WEBVTT 00:00.150 --> 00:05.850 Hello again! In this video, we are going to continue looking at removing algorithms. As is often the 00:05.850 --> 00:07.890 case, there are other versions of remove(). 00:08.530 --> 00:14.190 There is remove_if(), which takes a predicate, and _copy() versions, which will write the result 00:14.190 --> 00:15.660 to a destination container, 00:16.080 --> 00:18.600 instead of modifying the iterator range in place. 00:21.000 --> 00:27.920 The remove() algorithm uses the equality operator to decide whether to remove an element. With remove underscore 00:27.920 --> 00:31.440 if(), we can provide our own predicate as the third argument. 00:32.670 --> 00:37.200 This will be called with each element as the argument. And then, if this returns true, the element 00:37.200 --> 00:37.950 will be removed. 00:38.430 --> 00:41.010 And if it returns false, the element will be kept. 00:41.880 --> 00:47.880 In this example, we want to remove elements which are exactly divisible by 3, and keep the rest. 00:48.900 --> 00:54.900 The return value will be an iterator, again, to the first [removed] value, and we can pass that as a argument 00:54.900 --> 00:55.770 to erase(). 00:56.730 --> 00:58.440 So here is some code. 00:58.440 --> 00:59.490 We have our vector. 01:01.320 --> 01:06.090 We have the call to remove_if(), with the callable object as the third argument. 01:07.740 --> 01:13.920 Then we have the iterator as the return value. And then we call erase(), using that iterator. 01:18.410 --> 01:24.170 So we get a result in which the elements 3 and 9 have been removed, because they are divisible 01:24.170 --> 01:24.800 by 3. 01:25.100 --> 01:26.240 But the others have been kept. 01:28.250 --> 01:35.390 remove_copy() will write the results to a destination. It will write the data from this is iterator range 01:35.600 --> 01:40.220 into this destination, but it will omit any elements which have this value. 01:40.820 --> 01:46.700 So this will populate "vec2" with all these elements, except for the ones which have the value 1. 01:47.240 --> 01:53.060 If you remember the copy_if() algorithm, we could get that to do the same thing by copying all the 01:53.060 --> 01:54.680 elements which are not equal to 1. 01:55.190 --> 02:00.740 So these two are the same, but with the logic inverted. In this one, we say that 1 is the value we 02:00.740 --> 02:03.260 do not want to copy. In this. 02:03.590 --> 02:07.520 we say that anything except 1 is the value that we want to copy. 02:09.520 --> 02:13.120 So let's try this out. We have our call to remove_copy(). 02:13.840 --> 02:17.110 So this will remove the elements with the value 1. 02:17.770 --> 02:19.390 Then we have the equivalenct code with copy_if() 02:19.390 --> 02:22.950 So this will copy everything except the elements with value 1. 02:25.810 --> 02:26.950 So let's see what we get. 02:27.310 --> 02:32.020 So the new vector has those elements, without the elements with value one. 02:35.960 --> 02:38.720 And there is also remove_copy_if() which combines both these. 02:39.130 --> 02:45.200 It will take a lambda expression and do a copy of all the elements for which the expression is [true]. 02:47.260 --> 02:51.220 So that would give us a container in which the elements are, 1, 4, 1, 5. 02:55.040 --> 02:57.500 So here is our call to remove_copy_if(). 02:59.940 --> 03:06.000 There is the lambda expression. And there is the equivalent code using copy_if(). 03:10.310 --> 03:15.260 So this will populate the container with all the elements which are not exactly divisible by 3. 03:18.960 --> 03:21.120 The other main algorithm is unique(). 03:21.360 --> 03:24.810 This will remove duplicate elements, but they have to be adjacent. 03:25.350 --> 03:30.810 So this means that the elements in the iterator range must be sorted. Otherwise, unique() will not work properly. 03:31.470 --> 03:37.140 This has the similar behaviour, in that elements are only "logically" removed from the container and not physically 03:37.140 --> 03:37.590 removed. 03:38.220 --> 03:42.570 If we wants to physically remove them, we pass the returned iterartor to erase(). 03:46.090 --> 03:48.220 So we have code like this. We have our vector. 03:49.090 --> 03:58.720 We sort it, and then we call unique(). We get an iterator to the first removed element, and then 03:58.720 --> 04:00.010 we pass that to erase(). 04:03.260 --> 04:04.760 So there is our original vector. 04:05.060 --> 04:10.430 There is the sorted vector. It has an adjacent duplicates element, so that gets moved to the back 04:10.430 --> 04:11.030 of the container. 04:11.690 --> 04:14.660 And then we call erase(), and that element is destroyed. 04:15.470 --> 04:21.140 So the result is the elements, in order, without the duplicate elements with value 1. 04:26.520 --> 04:32.910 Unique uses the equality operator. We can also provide our own predicate for deciding what constitutes 04:32.910 --> 04:34.230 being a duplicate. 04:35.850 --> 04:41.040 The predicate will take 2 arguments, which correspond to successive elements. So this could be, 04:41.040 --> 04:44.460 for example, the first and second, or the second and third elements in the container. 04:45.480 --> 04:49.620 And in this case, we want to remove elements which differ by one. 04:50.040 --> 04:55.050 So if we have 3, followed by 4, then we want to remove the elements with value 4. 05:00.370 --> 05:02.410 So there is our call to unique(). 05:03.460 --> 05:05.140 Again, we have sorted the vector. 05:05.530 --> 05:06.130 (Do not forget that!) 05:07.300 --> 05:12.400 So we call unique with our predicate, and then we call erase() on the return value. 05:13.000 --> 05:14.410 So what do you think we will get? 05:17.090 --> 05:18.710 (Just go back to our original vector.) 05:21.380 --> 05:26.510 So there is our original version: there it is, sorted. 1 is followed by 1, which is not more than 1. 05:27.140 --> 05:33.560 1 is followed by 3, which is not one more than 1. 3 is followed by 1, which is one more 05:33.560 --> 05:34.850 than 3. 05:35.150 --> 05:40.550 So 4 gets moved to the end of the container and now 3 is followed by 5, which is not one 05:40.550 --> 05:40.840 more. 05:41.210 --> 05:43.190 And 5 is followed by 9, which is not one more. 05:43.610 --> 05:45.800 So this will only remove the element with value 05:45.800 --> 05:46.160 4. 05:46.460 --> 05:48.140 It keeps the elements with value 5. 05:51.810 --> 05:55.350 And again, we have the _copy version, which writes the result to a destination. 05:55.920 --> 05:59.040 So this will do the copy, but omit any duplicate elements. 06:01.170 --> 06:03.570 And we can also call it with a predicate. 06:07.410 --> 06:09.510 So here is the unique copy version. 06:14.540 --> 06:17.480 So this just copies the result, without the duplicate value 1. 06:19.760 --> 06:22.070 And this is the version with the predicate. 06:23.730 --> 06:27.240 So this will populate a new container with the result that we had before. 06:27.960 --> 06:29.640 Okay, so that is it for this video. 06:30.000 --> 06:32.850 I will see you next time, but until then, keep coding!