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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.
Type requirements
- `InputIt` must meet the requirements of InputIterator.
- `ForwardIt` must meet the requirements of ForwardIterator.

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 three 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

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 <vector>

int main()
{
    std::vector<int> v{ 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };

    // determine how many integers in a std::vector match a target value.
    int target1 = 3;
    int target2 = 5;
    int num_items1 = std::count(v.begin(), v.end(), target1);
    int num_items2 = std::count(v.begin(), v.end(), 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 = std::count_if(v.begin(), v.end(), [](int i){return i % 3 == 0;});
    std::cout << "number divisible by three: " << num_items3 << '\n';
}

Output:

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

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