std::count, std::count_if

 

C++

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Freestanding and hosted

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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)

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
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class InputIt, class T > typename iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T& value );			(until C++20)
template< class InputIt, class T > constexpr typename iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T& value );			(since C++20)
template< class ExecutionPolicy, class ForwardIt, class T > typename iterator_traits<ForwardIt>::difference_type     count( ExecutionPolicy&& policy,            ForwardIt first, ForwardIt last, const T& value );		(2)	(since C++17)
		(3)
template< class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type     count_if( InputIt first, InputIt last, UnaryPredicate p );				(until C++20)
template< class InputIt, class UnaryPredicate > constexpr typename iterator_traits<InputIt>::difference_type     count_if( InputIt first, InputIt last, UnaryPredicate p );				(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > typename iterator_traits<ForwardIt>::difference_type     count_if( ExecutionPolicy&& policy,               ForwardIt first, ForwardIt last, UnaryPredicate 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.

3) counts elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.

Parameters

first, last	-	the range of elements to examine
value	-	the value to search for
policy	-	the execution policy to use. See execution policy for details.
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.

Return value

number of elements satisfying the condition.

Complexity

exactly last - first comparisons / applications of the predicate

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.

Possible implementation

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

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

First version
template<class InputIt, class T> typename iterator_traits<InputIt>::difference_type count(InputIt first, InputIt last, const T& value) { typename iterator_traits<InputIt>::difference_type ret = 0; for (; first != last; ++first) { if (*first == value) { ret++; } } return ret; }
Second version
template<class InputIt, class UnaryPredicate> typename iterator_traits<InputIt>::difference_type count_if(InputIt first, InputIt last, UnaryPredicate p) { typename iterator_traits<InputIt>::difference_type ret = 0; for (; first != last; ++first) { if (p(*first)) { ret++; } } return ret; }

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <array>
 
int main()
{
    constexpr std::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 (const int target: {3, 4, 5}) {
        const int 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){return true;});
    };
    static_assert(distance(v.begin(), v.end()) == 10);
}

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

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 (niebloid)