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

 

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Non-modifying sequence operations
all_ofany_ofnone_of countcount_if	findfind_iffind_if_not find_end find_first_of	adjacent_find search search_n mismatch	equal for_each lexicographical_compare
Modifying sequence operations
copycopy_if copy_n copy_backward move move_backward	fill fill_n generate generate_n swap_ranges shuffle transform	removeremove_if replacereplace_if reverse rotate unique	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy
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is_partitioned partition_point	partition partition_copy	stable_partition
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is_sorted is_sorted_until	sort stable_sort	partial_sort partial_sort_copy	nth_element
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lower_bound	upper_bound	binary_search	equal_range
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merge inplace_merge	includes	set_difference set_intersection	set_symmetric_difference set_union
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is_heap is_heap_until	make_heap sort_heap	push_heap pop_heap
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max max_element	min min_element	minmax minmax_element
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is_permutation	next_permutation	prev_permutation

 

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

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; }

See also

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