std::ranges::count, std::ranges::count_if

 

C++

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Feature test macros(C++20)

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Concepts library(C++20)

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Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control 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
	(1)
template<std::input_iterator I, std::sentinel_for<I> S,           class T, class Proj =std::identity> requires std::indirect_binary_predicate              <ranges::equal_to, std::projected<I, Proj>, const T*> constexprstd::iter_difference_t<I>     count( I first, S last, const T& value, Proj proj ={});			(since C++20) (until C++26)
template<std::input_iterator I, std::sentinel_for<I> S,           class Proj =std::identity,           class T = std::projected_value_t<I, Proj>> requires std::indirect_binary_predicate              <ranges::equal_to, std::projected<I, Proj>, const T*> constexprstd::iter_difference_t<I>     count( I first, S last, const T& value, Proj proj ={});			(since C++26)
		(2)
template<ranges::input_range R, class T, class Proj =std::identity> requires std::indirect_binary_predicate              <ranges::equal_to,               std::projected<ranges::iterator_t<R>, Proj>, const T*> constexprranges::range_difference_t<R>     count( R&& r, const T& value, Proj proj ={});				(since C++20) (until C++26)
template<ranges::input_range R, class Proj =std::identity,           class T = std::projected_value_t<ranges::iterator_t<R>, Proj>> requires std::indirect_binary_predicate              <ranges::equal_to,               std::projected<ranges::iterator_t<R>, Proj>, const T*> constexprranges::range_difference_t<R>     count( R&& r, const T& value, Proj proj ={});				(since C++26)
template<std::input_iterator I, std::sentinel_for<I> S,           class Proj =std::identity,           std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexprstd::iter_difference_t<I>     count_if( I first, S last, Pred pred, Proj proj ={});			(3)	(since C++20)
template<ranges::input_range R, class Proj =std::identity,           std::indirect_unary_predicate<               std::projected<ranges::iterator_t<R>, Proj>> Pred > constexprranges::range_difference_t<R>     count_if( R&& r, Pred pred, Proj proj ={});			(4)	(since C++20)

Returns the number of elements in the range [first, last) satisfying specific criteria.

1) Counts the elements that are equal to value.

3) Counts elements for which predicate p returns true.

2,4) Same as (1,3), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Parameters

first, last	-	the iterator-sentinel pair defining the range of elements to examine
r	-	the range of the elements to examine
value	-	the value to search for
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements

Return value

Number of elements satisfying the condition.

Complexity

Exactly last - first comparisons and projection.

Notes

For the number of elements in the range without any additional criteria, see std::ranges::distance.

Feature-test macro	Value	Std	Feature
__cpp_lib_algorithm_default_value_type	202403	(C++26)	List-initialization for algorithms (1,2)

Possible implementation

count (1)
struct count_fn {template<std::input_iterator I, std::sentinel_for<I> S, class Proj =std::identity, class T = std::projected_value_t<I, Proj>> requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>, const T*>constexprstd::iter_difference_t<I> operator()(I first, S last, const T& value, Proj proj ={})const{std::iter_difference_t<I> counter =0;for(; first != last;++first)if(std::invoke(proj, *first)== value)++counter;return counter;}   template<ranges::input_range R, class Proj =std::identityclass T = std::projected_value_t<ranges::iterator_t<R>, Proj>> requires std::indirect_binary_predicate<ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*>constexprranges::range_difference_t<R> operator()(R&& r, const T& value, Proj proj ={})const{return(*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj));}};   inlineconstexpr count_fn count;
count_if (3)
struct count_if_fn {template<std::input_iterator I, std::sentinel_for<I> S, class Proj =std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred>constexprstd::iter_difference_t<I> operator()(I first, S last, Pred pred, Proj proj ={})const{std::iter_difference_t<I> counter =0;for(; first != last;++first)if(std::invoke(pred, std::invoke(proj, *first)))++counter;return counter;}   template<ranges::input_range R, class Proj =std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred>constexprranges::range_difference_t<R> operator()(R&& r, Pred pred, Proj proj ={})const{return(*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));}};   inlineconstexpr count_if_fn count_if;

Example

Run this code

#include <algorithm>#include <cassert>#include <complex>#include <iostream>#include <vector>   int main(){std::vector<int> v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10};   namespace ranges = std::ranges;   // determine how many integers in a std::vector match a target value.int target1 =3;int target2 =5;int num_items1 = ranges::count(v.begin(), v.end(), target1);int num_items2 = ranges::count(v, target2);std::cout<<"number: "<< target1 <<" count: "<< num_items1 <<'\n';std::cout<<"number: "<< target2 <<" count: "<< num_items2 <<'\n';   // use a lambda expression to count elements divisible by 3.int num_items3 = ranges::count_if(v.begin(), v.end(), [](int i){return i %3==0;});std::cout<<"number divisible by three: "<< num_items3 <<'\n';   // use a lambda expression to count elements divisible by 11.int num_items11 = ranges::count_if(v, [](int i){return i %11==0;});std::cout<<"number divisible by eleven: "<< num_items11 <<'\n';   std::vector<std::complex<double>> nums{{4, 2}, {1, 3}, {4, 2}};#ifdef __cpp_lib_algorithm_default_value_typeauto c = ranges::count(nums, {4, 2});#elseauto c = ranges::count(nums, std::complex<double>{4, 2});#endifassert(c ==2);}

Output:

number: 3 count: 2 number: 5 count: 0 number divisible by three: 3 number divisible by eleven: 0

See also

ranges::distance (C++20)	returns the distance between an iterator and a sentinel, or between the beginning and end of a range (algorithm function object)
views::counted (C++20)	creates a subrange from an iterator and a count (customization point object)
ranges::filter_viewviews::filter (C++20)	a view that consists of the elements of a range that satisfies a predicate (class template)(range adaptor object)
countcount_if	returns the number of elements satisfying specific criteria (function template)