std::ranges::count() algorithm

since C++20

Simplified

// (1)
constexpr std::iter_difference_t<I>
    count( I first, S last, const T& value, Proj proj = {} );

// (2)
constexpr ranges::range_difference_t<R>
    count( R&& r, const T& value, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::input_iterator
• S - std::sentinel_for<I>
• R - std::ranges::input_range
• T - (none)
• P - (none)

The Proj template argument has a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
• (2) - indirect_binary_predicate<ranges::equal_to, projected<ranges::iterator_t<R>, Proj>, const T*>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
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 = {} );

// (2)
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 = {} );

Returns the number of elements equal to value in the given range.

• (1) Counts the elements that are equal to value (using operator==).
• (2) Same as (1), 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.

Parameters

first last	The range of elements to examine.
r	The range of elements to examine.
value	The value to search for.
proj	Projection to apply to the elements.

Return value

• (1 - 2) The number of elements that are equal to value.

Complexity

Given N as ranges::distance(first, last):

• (1 - 2) exactly N comparisons and projections.

Exceptions

(none)

Possible implementation

ranges::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;

Notes

If you want to obtain the number of elements in range [first; last) or r, without any additional criteria, use ranges::distance.

Examples

Main.cpp

#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