640 --> 00:00:43.230 However, if you add more friends or you add Rolf 15 00:00:43.230 --> 00:00:45.620 to the abroad friends, of course this variable 16 00:00:45.620 --> 00:00:46.850 will not change. 17 00:00:46.850 --> 00:00:50.120 That's because you're saying that this variable contains 18 00:00:50.120 --> 00:00:52.970 a set with a string Rolf inside it. 19 00:00:52.970 --> 00:00:55.920 It is not related to these two sets. 20 00:00:55.920 --> 00:01:00.580 So instead of hard-coding these sets to be a single element, 21 00:01:00.580 --> 00:01:04.860 we're going to use these two sets to calculate who is local, 22 00:01:04.860 --> 00:01:07.000 which friends are not abroad. 23 00:01:07.000 --> 00:01:12.000 And the way to do that is by doing friends dot difference 24 00:01:12.210 --> 00:01:17.100 abroad, so what this does is it calls the difference 25 00:01:17.100 --> 00:01:20.520 function inside friends, and this function here takes in 26 00:01:20.520 --> 00:01:25.190 another set, and what this'll do is it'll take this set 27 00:01:25.190 --> 00:01:28.860 and remove from it the elements in this set. 28 00:01:28.860 --> 00:01:31.040 So we'll essentially calculate the difference between 29 00:01:31.040 --> 00:01:34.550 the two sets with this one as the starting point. 30 00:01:34.550 --> 00:01:38.293 So we'll take away Bob and Anne, leaving you just with Rolf. 31 00:01:39.500 --> 00:01:44.500 You can lead, print that out, and you'll see what I mean. 32 00:01:46.710 --> 00:01:49.541 line:15% So there we have our single set, Rolf. 33 00:01:49.541 --> 00:01:54.541 Notice that if you do abroad dot difference with friends, 34 00:01:55.230 --> 00:01:58.180 you're gonna get something different, because abroad 35 00:01:58.180 --> 00:02:01.970 has two elements, Bob and Anne, and what you're going to do 36 00:02:01.970 --> 00:02:04.900 is you're going to remove from this set 37 00:02:04.900 --> 00:02:06.880 the elements in this set. 38 00:02:06.880 --> 00:02:09.160 So you're gonna take away Bob and Anne, and Rolf as well, 39 00:02:09.160 --> 00:02:11.730 but Rolf is not even there, so you're gonna end up 40 00:02:11.730 --> 00:02:13.100 with an empty set. 41 00:02:13.100 --> 00:02:15.840 Indeed if we run this, you'll see that you get 42 00:02:15.840 --> 00:02:17.420 line:15% an empty set at the bottom. 43 00:02:17.420 --> 00:02:20.482 line:15% Now by the way, this is the notation for an empty set. 44 00:02:20.482 --> 00:02:23.420 line:15% If you want to create one that's empty with no elements 45 00:02:23.420 --> 00:02:26.950 line:15% at all, you have to use set and then normal brackets, 46 00:02:26.950 --> 00:02:29.800 line:15% and that is how also Python tells you about an empty set. 47 00:02:30.960 --> 00:02:35.850 So again, difference takes one set and removes the elements 48 00:02:35.850 --> 00:02:37.393 in this other set from it. 49 00:02:38.590 --> 00:02:41.450 If instead of a list of friends and a list of friends 50 00:02:41.450 --> 00:02:44.540 who are abroad, we have a list of local friends 51 00:02:44.540 --> 00:02:48.376 and a list of abroad friends, we can also calculate 52 00:02:48.376 --> 00:02:53.100 the total friends using another set operation. 53 00:02:53.100 --> 00:02:58.100 So what we'll say is friends is now local dot union abroad. 54 00:02:59.080 --> 00:03:02.780 And what union does is it unites two sets 55 00:03:02.780 --> 00:03:05.810 and it gives you the total of both. 56 00:03:05.810 --> 00:03:10.400 Here we will add to Rolf, Bob, and Anne, and we will end up 57 00:03:10.400 --> 00:03:13.050 with a set that contains Rolf, Bob, and Anne. 58 00:03:13.050 --> 00:03:15.600 line:15% So if we print friends, you'll see 59 00:03:15.600 --> 00:03:17.593 line:15% that that is indeed what we get back. 60 00:03:18.945 --> 00:03:22.260 Now lets say we've got a bunch of friends, some of whom 61 00:03:22.260 --> 00:03:24.990 study art and some of whom study science. 62 00:03:24.990 --> 00:03:28.140 So the friends who study art are Bob, Jen, Rolf, and Charlie 63 00:03:28.140 --> 00:03:30.890 and the ones who study science are Bob, Jen, Adam, and Anne. 64 00:03:30.890 --> 00:03:33.560 So you can see that Bob and Jen actually study both art 65 00:03:33.560 --> 00:03:36.360 and science, Rolf and Charlie study art only, 66 00:03:36.360 --> 00:03:38.713 and Adam and Anne study science only. 67 00:03:40.010 --> 00:03:43.100 You already know how to calculate from these two sets 68 00:03:43.100 --> 00:03:45.273 the ones that study art only, and the ones 69 00:03:45.273 --> 00:03:48.680 that study science only, by using the previous operations. 70 00:03:48.680 --> 00:03:51.850 But what we don't know is how to find out which ones study 71 00:03:51.850 --> 00:03:55.823 both subjects, so that is where intersection comes in. 72 00:03:56.780 --> 00:03:58.070 We can see that another variable, 73 00:03:58.070 --> 00:04:02.103 both is art dot intersection science, 74 00:04:03.240 --> 00:04:05.510 and then if we print both, 75 00:04:05.510 --> 00:04:08.223 line:15% you'll see that the output will be Bob and Jen. 76 00:04:09.820 --> 00:04:11.690 I encourage you to have a play-around 77 00:04:11.690 --> 00:04:13.980 with these set operations, because they can be 78 00:04:13.980 --> 00:04:17.650 a little tricky to understand fully, but once 79 00:04:17.650 --> 00:04:19.650 you've given them a go and you've tried them out, 80 00:04:19.650 --> 00:04:22.300 I'm sure it'll all be much clearer. 81 00:04:22.300 --> 00:04:24.060 line:15% In the resources section of this lecture, 82 00:04:24.060 --> 00:04:26.230 line:15% or in the description below in the video, I'm going to link 83 00:04:26.230 --> 00:04:28.740 line:15% a couple of blog posts that we've written 84 00:04:28.740 --> 00:04:32.730 line:15% on these set operations that also add a few more 85 00:04:32.730 --> 00:04:36.300 line:15% that can be helpful as well, such as symmetric difference. 86 00:04:36.300 --> 00:04:37.770 line:15% Thanks for joining me in this video, 87 00:04:37.770 --> 00:04:39.420 line:15% and I'll see you in the next one.