std::ranges::for_each_n, std::ranges::for_each_n_result

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

Defined in header <algorithm>
Call signature
template< std::input_iterator I, class Proj = std::identity,           std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > constexpr for_each_n_result<I, Fun>     for_each_n( I first, std::iter_difference_t<I> n, Fun f, Proj proj = {});	(1)	(since C++20)
Helper types
template< class I, class F > using for_each_n_result = ranges::in_fun_result<I, F>;	(2)	(since C++20)

1) Applies the given function object f to the projected result by proj of dereferencing each iterator in the range [first, first + n), in order.

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.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first	-	iterator denoting the begin of the range to apply the function to
n	-	the number of elements to apply the function to
f	-	the function to apply to the projected range [first, first + n)
proj	-	projection to apply to the elements

Return value

An object {first + n, std::move(f)}, where first + n may be evaluated as std::ranges::next(std::move(first), n) depending on iterator category.

Complexity

Exactly n applications of f and proj.

Possible implementation

struct for_each_n_fn
{
    template<std::input_iterator I, class Proj = std::identity,
             std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
    constexpr for_each_n_result<I, Fun>
        operator()(I first, std::iter_difference_t<I> n, Fun fun, Proj proj = Proj{}) const
    {
        for (; n-- > 0; ++first)
            std::invoke(fun, std::invoke(proj, *first));
        return {std::move(first), std::move(fun)};
    }
};
 
inline constexpr for_each_n_fn for_each_n {};

Example

Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <ranges>
#include <string_view>
 
struct P
{
    int first;
    char second;
    friend std::ostream& operator<<(std::ostream& os, const P& p)
    {
        return os << '{' << p.first << ",'" << p.second << "'}";
    }
};
 
auto print = [](std::string_view name, auto const& v)
{
    std::cout << name << ": ";
    for (auto n = v.size(); const auto& e : v)
        std::cout << e << (--n ? ", " : "\n");
};
 
int main()
{
    std::array a {1, 2, 3, 4, 5};
    print("a", a);
    // Negate first three numbers:
    std::ranges::for_each_n(a.begin(), 3, [](auto& n) { n *= -1; });
    print("a", a);
 
    std::array s { P{1,'a'}, P{2, 'b'}, P{3, 'c'}, P{4, 'd'} };
    print("s", s);
    // Negate data members 'P::first' using projection:
    std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first);
    print("s", s);
    // Capitalize data members 'P::second' using projection:
    std::ranges::for_each_n(s.begin(), 3, [](auto& c) { c -= 'a'-'A'; }, &P::second);
    print("s", s);
}

Output:

a: 1, 2, 3, 4, 5
a: -1, -2, -3, 4, 5
s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}

See also

range-for loop(C++11)	executes loop over range
ranges::for_each (C++20)	applies a function to a range of elements (niebloid)
for_each_n (C++17)	applies a function object to the first N elements of a sequence (function template)
for_each	applies a function to a range of elements (function template)