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std::for_each_n

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

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>>` is `true`.	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`.	(since C++20)

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.
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.
-`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

#include <algorithm>
#include <iostream>
#include <vector>
 
void println(auto const& v)
{
    for (auto count{v.size()}; auto const& 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

transform	applies a function to a range of elements, storing results in 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 (C++20)	applies a function object to the first n elements of a sequence (niebloid)