WEBVTT 00:00.120 --> 00:06.150 Hello again! In this video, we are going to look at write algorithms. A write algorithm is one which can 00:06.150 --> 00:08.430 change the elements in an iterator range. 00:09.300 --> 00:11.460 The first one we are going to look at is replace(). 00:12.090 --> 00:14.910 This will take two arguments, as well as the iterator range. 00:15.690 --> 00:20.910 If there is any element which has the first value, then it will be replaced by an element with the 00:20.910 --> 00:21.570 second value. 00:22.740 --> 00:29.640 For example, if we have a vector and we pass begin() and end(), and 1 and 2, then any elements in this 00:29.640 --> 00:35.700 vector which have the value 1 will have the value 2. If we write this as a loop, 00:35.910 --> 00:41.610 then if the value is 1, set it to 2, otherwise leave it unchanged. 00:44.270 --> 00:46.350 So here is some code to try that out. 00:47.030 --> 00:48.050 We have our vector. 00:50.550 --> 00:56.130 We have the replace() call, so replace elements with value 1 by elements with value 2. 00:57.300 --> 00:58.650 Then we have the handwritten loop. 01:01.700 --> 01:08.700 If the element value is 1, make it 2. Otherwise do nothing. And there we are: the elements with 01:08.700 --> 01:10.920 value 1, now have value 2. 01:12.680 --> 01:15.290 There is also a version which takes a predicate. 01:15.770 --> 01:22.070 If the predicate returns true, then set the value of the element to the fourth argument. So we 01:22.070 --> 01:26.210 can call replace_if() with the iterator range, and then a callable object. 01:26.750 --> 01:33.350 In this case, a lambda expression. And for a change, we are going to work on even numbers: if they are 01:33.380 --> 01:34.820 exactly divisible by 2, 01:35.120 --> 01:36.770 then set the value to 6. 01:38.560 --> 01:44.800 And we could also write a loop in which, if the number is divisible by 2, then change its value. 01:48.960 --> 01:54.030 So there is the call to replace_if() with the lambda expression and the new value of 6. 01:57.750 --> 02:00.690 So there we are. There was 1 even element, which was 4. 02:01.050 --> 02:02.580 And now it is 6. 02:03.180 --> 02:09.330 When an algorithm modifies the data in an iterator range, the library usually provides two versions. 02:10.170 --> 02:13.170 There is a base version which will make the changes in-place. 02:13.500 --> 02:14.640 So this will overwrite 02:14.640 --> 02:16.870 the data in the original iterator range. 02:17.640 --> 02:23.160 And there is another version with underscore copy in the name, and this will write the modified data 02:23.520 --> 02:24.840 to a different iterator range. 02:25.290 --> 02:27.720 So the original data is unchanged. 02:29.390 --> 02:33.800 So this is useful for working with data which is important, or you want to use later. 02:34.550 --> 02:39.110 And also, it makes life easier when you are working with associative containers. As you will see 02:39.110 --> 02:44.120 when we get onto those, modifying the elements of an associative container can be a little bit more 02:44.120 --> 02:44.600 involved. 02:45.470 --> 02:50.330 The replace() algorithm has a _copy version, which will write the data to some destination. 02:51.680 --> 02:55.610 So we need to give an iterator, to tell the algorithm where to write this data. 02:56.000 --> 02:58.040 And usually, that will be an insert iterator. 02:59.120 --> 03:04.520 So we have the iterator range that we are processing. So we can use const iterators here, because 03:04.520 --> 03:05.810 this is not going to be modified. 03:06.500 --> 03:09.350 Then we have the iterator for where we are writing the data. 03:10.130 --> 03:11.090 Then we have the value 03:11.090 --> 03:13.790 we want to modify. And then we have the new value. 03:14.540 --> 03:20.780 So this is going to leave the original vector unchanged and write out a new vector with the same elements. 03:20.900 --> 03:22.940 Except that elements with the value 1, 03:22.970 --> 03:23.930 now have the value 2. 03:28.760 --> 03:31.220 So here is our call to replace_copy(). 03:32.660 --> 03:36.770 We are going to iterate through the elements of this vector and copy them to a new vector. 03:37.400 --> 03:42.800 If the element value is 1, then the new vector will have 2. Otherwise, it will have the same value. 03:45.050 --> 03:50.630 If we were writing this by hand, we would push back a different value, depending on the value of the element. 03:50.960 --> 03:53.720 If the element is 1, then we would push back 2. 03:54.170 --> 03:56.960 Otherwise, we just push back the original value. 04:00.600 --> 04:01.260 And there we are. 04:01.470 --> 04:08.010 So we have a new vector in which 3, 4, 5, 9 are the same, but 1 is replaced by 2. 04:10.100 --> 04:16.250 And there is also a copy version of replace_if(). It is called replace_copy_if(). So this will take 04:16.250 --> 04:18.260 the iterator range from the original vector. 04:18.680 --> 04:21.860 The destination iterator. The lambda expression. 04:22.580 --> 04:27.350 And if this lambda expression returns true, the element in the new vector will have the value 04:27.370 --> 04:30.800 six. Otherwise, it have the same value that it does in "vec1". 04:34.820 --> 04:36.800 So there it is, replace_copy_if(). 04:38.260 --> 04:44.470 The lambda expression: if the number is even, it will have the value 6, otherwise it will have the original 04:44.470 --> 04:44.800 value. 04:47.460 --> 04:51.870 And again, we could write a loop with if statements and push_backs. 04:54.630 --> 05:00.260 So there we are, we have the even value 4 in the original vector and in the new vector, that is 05:00.270 --> 05:00.720 six. 05:00.990 --> 05:02.760 But the other elements all have the same values. 05:04.230 --> 05:05.820 OK, so that is it for this video. 05:06.300 --> 05:07.140 I will see you next time. 05:07.140 --> 05:09.240 But until then, keep coding!