std::ranges...s::count_if(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 std::ranges...s::count_if(3) 💖

Okay bestie, welcome to the C++ Standard Libary manual page for std::ranges...s::count_if(3) — yes, that's the title and that's the manual name. This page is all about the std::ranges::count and std::ranges::count_if niebloids, and honestly they ate. Let's get into it 💅

🌸 NAME 🌸

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

✨ Synopsis ✨

Defined in header <algorithm>
Call signature

// count (1) — since C++20, until C++26
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 = {} );

// count (1) — since 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 = {} );

// count (2) — since C++20, until C++26
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 = {} );

// count (2) — since 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 = {} );

// count_if (3) — 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 = {} );

// count_if (4) — 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 = {} );

Returns the number of elements in the range [first, last) satisfying specific criteria.

  1. Counts the elements that are equal to value.
  2. 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:

  • 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.

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.

🧪 Feature-test macro 🧪

  • __cpp_lib_algorithm_default_value_type — value 202403 — Std (C++26) — feature: 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

🦋 See also 🦋

  • ranges::distance (C++20) — returns the distance between an iterator and a sentinel, or between the beginning and end of a range (niebloid)
  • 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)

okay that's the whole page, bestie 💖 — source: http://cppreference.com, version/date: 2024.06.10, page: std::ranges...s::count_if(3). Mwah 💋

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