# std::ranges::count, std::ranges::count_if

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate< ranges::equal_to, std::projected<I, Proj>, const T*> constexpr std::iter_difference_t<I> count( I first, S last, const T& value, Proj proj = {} ); | (1) | (since C++20)
template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate< ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::range_difference_t<R> count( R&& r, const T& value, Proj proj = {} ); | (2) | (since C++20)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr std::iter_difference_t<I> count_if( I first, S last, Pred pred, Proj proj = {} ); | (3) | (since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::range_difference_t<R> count_if( R&& r, Pred pred, Proj proj = {} ); | (4) | (since C++20)

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 uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of the elements to examine
value | - | the value to search for
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

Number of elements satisfying the condition.

### Complexity

Exactly last - first comparisons and projection.

### Notes

For the number of elements in the range without any additional criteria, see std::ranges::distance.

### Possible implementation

count
    struct count_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 class T, class Proj = std::identity>
        requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
                                                const T*>
        constexpr std::iter_difference_t<I>
            operator()(I first, S last, const T& value, Proj proj = {}) const
        {
            std::iter_difference_t<I> counter = 0;
            for (; first != last; ++first)
                if (std::invoke(proj, *first) == value)
                    ++counter;
            return counter;
        }

        template<ranges::input_range R, class T, class Proj = std::identity>
        requires std::indirect_binary_predicate<ranges::equal_to,
                                                std::projected<ranges::iterator_t<R>, Proj>,
                                                const T*>
        constexpr ranges::range_difference_t<R>
            operator()(R&& r, const T& value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj));
        }
    };

    inline constexpr count_fn count;
count_if
    struct count_if_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr std::iter_difference_t<I>
            operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            std::iter_difference_t<I> counter = 0;
            for (; first != last; ++first)
                if (std::invoke(pred, std::invoke(proj, *first)))
                    ++counter;
            return counter;
        }

        template<ranges::input_range R, class Proj = std::identity,
                 std::indirect_unary_predicate<
                     std::projected<ranges::iterator_t<R>, Proj>> Pred>
        constexpr ranges::range_difference_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r),
                           std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr count_if_fn count_if;

### 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};

        namespace ranges = std::ranges;

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

        // use a lambda expression to count elements divisible by 11.
        int num_items11 = ranges::count_if(v, [](int i) {return i % 11 == 0;});
        std::cout << "number divisible by eleven: " << num_items11 << '\n';
    }

Output:

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

### See also

ranges::distance (C++20) | returns the distance between an iterator and a sentinel, or between the beginning and end of a range (niebloid)
views::counted (C++20) | creates a subrange from an iterator and a count (customization point object)
ranges::filter_view views::filter (C++20) | a `view` that consists of the elements of a `range` that satisfies a predicate (class template) (range adaptor object)
count count_if | returns the number of elements satisfying specific criteria (function template)

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