std::ranges::count_if() algorithm

od C++20

Simplified

// (1)
constexpr std::iter_difference_t<I>
    count_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::range_difference_t<R>
    count_if( R&& r, Pred pred, 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
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::projected<ranges::iterator_t<R>, Proj>>
• P - (none)

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

Detailed

// (1)
template<
	std::input_iterator I,
	std::sentinel_for<I> S,
	std::indirect_unary_predicate<std::projected<I, Proj>> Pred
	class Proj = std::identity
>
constexpr std::iter_difference_t<I>
    count_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
template<
	ranges::input_range R,
	std::indirect_unary_predicate<std::projected<I, Proj>> Pred
	class Proj = std::identity
>
constexpr ranges::range_difference_t<R>
    count_if( R&& r, Pred pred, Proj proj = {} );

Returns the number of elements in the given range that satisfy specific criteria.

• (1) Counts elements for which predicate p returns true.
• (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.
pred	Predicate to apply to the projected elements.
proj	Projection to apply to the elements.

Return value

• (1 - 2) The number of elements for which the predicate p holds true.

Complexity

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

• (1 - 2) exactly N calls to p and projections.

Exceptions

(none)

Possible implementation

ranges::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;

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_if(v.begin(), v.end(), target1);
    int num_items2 = ranges::count_if(v, target2);
    std::cout << "number: " << target1 << " count_if: " << num_items1 << '\n';
    std::cout << "number: " << target2 << " count_if: " << num_items2 << '\n';

    // use a lambda expression to count_if elements divisible by 3.
    int num_items3 = ranges::count_if_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_if elements divisible by 11.
    int num_items11 = ranges::count_if_if(v, [](int i) {return i % 11 == 0;});
    std::cout << "number divisible by eleven: " << num_items11 << '\n';
}

Output

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