Is it possible to copy elements from a range to a destination with a different ordering, not just reversed or rotated?

Question

Ryan McCombe · Accepted Answer

Yes, it is possible to copy elements from a range to a destination with a different ordering, not just reversed or rotated.

You can achieve this by using custom sorting or shuffling algorithms before or during the copy process.

Custom Sorting Before Copy

One approach is to sort the elements in the desired order before copying them to the destination container. Here's an example:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> Source{5, 3, 1, 4, 2};
7  std::vector<int> Destination(5);
8
9  // Sort the source range in ascending order
10  std::sort(Source.begin(), Source.end());
11
12  // Copy the sorted range to the destination
13  std::ranges::copy(Source, Destination.begin());
14
15  for (int Value : Destination) {
16    std::cout << Value << ", ";
17  }
18}

11, 2, 3, 4, 5,

Custom Order During Copy

Another approach is to define a custom order during the copy process using a permutation or a custom transformation.

Using a Permutation Index

You can use an index array to define a custom order and apply this order during the copy process:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> Source{10, 20, 30, 40, 50};
7  std::vector<int> Destination(5);
8  std::vector<size_t> Permutation{4, 2, 0, 3, 1};
9
10  for (size_t i = 0; i < Source.size(); ++i) {
11    Destination[i] = Source[Permutation[i]]; 
12  }
13
14  for (int Value : Destination) {
15    std::cout << Value << ", ";
16  }
17}

150, 30, 10, 40, 20,

Using Transform Iterator

For more complex transformations, you can use a transform iterator to apply a function to each element during the copy:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4#include <iterator>
5
6int main() {
7  std::vector<int> Source{1, 2, 3, 4, 5};
8  std::vector<int> Destination(5);
9
10  std::transform(Source.begin(), Source.end(),
11                 Destination.begin(),
12                 [](int x) { return x * x; }); 
13
14  for (int Value : Destination) {
15    std::cout << Value << ", ";
16  }
17}

11, 4, 9, 16, 25,

Using Custom Comparison

You can also use a custom comparison function to define a specific order and then copy the elements:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5bool customCompare(int a, int b) {
6  // Group even and odd numbers
7  return (a % 2) < (b % 2); 
8}
9
10int main() {
11  std::vector<int> Source{1, 2, 3, 4, 5};
12  std::vector<int> Destination(5);
13
14  std::sort(Source.begin(), Source.end(),
15    customCompare);
16
17  std::ranges::copy(Source, Destination.begin());
18
19  for (int Value : Destination) {
20    std::cout << Value << ", ";
21  }
22}

12, 4, 1, 3, 5,

By using these techniques, you can copy elements in a custom order that suits your specific requirements.

Copying Algorithms

Copying with Different Ordering