std::for_each_n

 

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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 all following conditions are satisfied:

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
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(autoconst& v){for(auto count{v.size()};constauto& 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 unary function object to elements from a range (function template)
ranges::for_each_n (C++20)	applies a function object to the first N elements of a sequence (algorithm function object)