WEBVTT 00:00.150 --> 00:06.270 Hello again! In this video, we are going to look at algorithms which have underscore if versions. Many 00:06.270 --> 00:08.430 algorithms actually come in two versions. 00:08.730 --> 00:14.640 There is a base version which takes a value argument, and another version with underscore if after the 00:14.640 --> 00:17.310 name, which takes a predicate argument. 00:18.570 --> 00:23.730 We know about the find algorithm, which takes a value and searches for the first element which 00:23.730 --> 00:24.870 is equal to that value. 00:25.650 --> 00:33.200 There is also a find underscore if algorithm, which takes a predicate function, and it will call 00:33.210 --> 00:38.940 this predicate function on each element in turn, until it finds an element for which the predicate 00:38.940 --> 00:42.930 returns true. And then it will return an iterator to that element. 00:45.470 --> 00:47.930 We need to provide this predicate function. 00:48.200 --> 00:51.200 So it is going to be called with a single argument and it returns bool. 00:52.370 --> 00:54.650 One way we can do this is with a functor. 00:58.290 --> 01:04.770 So here is a functor class. It is called "ge5", for reasons which will become apparent! 01:06.360 --> 01:11.670 It just has one member, which is a public function. The function call operation. 01:12.360 --> 01:15.840 This takes a single string as argument and returns bool. 01:17.010 --> 01:20.790 And it will return true if the argument has more than 5 characters in it. 01:23.500 --> 01:25.870 Then we have our vector of names again. 01:26.740 --> 01:29.920 And first we are going to do this by writing a manual loop. 01:30.400 --> 01:34.720 So we are going to iterate over the vector elements using a range for loop. 01:35.380 --> 01:39.880 And then we call the functor, with each of these elements as its argument. 01:40.150 --> 01:43.450 So we create an object of the functor, outside the loop. 01:45.400 --> 01:49.360 And then, when this function returns true, the loop will end. 01:52.780 --> 01:58.960 I have also written an algorithm call. So this is going to take the begin() and end() of the vector as 01:58.960 --> 02:02.620 arguments, and it is going to be passed an object of this functor. 02:03.310 --> 02:06.460 Well, we could actually use that one, but it does not really matter. 02:07.540 --> 02:14.290 So find_if() it will call if this functor with each element in turn, as the argument. And when it finds 02:14.290 --> 02:18.090 one which returns true, then the find_if() call it will return. 02:18.090 --> 02:19.540 An iterator to that element. 02:20.710 --> 02:25.630 Then we can check if we have a valid element. If we found a match. And then we can do something with 02:25.630 --> 02:26.320 that iteration. 02:30.310 --> 02:34.180 So there we are, the first name with greater then 5 characters is "Dilbert". 02:36.380 --> 02:36.860 Yes. 02:37.370 --> 02:40.640 Perhaps I should have put them in a different order. Never mind! 02:42.190 --> 02:47.530 So you can imagine the code for the find_if() algorithm looks like this. So we have a pair of iterators, 02:47.530 --> 02:49.030 the iterator range. 02:49.480 --> 02:53.610 We have some callable function and we return another iterator. 02:54.460 --> 02:55.780 Then it loops over the elements. 02:56.140 --> 03:00.220 It calls the function with the current element as the argument. 03:00.910 --> 03:07.180 And then if that is true, then it returns that iterator and ends the loop. If it gets to the end of 03:07.180 --> 03:11.170 the loop without finding a match, then it returns the end of the range iterator. 03:13.840 --> 03:18.970 Incidentally, there is a find_if_not() algorithm, so we can exactly the same code. 03:19.510 --> 03:25.150 And this will look for an element which causes the predicate to return false. 03:25.720 --> 03:30.020 So this is going to call the functor for every element in the vector. 03:30.700 --> 03:36.130 And when it finds one with fewer than 5 characters, it will return an iterator to that element. 03:37.690 --> 03:38.870 So let's try this out. 03:43.360 --> 03:47.410 So the first word with fewer than 5 characters is "PHB". 03:51.780 --> 03:57.660 Let's suppose now that we want to generalize this. So being able to look for 5 characters is good, 03:57.660 --> 04:03.300 but we would like to be able to look for any number of characters. And we can do that by adding state 04:03.300 --> 04:04.260 to our functor. 04:04.920 --> 04:09.600 So we now call it "ge_n", so it is greater than or equal to "n" characters. 04:10.320 --> 04:16.710 We have this member "n", which is the number of characters we are looking for, and we set this in 04:16.710 --> 04:17.460 the constructor. 04:18.750 --> 04:24.900 And then in the operator, we just compare the number of characters in the data to this value "n". 04:28.050 --> 04:33.990 When we call the find_if() algorithm, we need to provide an object, and we need to pass the value of this 04:33.990 --> 04:35.970 number "n" to the constructor. 04:36.570 --> 04:38.820 So that is now an object with 5 as the argument. 04:39.210 --> 04:43.380 So that is going to do the same thing as the one we had before. It is going to find the first element with 04:43.380 --> 04:44.760 more than 5 characters in it. 04:49.200 --> 04:50.490 And it is "Dilbert" again. 04:52.650 --> 04:54.810 So let's just change these around, slightly. 05:05.290 --> 05:12.280 (So it should still be - actually let's change those around.) Just to prove it is not printing out "Dilbert" all the time! 05:16.860 --> 05:20.180 So there we are. The first word with more than 5 characters is "Dogbert". 05:22.380 --> 05:24.840 And then we can change that. So we make that 3. 05:30.480 --> 05:32.700 So the first one is "Asok". 05:32.920 --> 05:35.120 (I think it is pronounced "Ashok", actually.) 05:35.730 --> 05:37.050 And you can experiment with that. 05:37.770 --> 05:40.470 (Okay, so let's just put that back to actually how it was before.) 05:43.650 --> 05:45.600 Okay, so that is it for this video. 05:46.080 --> 05:46.910 I'll see you next time. 05:47.130 --> 05:48.870 Until then, keep coding!