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

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

NAME

std::experimental::ranges::count,std::experimental::ranges::count_if - std::experimental::ranges::count,std::experimental::ranges::count_if

Synopsis

Defined in header <experimental/ranges/algorithm>
template< InputIterator I, Sentinel<I> S, class T, class Proj = ranges::identity >
requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*> (1) (ranges TS)
ranges::difference_type_t<I> count(I first, S last, const T& value, Proj proj = Proj{});
template< InputRange R, class T, class Proj = ranges::identity >
requires IndirectRelation<ranges::equal_to<>,
projected<ranges::iterator_t<R>, Proj>, const T*> (2) (ranges TS)
ranges::difference_type_t<ranges::iterator_t<R>>
count(R&& r, const T& value, Proj proj = Proj{});
template< InputIterator I, Sentinel<I> S, class Proj = ranges::identity,
IndirectUnaryPredicate<projected<I, Proj>> Pred > (3) (ranges TS)
ranges::difference_type_t<I> count_if(I first, S last, Pred pred, Proj proj = Proj{});
template< InputRange R, class Proj = ranges::identity,
IndirectUnaryPredicate<projected<ranges::iterator_t<R>, Proj>> Pred > (4) (ranges TS)
ranges::difference_type_t<ranges::iterator_t<R>>
count_if(R&& r, Pred pred, Proj proj = Proj{});

Returns the number of elements in the range [first, last) satisfying specific criteria.
1) counts the elements whose projected values are equal to value (i.e., ranges::invoke(proj, *i) == value).
3) counts the elements whose projected values satisfy the predicate pred (i.e., ranges::invoke(pred, ranges::invoke(proj, *i)) != false).
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.
Notwithstanding the declarations depicted above, the actual number and order of template parameters for algorithm declarations is unspecified. Thus, if explicit template arguments are used when calling an algorithm, the program is probably non-portable.

Parameters

first, last - the range of elements to examine
r - the range of 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 / applications of the predicate, and the same number of applications of the projection.

Notes

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

Possible implementation

First version

  template< InputIterator I, Sentinel<I> S, class T, class Proj = ranges::identity >
    requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*>
  ranges::difference_type_t<I> count(I first, S last, const T& value, Proj proj = Proj{})
  {
      ranges::difference_type_t<I> ret = 0;
      for (; first != last; ++first) {
          if (ranges::invoke(proj, *first) == value) {
              ++ret;
          }
      }
      return ret;
  }

Second version

  template< InputIterator I, Sentinel<I> S, class Proj = ranges::identity,
            IndirectUnaryPredicate<projected<I, Proj>> Pred >
  ranges::difference_type_t<I> count_if(I first, S last, Pred pred, Proj proj = Proj{})
  {
      ranges::difference_type_t<I> ret = 0;
      for (; first != last; ++first) {
          if (ranges::invoke(pred, ranges::invoke(proj, *i))) {
              ++ret;
          }
      }
      return ret;
  }

Example

 This section is incomplete
 Reason: no example