c++: Swap first and second element for all tuples in a vector

Question

is there a way to swap the first and second elements for all tuples in a vector? Let's say I have something like this:

#include<vector>
#include<tuple>

int main()
{
std::vector<std::tuple<int,int>> my_vector;
my_vector.push_back(std::make_tuple(1,2));
my_vector.push_back(std::make_tuple(3,4));
}

The first elements of the tuple are now 1 and 3 with the second elements being 2 and 4. Is there an easy to make 2 and 4 the first elements?

`std::tuple` can have more than two elements. Why don't you use `std::pair` instead? — NutCracker, Jan 09 '20 at 14:03
So your question is actually: "How can I exchange the first and the second value of a 2 element tuple?" — Jabberwocky, Jan 09 '20 at 14:03
*is there a way to swap the first and second elements for all tuples in a vector?* -- If the answer to that was "no", C++ would be a very weak language. — PaulMcKenzie, Jan 09 '20 at 14:04
@Jabberwocky yes this is basically my question. My english is not that great. — AkariYukari, Jan 09 '20 at 14:05
I think Jabberwocky just wanted to point out that it doesnt matter that the tuples are in a vector. If you know how to do it with one you also can do it in a loop with many — 463035818_is_not_an_ai, Jan 09 '20 at 14:05
@AkariYukari -- *all tuples in a vector* -- Use `std::transform` and the appropriate lambda to swap. — PaulMcKenzie, Jan 09 '20 at 14:05
Use `std::swap` and pass the result of `std::get<...>` for each value in the tuple? — Some programmer dude, Jan 09 '20 at 14:06

Evg · Accepted Answer · 2020-01-09T14:22:15.567

You can use std::for_each algorithm with a lambda:

std::for_each(my_vector.begin(), my_vector.end(), [](auto& tuple) {
    std::swap(std::get<0>(tuple), std::get<1>(tuple));
});

You can also use a range-based for:

for (auto& tuple : my_vector)
    std::swap(std::get<0>(tuple), std::get<1>(tuple));

If you decide to replace std::tuple with std::pair, this code will also work, because std::pair's members first and second can be accessed using std::get<0> and std::get<1>, respectively.

Instead of std::swap(...) you might want to write it this way:

using std::swap;
swap(std::get<0>(tuple), std::get<1>(tuple));

This is useful if instead of int, you have some user-defined type T, for which void swap(T&, T&) function is implemented (being presumably more performant than std::swap) and can be found by argument-dependent lookup.

NutCracker · Answer 2 · 2020-01-09T14:09:43.617

Since your std::tuples are containing only two values, I would suggest using std::pair instead.

#include <iostream>
#include <vector>
#include <utility>

int main()
{
    std::vector<std::pair<int,int>> my_vector;
    my_vector.push_back(std::make_pair(1,2));
    my_vector.push_back(std::make_pair(3,4));

    for (auto const& p : my_vector) {
        std::cout << p.first << " " << p.second << std::endl;
    }

    for (auto& p : my_vector) {
        std::swap(p.first, p.second);
    }

    for (auto const& p : my_vector) {
        std::cout << p.first << " " << p.second << std::endl;
    }
}

Live example

@formerlyknownas_463035818 sure. I was planning to add that. — NutCracker, Jan 09 '20 at 14:07

c++: Swap first and second element for all tuples in a vector

2 Answers2