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

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(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 = { } ) ;			(since C++20) (until C++26)
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 > count ( I first, S last, const T & value, Proj proj = { } ) ;			(since C++26)
		(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 = { } ) ;				(since C++20) (until C++26)
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 > count ( R && r, const T & value, Proj proj = { } ) ;				(since C++26)
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 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.

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 .

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithms ( 1,2 )

Possible implementation

count (1)

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;

count_if (3)

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 <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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Technical specifications
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External libraries

ranges::distance (C++20)	returns the distance between an iterator and a sentinel, or between the beginning and end of a range (algorithm function object)
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)