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

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

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class T >
                        (until
   typename iterator_traits<InputIt>::difference_type C++20)
   count( InputIt first, InputIt last, const T& value );
   template< class InputIt, class T >
                        (since
   constexpr typename iterator_traits<InputIt>::difference_type C++20)
   count( InputIt first, InputIt last, const T& value );
   template< class ExecutionPolicy, class ForwardIt, class T >
   typename iterator_traits<ForwardIt>::difference_type (2) (since
   count( ExecutionPolicy&& policy, C++17)
   ForwardIt first, ForwardIt last, const T& value );
   template< class InputIt, class UnaryPredicate > (1)
                        (until
   typename iterator_traits<InputIt>::difference_type C++20)
   count_if( InputIt first, InputIt last, UnaryPredicate p );
   template< class InputIt, class UnaryPredicate >
                        (since
   constexpr typename iterator_traits<InputIt>::difference_type C++20)
                        (3)
   count_if( InputIt first, InputIt last, UnaryPredicate p );
   template< class ExecutionPolicy, class ForwardIt, class
   UnaryPredicate >
                        (since
   typename iterator_traits<ForwardIt>::difference_type (4) C++17)
   count_if( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last, UnaryPredicate p );
   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 executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of elements to examine
   value - the value to search for
   policy - the execution policy to use. See execution policy for details.
                 unary predicate which returns true for the required elements.
                 The expression p(v) must be convertible to bool for every argument v
   p - of type (possibly const) VT, where VT is the value type of InputIt,
                 regardless of value category, and must not modify v. Thus, a parameter
                 type of VT&is not allowed
                 , nor is VT unless for VT a move is equivalent to a copy
                 (since C++11).

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
   number of elements satisfying the condition.

Complexity
   exactly last - first comparisons / applications of the predicate

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes
   For the number of elements in the range [first, last) without any additional
   criteria, see std::distance.

Possible implementation
   See also the implementations of count in libstdc++ and libc++.
   See also the implementations of count_if in libstdc++ and libc++.

First version
   template<class InputIt, class T>
   typename iterator_traits<InputIt>::difference_type
       count(InputIt first, InputIt last, const T& value)
   {
       typename iterator_traits<InputIt>::difference_type ret = 0;
       for (; first != last; ++first) {
           if (*first == value) {
               ret++;
           }
       }
       return ret;
   }

Second version
   template<class InputIt, class UnaryPredicate>
   typename iterator_traits<InputIt>::difference_type
       count_if(InputIt first, InputIt last, UnaryPredicate p)
   {
       typename iterator_traits<InputIt>::difference_type ret = 0;
       for (; first != last; ++first) {
           if (p(*first)) {
               ret++;
           }
       }
       return ret;
   }

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <array>
 int main()
 {
     constexpr std::array v = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
     std::cout << "v: ";
     std::copy(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " "));
     std::cout << '\n';
     // determine how many integers match a target value.
     for (const int target: {3, 4, 5}) {
         const int num_items = std::count(v.cbegin(), v.cend(), target);
         std::cout << "number: " << target << ", count: " << num_items << '\n';
     }
     // use a lambda expression to count elements divisible by 4.
     int count_div4 = std::count_if(v.begin(), v.end(), [](int i){return i % 4 == 0;});
     std::cout << "numbers divisible by four: " << count_div4 << '\n';
     // A simplified version of `distance` with O(N) complexity:
     auto distance = [](auto first, auto last) {
         return std::count_if(first, last, [](auto){return true;});
     };
     static_assert(distance(v.begin(), v.end()) == 10);
 }

Output:
 v: 1 2 3 4 4 3 7 8 9 10
 number: 3, count: 2
 number: 4, count: 2
 number: 5, count: 0
 numbers divisible by four: 3

See also
   distance returns the distance between two iterators
                    (function template)
   ranges::count
   ranges::count_if returns the number of elements satisfying specific criteria
   (C++20) (niebloid)
   (C++20)