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Showing content from https://en.cppreference.com/w/cpp/algorithm/../algorithm/../iterator/../algorithm/ranges/count.html below:

std::ranges::count, std::ranges::count_if - cppreference.com

Call signature

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.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Number of elements satisfying the condition.

Exactly last - first comparisons and projection.

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

struct count_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T = std::projected_value_t<I, Proj>>
    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 Proj = std::identity
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    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;

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;

#include <algorithm>
#include <cassert>
#include <complex>
#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';
 
    std::vector<std::complex<double>> nums{{4, 2}, {1, 3}, {4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto c = ranges::count(nums, {4, 2});
    #else
        auto c = ranges::count(nums, std::complex<double>{4, 2});
    #endif
    assert(c == 2);
}

Output:

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

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