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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.

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.

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

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

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';
}

Output:

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

transform	applies a function to a range of elements (function template)
range-for loop	executes loop over range (since C++11)
for_each	applies a function to a range of elements (function template)