NAME¶

std::for_each_n - std::for_each_n

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class Size, class UnaryFunc > (since C++17)
InputIt for_each_n( InputIt first, Size n, UnaryFunc f (1) (constexpr since C++20)
);
template< class ExecutionPolicy,

class ForwardIt, class Size, class UnaryFunc >
ForwardIt for_each_n( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, Size n, UnaryFunc
f );

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.
If UnaryFunc is not MoveConstructible, the behavior is undefined.
2) f might not be applied in order. The algorithm is executed according to policy.
This overload participates in overload resolution only if

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 UnaryFunc is not CopyConstructible, the behavior is undefined.

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. 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.
-
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¶

// Run this code

#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¶

applies a function to a range of elements, storing results in
transform a destination range
(function template)
range-for loop(C++11) executes loop over range
for_each applies a function to a range of elements
(function template)
ranges::for_each_n applies a function object to the first N elements of a
(C++20) sequence
(niebloid)