25 Algorithms library [algorithms]

25.5 Non-modifying sequence operations [alg.nonmodifying]

25.5.9 Count [alg.count]

template<class InputIterator, class T> constexpr typename iterator_traits<InputIterator>::difference_type count(InputIterator first, InputIterator last, const T& value); template<class ExecutionPolicy, class ForwardIterator, class T> typename iterator_traits<ForwardIterator>::difference_type count(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, const T& value); template<class InputIterator, class Predicate> constexpr typename iterator_traits<InputIterator>::difference_type count_if(InputIterator first, InputIterator last, Predicate pred); template<class ExecutionPolicy, class ForwardIterator, class Predicate> typename iterator_traits<ForwardIterator>::difference_type count_if(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, Predicate pred); template<input_iterator I, sentinel_for<I> S, class T, class Proj = identity> requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*> constexpr iter_difference_t<I> ranges::count(I first, S last, const T& value, Proj proj = {}); template<input_range R, class T, class Proj = identity> requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T*> constexpr range_difference_t<R> ranges::count(R&& r, const T& value, Proj proj = {}); template<input_iterator I, sentinel_for<I> S, class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred> constexpr iter_difference_t<I> ranges::count_if(I first, S last, Pred pred, Proj proj = {}); template<input_range R, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> constexpr range_difference_t<R> ranges::count_if(R&& r, Pred pred, Proj proj = {});
Let E be:
  • *i == value for the overloads with no parameter pred or proj,
  • pred(*i) != false for the overloads with a parameter pred but no parameter proj,
  • invoke(proj, *i) == value for the overloads with a parameter proj but no parameter pred,
  • invoke(pred, invoke(proj, *i)) != false for the overloads with both parameters proj and pred.
Effects: Returns the number of iterators i in the range [first, last) for which E holds.
Complexity: Exactly last - first applications of the corresponding predicate and any projection.