WEBVTT 00:00.180 --> 00:05.420 Hello again! In this video, we are going to look at the less-than operator. The less-than operator 00:05.430 --> 00:06.810 is a non-member function. 00:07.080 --> 00:09.720 It takes two arguments and it returns a bool. 00:10.410 --> 00:16.950 If the first argument, the object on the left, is less than the second argument, the object on the 00:16.950 --> 00:20.070 right, it returns true, otherwise false. 00:22.360 --> 00:25.150 This is actually a very important operator. 00:25.750 --> 00:31.030 The Standard Library uses the less-than operator to sort and order the elements in a container. 00:31.780 --> 00:36.570 If we have a container and the elements do not implement the less-than operator, then we cannot 00:36.610 --> 00:38.350 sort the elements in that container. 00:38.980 --> 00:44.740 If you try to sort it, the code does not compile and you get a huge, confusing template error! 00:46.640 --> 00:51.950 There are also some containers which sort their elements. They store their objects in an order 00:51.950 --> 00:57.650 which is sorted. If we try to insert an object which does not support the less-than operator into one 00:57.650 --> 00:58.460 of these containers, 00:58.910 --> 01:00.380 then again, that does not compile. 01:03.020 --> 01:06.290 You may wonder what the Standard Library does about other comparisons. 01:06.710 --> 01:11.090 And the answer is that you can generate any comparison operator from the less-than operator. 01:11.870 --> 01:17.870 For example, if a less-than b is false and b less-than a is also false, then a must be equal to 01:17.870 --> 01:18.170 b. 01:18.620 --> 01:20.240 So that is your equality operator! 01:21.530 --> 01:25.400 If a less-than b is false, then a must be greater than or equal to b. 01:26.390 --> 01:32.090 And if a less-than b is false and a is not equal to b, then a must be greater than b. And so on. 01:33.540 --> 01:38.030 So I have added the less-than operator to our student class. 01:42.750 --> 01:45.510 We are going to be using the name to start with. 01:46.410 --> 01:49.410 So this is going to compare the name members of the objects. 01:49.680 --> 01:51.180 These are standard strings. 01:51.480 --> 01:56.830 So this will call the less-than operator for the string class. That will perform an alphabetical sort, 01:56.850 --> 02:01.620 so a name at the beginning of the alphabet is less than a name which is later on in the alphabet. 02:02.190 --> 02:03.660 And it is also case sensitive. 02:04.200 --> 02:08.430 So a name with a capital letter is less than a name with a lower case letter. 02:10.710 --> 02:14.460 And the main function is the same, except we are using the less-than operator this time. 02:15.510 --> 02:16.470 So let's try that out. 02:18.090 --> 02:21.590 So remember, this is just looking at the names. It completely ignores the IDs. 02:21.600 --> 02:27.570 These John Smiths are regarded as having the same value by the less-than operator, so they are not less than 02:27.570 --> 02:27.990 each other. 02:28.680 --> 02:31.030 Jack Jones IS less than John Smith. 02:31.500 --> 02:39.330 So "stu3" is less than the other two, and John Smith is not less than Jack Jones, so everything 02:39.330 --> 02:40.200 else is false. 02:41.010 --> 02:42.420 So that all works as expected. 02:43.140 --> 02:48.450 By the way, if you define a less-than operator for a class and also the inequality operators, you should 02:48.450 --> 02:49.920 make these behave consistently. 02:50.580 --> 02:57.780 So if a is less than b, then a cannot be equal to b, and a must be not equal to b. If a is equal 02:57.780 --> 03:00.660 to be, then a cannot be less than b and b cannot be... 03:01.260 --> 03:03.060 So that is just something to bear in mind. 03:06.000 --> 03:12.540 Talking of sorting, the C++ library has a sort() function, std::sort, in the header. 03:13.020 --> 03:16.530 And this takes an iterator range and it will sort the elements in that range. 03:17.550 --> 03:22.290 For example, we can source all the elements in a vector, by passing the return values from begin() 03:22.290 --> 03:23.160 and end(). 03:25.250 --> 03:32.300 Like that. And this will sort the elements in ascending order. The sort function will use 03:32.300 --> 03:35.270 the less-than operator of the elements to determine the order. 03:37.490 --> 03:42.350 If the elements are string, as they were with the students, then this is going to perform a sort in 03:42.350 --> 03:43.700 increasing alphabetical order. 03:44.720 --> 03:49.880 And we are using begin() and not cbegin() because the source algorithm will move the elements around. 03:50.180 --> 03:51.950 So the iterators cannot be constant. 03:56.730 --> 04:01.230 So here's an example of where we have a vector of names, of characters in a well-known cartoon. 04:02.640 --> 04:07.170 We start by printing out the elements using a range-for loop. 04:08.250 --> 04:09.930 Then we call our sort. 04:10.620 --> 04:14.880 So that is going to sort all the elements in the vector. And then we print out all the elements again. 04:15.150 --> 04:17.520 So these should be sorted into alphabetical order. 04:19.800 --> 04:20.700 And that is what we get. 04:20.730 --> 04:24.600 So that is the vector before, and that is the vector after, in alphabetical order. 04:27.480 --> 04:32.820 As with the equality operator, you may need to think about what it means for one of your objects to 04:32.820 --> 04:37.210 be less than another one. With the student class we have two possibilities. 04:37.860 --> 04:44.610 We can use the names, so we can use alphabetical ordering to decide which comes first. 04:45.420 --> 04:50.520 And that might be more convenient if you're a teacher or lecturer and you want a list of students in 04:51.090 --> 04:57.180 order of their names. You could also use the ID, to order the elements. 04:57.630 --> 05:00.540 And that will sort the students in numerical order. 05:01.380 --> 05:06.040 And that might be more convenient if you are an administrator for the school or college or university 05:06.040 --> 05:06.540 or whatever. 05:08.530 --> 05:12.430 So going back to our student class again. This is all the same. 05:15.490 --> 05:21.880 And then in the main function, we are going to create a vector of these students. And then we are going 05:21.880 --> 05:24.580 to sort it, so we put them out before and after, again. 05:28.520 --> 05:30.240 So we have these in alphabetical order. 05:30.260 --> 05:36.560 We have Jack Jones first and then the John Smith's. And again, the sort does not take the ID into 05:36.560 --> 05:37.280 consideration. 05:37.820 --> 05:39.320 So these could be the other way around. 05:45.510 --> 05:48.630 If we sort by the ID, so we need to change this. 05:58.460 --> 06:02.480 (The problem with comments is they are not checked by the compiler, so they easily get out of date) 06:03.860 --> 06:08.570 And we want to make sure that the equality operator is consistent, so we use the ID there as well. 06:09.080 --> 06:12.980 We do not actually call it in this program, but we might use this code later in some way where we do 06:12.980 --> 06:14.450 need the equality operator. 06:16.040 --> 06:17.990 So let's try calling our sort again. 06:20.200 --> 06:20.920 And there you are. 06:20.950 --> 06:26.920 Now they are ordered by the ID. So John Smith with the lowest ID comes first and then Jack Smith 06:26.920 --> 06:30.670 comes last because he has the highest ID. Sorry, Jack Jones. 06:30.670 --> 06:31.330 Not Jack Smith. 06:32.950 --> 06:34.900 Okay, so that's it for this video. 06:35.380 --> 06:36.190 I'll see you next time. 06:36.190 --> 06:38.230 But until then, keep coding!