std::ranges::count,std::ranges::count_if(3)

NAME
std::ranges::count,std::ranges::count_if - std::ranges::count,std::ranges::count_if

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S,
             class T, class Proj = std::identity >
   requires std::indirect_binary_predicate (since
                <ranges::equal_to, std::projected<I, Proj>, C++20)
   const T*> (until
   constexpr std::iter_difference_t<I> C++26)
       count( I first, S last, const T& value, Proj proj = {}
   );
   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 (since
                <ranges::equal_to, std::projected<I, Proj>, C++26)
   const T*>
   constexpr std::iter_difference_t<I>
       count( I first, S last, const T& value, Proj proj = {}
   );
   template< ranges::input_range R, class T, class Proj =
   std::identity >
   requires std::indirect_binary_predicate (since
                <ranges::equal_to, C++20)
                 std::projected<ranges::iterator_t<R>, Proj>, (until
   const T*> C++26)
   constexpr ranges::range_difference_t<R> (1)
       count( R&& r, const T& value, Proj proj = {} );
   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 (since
                <ranges::equal_to, C++26)
                 std::projected<ranges::iterator_t<R>, Proj>,
   const T*>
   constexpr ranges::range_difference_t<R> (2)
       count( R&& r, const T& value, Proj proj = {} );
   template< std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, (3) (since
   Proj>> Pred > C++20)
   constexpr std::iter_difference_t<I>
       count_if( I first, S last, Pred pred, Proj proj = {} );
   template< ranges::input_range R, class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> (4) (since
   Pred > C++20)
   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.
   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 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 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

See also
   ranges::distance returns the distance between an iterator and a sentinel, or
   (C++20) between the beginning and end of a range
                       (niebloid)
   views::counted creates a subrange from an iterator and a count
   (C++20) (customization point object)
   ranges::filter_view a view that consists of the elements of a range that satisfies a
   views::filter predicate
   (C++20) (class template) (range adaptor object)
   count returns the number of elements satisfying specific criteria
   count_if (function template)