std::equal() vs std::is_permutation()

The main difference between std::ranges::equal() and std::ranges::is_permutation() lies in how they compare collections.

std::ranges::equal() checks if two collections have the same elements in the same order. If any element differs or is out of order, the function returns false. For example:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> A{1, 2, 3};
7  std::vector<int> B{1, 2, 3};
8
9  if (std::ranges::equal(A, B)) {
10    std::cout << "Ranges are equal";
11  } else {
12    std::cout << "Ranges are not equal";
13  }
14}

1Ranges are equal

In this case, std::ranges::equal() returns true because both collections contain the same elements in the same order.

On the other hand, std::ranges::is_permutation() checks if two collections contain the same elements, regardless of order. This means that even if the elements are shuffled, the function will return true if all elements match:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> A{1, 2, 3};
7  std::vector<int> B{3, 2, 1};
8
9  if (std::ranges::is_permutation(A, B)) {
10    std::cout << "Ranges are a permutation";
11  } else {
12    std::cout << "Ranges are not a permutation";
13  }
14}

1Ranges are a permutation

Here, std::ranges::is_permutation() returns true because both collections contain the same elements, even though they are in a different order.

To summarize:

• Use std::ranges::equal() when the order of elements matters.
• Use std::ranges::is_permutation() when the order of elements does not matter, and you only care about the presence of the same elements.

These functions are useful in different scenarios depending on whether the sequence of elements is important for your application.