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std::for_each_n - cppreference.com

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Defined in header <algorithm>

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

 (1) (since C++17)
(constexpr since C++20)

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

 (2) (since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n). 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.

This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

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

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.

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 TriviallyCopyable.

Parameters

first - the beginning of the range to apply the function to
n - the number of elements to apply the function to
policy - the execution policy to use
f - function object, to be applied to the result of dereferencing every iterator in the range [first, first + n)

The signature of the function should be equivalent to the following:

void fun(const Type &a);

The signature does not need to have const &.
The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.

​

Type requirements
-InputIt must meet the requirements of LegacyInputIterator.
-ForwardIt must meet the requirements of LegacyForwardIterator.
-Size must be convertible to an integral type.

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

Example

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

void println(auto const& 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);
}

Output: 

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

See also