std::for_each_n
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Defined in header <algorithm>


template< class InputIt, class Size, class UnaryFunction > InputIt for_each_n( InputIt first, Size n, UnaryFunction f ); 
(1)  (since C++17) 
template< class ExecutionPolicy, class ForwardIt, class Size, class UnaryFunction2 > InputIt 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 &. 
Type requirements  
InputIt must meet the requirements of InputIterator .
 
ForwardIt must meet the requirements of ForwardIterator .
 
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 three standard policies, std::terminate is called. For any otherExecutionPolicy
, the behavior is implementationdefined.  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
This section is incomplete Reason: no example 
See also
applies a function to a range of elements (function template)  
rangefor loop  executes loop over range (since C++11) 
applies a function to a range of elements (function template) 