std::for_each_n

 

C++

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Feature test macros (C++20)

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
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)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template< class InputIt, class Size, class UnaryFunc > InputIt for_each_n( InputIt first, Size n, UnaryFunc f );	(1)	(since C++17) (constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt, class Size, class UnaryFunc > ForwardIt for_each_n( ExecutionPolicy&& policy,                       ForwardIt first, Size n, UnaryFunc f );	(2)	(since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n). If f returns a result, the result is ignored.

1) f is applied in order starting from first.

If UnaryFunc is not MoveConstructible, the behavior is undefined.

2) f might not be applied in order. The algorithm is executed according to policy.

This overload participates in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

If UnaryFunc is not CopyConstructible, the behavior is undefined.

If n >= 0 is not true, the behavior is undefined.

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.

Unlike the rest of the parallel algorithms, for_each_n is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

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.
- Size must be convertible to an integral type.

Return value

An iterator equal to first + n, or more formally, to std::advance(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

See also the implementation in libstdc++, libc++ and MSVC stdlib.

template<class InputIt, class Size, class UnaryFunc> InputIt for_each_n(InputIt first, Size n, UnaryFunc f) { for (Size i = 0; i < n; ++first, (void) ++i) f(*first);   return first; }

Example

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
void println(auto const& v)
{
    for (auto count{v.size()}; const auto& e : v)
        std::cout << e << (--count ? ", " : "\n");
}
 
int main()
{
    std::vector<int> vi{1, 2, 3, 4, 5};
    println(vi);
 
    std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });
    println(vi);
}

Output:

1, 2, 3, 4, 5
2, 4, 6, 4, 5

See also

transform	applies a function to a range of elements, storing results in a destination range (function template)
range-for loop(C++11)	executes loop over range
for_each	applies a function to a range of elements (function template)
ranges::for_each_n (C++20)	applies a function object to the first N elements of a sequence (niebloid)