std::for_each_n

 

C++

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Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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

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,

See also

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)