Equal vs Lexicographical Compare

Use std::lexicographical_compare when you need to determine the relative ordering of two ranges, rather than just checking for equality. This function compares elements in a lexicographical (dictionary) order, which means it checks each element pair until it finds a mismatch.

If it finds that an element in the first range is less than the corresponding element in the second range, it returns true. Here's an 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, 4};
8
9  if (std::lexicographical_compare(
10    A.begin(), A.end(), B.begin(), B.end())) {
11    std::cout << "A is less than B";
12  } else {
13    std::cout << "A is not less than B";
14  }
15}

1A is less than B

In contrast, std::ranges::equal() simply checks if two ranges have the same elements in the same order:

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

Use std::lexicographical_compare():

• When you need to sort or compare ranges based on order
• When implementing algorithms that depend on ordering, such as binary search or merge algorithms

Use std::ranges::equal():

• When you need to check if two ranges are exactly the same
• For equality checks where the order and presence of elements must match exactly

In summary, std::lexicographical_compare() is useful for determining if one range is less than another based on their order, while std::ranges::equal() is used to check for exact equality of ranges.