std::count, std::count_if

 

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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>
	(1)
template<class InputIt, class T > typenamestd::iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T& value );			(constexpr since C++20) (until C++26)
template<class InputIt, class T =typenamestd::iterator_traits                                        <InputIt>::value_type> constexprtypenamestd::iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T& value );			(since C++26)
		(2)
template<class ExecutionPolicy, class ForwardIt, class T > typenamestd::iterator_traits<ForwardIt>::difference_type     count( ExecutionPolicy&& policy,            ForwardIt first, ForwardIt last, const T& value );				(since C++17) (until C++26)
template<class ExecutionPolicy,           class ForwardIt, class T =typenamestd::iterator_traits                                          <ForwardIt>::value_type> typenamestd::iterator_traits<ForwardIt>::difference_type     count( ExecutionPolicy&& policy,            ForwardIt first, ForwardIt last, const T& value );				(since C++26)
template<class InputIt, class UnaryPred > typenamestd::iterator_traits<InputIt>::difference_type     count_if( InputIt first, InputIt last, UnaryPred p );			(3)	(constexpr since C++20)
template<class ExecutionPolicy, class ForwardIt, class UnaryPred > typenamestd::iterator_traits<ForwardIt>::difference_type     count_if( ExecutionPolicy&& policy,               ForwardIt first, ForwardIt last, UnaryPred p );			(4)	(since C++17)

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

1) Counts the elements that are equal to value (using operator==).

3) Counts elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy.

These overloads participate 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)

Parameters

first, last	-	the pair of iterators defining the range of elements to examine
value	-	the value to search for
policy	-	the execution policy to use
p	-	unary predicate which returns ​true for the required elements. The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​
Type requirements
- InputIt must meet the requirements of LegacyInputIterator.
- ForwardIt must meet the requirements of LegacyForwardIterator.
- UnaryPred must meet the requirements of Predicate.

Return value

The number of iterators it in the range[first, last) satisfying the following condition:

1,2)*it == value is true.

3,4)p(*it)!=false is true.

Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.

3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

Exceptions

The overloads with a template parameter named ExecutionPolicy report 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.

Notes

For the number of elements in the range [first, last) without any additional criteria, see std::distance.

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

Possible implementation

See also the implementations of count in libstdc++ and libc++.

See also the implementations of count_if in libstdc++ and libc++.

count (1)
template<class InputIt, class T =typenamestd::iterator_traits<InputIt>::value_type>typenamestd::iterator_traits<InputIt>::difference_type count(InputIt first, InputIt last, const T& value){typenamestd::iterator_traits<InputIt>::difference_type ret =0;for(; first != last;++first)if(*first == value)++ret;return ret;}
count_if (3)
template<class InputIt, class UnaryPred>typenamestd::iterator_traits<InputIt>::difference_type count_if(InputIt first, InputIt last, UnaryPred p){typenamestd::iterator_traits<InputIt>::difference_type ret =0;for(; first != last;++first)if(p(*first))++ret;return ret;}

Example

Run this code

#include <algorithm>#include <array>#include <cassert>#include <complex>#include <iostream>#include <iterator>   int main(){constexprstd::array v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10};std::cout<<"v: ";std::copy(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " "));std::cout<<'\n';   // Determine how many integers match a target value.for(constint target :{3, 4, 5}){constint num_items = std::count(v.cbegin(), v.cend(), target);std::cout<<"number: "<< target <<", count: "<< num_items <<'\n';}   // Use a lambda expression to count elements divisible by 4.int count_div4 = std::count_if(v.begin(), v.end(), [](int i){return i %4==0;});std::cout<<"numbers divisible by four: "<< count_div4 <<'\n';   // A simplified version of `distance` with O(N) complexity:auto distance =[](auto first, auto last){return std::count_if(first, last, [](auto){returntrue;});}; static_assert(distance(v.begin(), v.end())==10);   std::array<std::complex<double>, 3> nums{{{4, 2}, {1, 3}, {4, 2}}};#ifdef __cpp_lib_algorithm_default_value_type// T gets deduced making list-initialization possibleauto c = std::count(nums.cbegin(), nums.cend(), {4, 2});#elseauto c = std::count(nums.cbegin(), nums.cend(), std::complex<double>{4, 2});#endifassert(c ==2);}

Output:

v: 1 2 3 4 4 3 7 8 9 10 number: 3, count: 2 number: 4, count: 2 number: 5, count: 0 numbers divisible by four: 3

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be EqualityComparable, but the value type of InputIt is not always T	removed the requirement

See also

distance	returns the distance between two iterators (function template)
ranges::countranges::count_if (C++20)(C++20)	returns the number of elements satisfying specific criteria (algorithm function object)