Difference between revisions of "cpp/algorithm/for each n"

Revision as of 15:15, 29 December 2022

For both overloads, if the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored. If n is less than zero, the behavior is undefined.

Unlike the rest of the parallel algorithms, for_each_n is not allowed to make copies of the elements in the sequence even if they are trivially copyable.

Parameters

Return value

An iterator equal to first + n, or more formally, to std::advance(first, n).

Complexity

Exactly n applications of f

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

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

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

Example

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> ns{1, 2, 3, 4, 5};
 
    for (auto n: ns)
        std::cout << n << ", ";
    std::cout << '\n';
 
    std::for_each_n(ns.begin(), 3, [](auto& n){ n *= 2; });
    for (auto n: ns)
        std::cout << n << ", ";
    std::cout << '\n';
}

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

See also