Sort a vector of pairs by first element then by second element of the pair in C++? [duplicate]

Answer 1

pairs by default compare by first element, then second. So, if you don't care about preserving the order when the first elements compare equal, then you can just use std::sort:

std::sort(v.begin(), v.end());

Answer 2

std::pairs comparison operators compare pairs lexicographically, it first compares the first elements, then the second elements if the first elements are equal.

Here is an example of using std::vector<std::pair<int, int>> and std::sort.

Using std::sort that way uses std::pair's operator <, which, as said above, compares the pairs lexicographically.

UPDATE: Here is an example using std::stable_sort and a custom comparison function that compares only the first element.

By using std::stable_sort, you are guaranteed that the relative order of equal elements are preserved. That is, even if the first elements of the std::pairs are equal, the original relative order is still retained.