std::for_each_n (3) - Linux Manuals

std::for_each_n: std::for_each_n

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 > (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,