std::count,std::count_if (3) - Linux Manuals

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

NAME

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

Synopsis

Defined in header <algorithm>
template< class InputIt, class T >
typename iterator_traits<InputIt>::difference_type (until C++20)
count( InputIt first, InputIt last, const T &value );
template< class InputIt, class T >
constexpr typename iterator_traits<InputIt>::difference_type (since 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 C++17)
count( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T &value ); (1)
template< class InputIt, class UnaryPredicate >
typename iterator_traits<InputIt>::difference_type (until C++20)
count_if( InputIt first, InputIt last, UnaryPredicate p );
template< class InputIt, class UnaryPredicate >
constexpr typename iterator_traits<InputIt>::difference_type (3) (since C++20)
count_if( InputIt first, InputIt last, UnaryPredicate p );
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
typename iterator_traits<ForwardIt>::difference_type (4) (since 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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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.
p - The expression p(v) must be convertible to bool for every argument v 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

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 <vector>

  int main()
  {
      std::vector<int> v{ 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };

      // determine how many integers in a std::vector match a target value.
      int target1 = 3;
      int target2 = 5;
      int num_items1 = std::count(v.begin(), v.end(), target1);
      int num_items2 = std::count(v.begin(), v.end(), 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 = std::count_if(v.begin(), v.end(), [](int i){return i % 3 == 0;});
      std::cout << "number divisible by three: " << num_items3 << '\n';
  }

Output:

  number: 3 count: 2
  number: 5 count: 0
  number divisible by three: 3