
Boost : 
Subject: Re: [boost] [ICL] cardinality of float interval
From: Joachim Faulhaber (afojgo_at_[hidden])
Date: 20110217 19:01:14
2011/2/17 John Reid <j.reid_at_[hidden]>:
> cardinality( interval< float >( 1, 5 ) ) == 18446744073709551615
>
> Can this be true?
No, the only truth is 42 ;)
> Is this by design?
Yes it is.
cardinality(x) gives the number of elements in a set, container or
interval. In the STL the number of elements of a container is
represented by the type std::size_t. For an interval representing a
set we face the situation that the number of elements can be infinite
if the element type is continuous.
Since float represents a real number it is assumed to be continuous.
So the cardinality of interval<float>::closed(1,5) is infinite. Since
std::size_t has no INF value, I chose
(std::numeric_limits<std::size_t>::max)() as representation for
infinity. You can use icl::infinity<T>::value() to check for this
representation of infinity.
typedef size_type_of<interval<float>::type>::type itv_float_size_type;
BOOST_CHECK( (is_same<itv_float_size_type, std::size_t>::value) );
BOOST_CHECK_EQUAL(cardinality(interval<float>::closed(5,5)), 1);
BOOST_CHECK_EQUAL(
cardinality(interval<float>::closed(1,5)),
icl::infinity<itv_float_size_type>::value()
);
I know ... this design is questionable.
I could write a function template
template<class IntervalT>
boost is_infinite(IntervalT const&);
and make the cardinality function partial instead.
Are you using the cardinality function on float intervals in real use cases?
Best regards,
Joachim
 Interval Container Library [Boost.Icl] http://www.joachimfaulhaber.de
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