# std::count, std::count_if

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< cpp‎ | algorithm

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 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

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

### See also

distance | returns the distance between two iterators (function template)

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