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) 
f
to the result of dereferencing every iterator in the range [first, first + n)
, in order. 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.
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:
The signature does not need to have 
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. 
first + n
.
Exactly n
applications of f
.
The overload with a template parameter named ExecutionPolicy
reports errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. 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; } 
#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,
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) 
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