std::count, std::count_if

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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
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)
Atomic operations library (C++11)
Thread support library (C++11)
Technical specifications
Symbols index
External libraries

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)