std::for_each_n - cppreference.com

template< class InputIt, class Size, class UnaryFunc >
InputIt for_each_n( InputIt first, Size count, UnaryFunc f );

template< class ExecutionPolicy,
          class ForwardIt, class Size, class UnaryFunc >
ForwardIt for_each_n( ExecutionPolicy&& policy,
                      ForwardIt first, Size count, UnaryFunc f );

Applies the given invocable object f to each element in the target range [first, std::next(first, count)). If f returns a result, the result is ignored.

1) f is applied in order starting from first.

2) f might not be applied in order. The algorithm is executed according to policy.

Unlike other parallel algorithms, for_each_n is not allowed to make arbitrary copies of elements from the target range.

If count >= 0 is not true, the behavior is undefined.

Parameters

Return value

std::next(first, count)

Complexity

Exactly count applications of f.

Exceptions

Notes

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements in the target range.

Possible implementation

See also the implementation in libstdc++, libc++ and MSVC stdlib.

template<class InputIt, class Size, class UnaryFunc>
InputIt for_each_n(InputIt first, Size count, UnaryFunc f)
{
    for (Size i = 0; i < count; ++first, (void) ++i)
        f(*first);
    
    return first;
}

Example

#include <algorithm>
#include <iostream>
#include <vector>

void println(const auto& v)
{
    for (auto count{v.size()}; const auto& e : v)
        std::cout << e << (--count ? ", " : "\n");
}

int main()
{
    std::vector<int> vi{1, 2, 3, 4, 5};
    println(vi);
    
    std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });
    println(vi);
}

1, 2, 3, 4, 5
2, 4, 6, 4, 5

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also