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>, (1) (ranges TS)
const T*>

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

See also¶

count returns the number of elements satisfying specific criteria
count_if (function template)
returns the distance between an iterator and a sentinel, or between the
distance beginning and the end of a range
(function template)