Le jeudi 19 octobre 2006 à 01:03 +0800, tirath a écrit :
Hi all,
The query... Before I started using the library, I had this assumption: let A be a float with an exact representation (i.e. width=0). If we cast A to a double, the resulting double will have a non-zero width - specifically the width will be equal to the extra precision afforded to doubles vs floats.
If you have a set of numbers that is exactly representable as a singleton interval<float>, then this set of numbers is also representable as a singleton interval<double>. There is no reason to use a bigger interval than necessary to represent this set of numbers.
Unless I've messed something up, I find that the library does not actually behave this way, and to be frank, I think it ought to! :-) Here's the simple testcase:
typedef interval<float> I1; typedef interval<double> I2; I1 y = 1.0; I2 yy = (I1)y; cout << yy.upper() - yy.lower();
I expected yy.upper()-yy.lower() to be non-zero, but it was zero.
This is the expected behavior. Interval "y" is a set with a single element 1 and you assign it to interval "yy". So interval "yy" has to contain 1 and it doesn't have to contain any other value. The best interval that satisfies this property is the interval [1,1], and this is the one the library computes.
I had a look at the policies but couldn't spot anything obviously relevant to this situation. Any thoughts on this behaviour per se, or any ideas on a neat and elegant way to implement my desired behaviour?
Could you detail why you desire this behavior? I can't think of a situation where you would like the intervals to get worse when you convert from interval<float> to interval<double>. Intervals may get worse when you convert from interval<double> to interval<float>, but not the other way around. Even for the application you described, I don't understand why you expect this behavior. A conversion from float to double is exact, so you want a conversion from interval<float> to interval<double> to be exact too, in order for your simulations to be useful. Best regards, Guillaume