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

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:

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

#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

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

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