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

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, std::projected<I, Proj>, const T*> constexpr std::iter_difference_t<I> count( I first, S last, const T& value, Proj proj = {} );	(1)	(since C++20)
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 = {} );	(2)	(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 = {} );	(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 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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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_viewviews::filter (C++20)	a `view` that consists of the elements of a `range` that satisfies a predicate (class template) (range adaptor object)
countcount_if	returns the number of elements satisfying specific criteria (function template)