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

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C++
Experimental
Ranges
Algorithms library
Non-modifying sequence operations
Modifying sequence operations
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Sorting operations
Binary search operations
Set operations (on sorted ranges)
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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;
}

Example

This section is incomplete
Reason: no example

See also

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