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

From cppreference.com
< cpp‎ | experimental‎ | ranges

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;
    }

### Example

| This section is incomplete Reason: no example

### See also

count count_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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/experimental/ranges/algorithm/count&oldid=110305"