Std::for_each_n (3) Linux Manual Page

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 >                                   (1)     (since C++20)

constexpr InputIt for_each_n( InputIt first, Size n, UnaryFunction f );

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>> 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.
         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

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

// 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,

std::for_each_n (3) Linux Manual Page