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<ranges::equal_to, (since
   std::projected<I, Proj>, (1) C++20)
   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<ranges::equal_to,
   std::projected<ranges::iterator_t<R>, Proj>, (2) (since
   const T*> C++20)
   constexpr ranges::range_difference_t<R>
   count( R&& r, const T& value, Proj proj = {} );
   template< std::input_iterator I, std::sentinel_for<I> S,
   class Proj = std::identity, (since
   std::indirect_unary_predicate<std::projected<I, Proj>> Pred > (3) 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>, (since
   Proj>> Pred > (4) 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 may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   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.

Possible implementation

First version
struct count_fn {
  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>
    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 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>
    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;

Second version
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 <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';
 }

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)