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)

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)

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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 objectf to the result of dereferencing every iterator in the range[first, first+ n). Iff returns a result, the result is ignored.

1)f is applied in order starting fromfirst.

IfUnaryFunc is notMoveConstructible, the behavior is undefined.

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

This overload participates in overload resolution only if all following conditions are satisfied:

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

IfUnaryFunc is notCopyConstructible, the behavior is undefined.

Ifn>=0 is nottrue, 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 areTriviallyCopyable.

[edit]Parameters

first	-	the beginning of the range to apply the function to
n	-	the number of elements to apply the function to
policy	-	theexecution 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 haveconst&. The type Type must be such that an object of typeInputIt can be dereferenced and then implicitly converted to Type.
Type requirements
- `InputIt` must meet the requirements ofLegacyInputIterator.
- `ForwardIt` must meet the requirements ofLegacyForwardIterator.
- `Size` must be convertible to an integral type.

[edit]Return value

An iterator equal tofirst+ n, or more formally, tostd::advance(first, n).

[edit]Complexity

Exactlyn applications off.

[edit]Exceptions

The overload with a template parameter namedExecutionPolicy reports errors as follows:

If execution of a function invoked as part of the algorithm throws an exception andExecutionPolicy is one of thestandard policies,std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory,std::bad_alloc is thrown.

[edit]Possible implementation

See also the implementation inlibstdc++,libc++ andMSVC 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;}

[edit]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, 52, 4, 6, 4, 5

[edit]See also

transform	applies a function to a range of elements, storing results in a destination range (function template)[edit]
range-`for` loop(C++11)	executes loop over range[edit]
for_each	applies a unaryfunction object to elements from arange (function template)[edit]
ranges::for_each_n (C++20)	applies a function object to the first N elements of a sequence (algorithm function object)[edit]