NAME

std::for_each_n - std::for_each_n

Synopsis

Defined in header <algorithm>
template< class InputIt, class Size, class UnaryFunction > (since C++17)
InputIt for_each_n( InputIt first, Size n, UnaryFunction f ); (until C++20)
template< class InputIt, class Size, class UnaryFunction >
constexpr InputIt for_each_n( InputIt first, Size n, (since C++20)
UnaryFunction f ); (1)
template< class ExecutionPolicy, class ForwardIt, class Size,
class UnaryFunction2 > (2) (since C++17)
ForwardIt for_each_n( ExecutionPolicy&& policy, ForwardIt
first, Size n, UnaryFunction2 f );

1) Applies the given function object f to the result of dereferencing every iterator
in the range [first, first + n), in order.
2) Applies the given function object f to the result of dereferencing every iterator
in the range [first, first + n) (not necessarily in order). The algorithm is
executed according to policy. This overload does not participate in overload
resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

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

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. See execution policy for details.
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:

f - 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.
-
UnaryFunction must meet the requirements of MoveConstructible. Does not have to be
CopyConstructible
-
UnaryFunction2 must meet the requirements of CopyConstructible.

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

// Run this code

#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';
}

Output:

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