On Tue, 3 Feb 2009, Roberto Bagnara wrote:
The comparison between mpz_class and double works fine with +inf and -inf, while the comparison between mpq_class and double gives a floating point exception, due to inappropriate mpq_set_d call in eval specialization.
in the Parma Polyhedra Library, we provide numerical abstractions that are parametric with respect to several (extended) number families. This includes all the native integer and floating point types and GMP's integers and rationals, possibly augmented with special values for -inf, +inf and NaN.
Oh, so you extended gmp types with +-inf and NaN, that is interesting. Out of curiosity, how do you do that?
On these numerical types, we provide several operations, including safe (modulo bugs) comparisons. To implement those we of course rely on what (we believe) is provided by the underlying base type.
Ok, so your goal is primarily completeness and correctness for those extended types, this is a fine goal, I am all for it.
In this case, we were spoiled by the fact that GMP supports comparisons between mpz and double via mpz_cmp_d (which can be called with an infinity).
Note that you still need to handle NaN separately (you could consider it another gmpxx bug that comparison (<,>,<=,>=) with NaN may do something other than return false).
Without looking at the manual (tsk, tsk, ...) we assumed the same functionality was provided for mpq. However, it seems there is no such a thing as mpq_cmp_d and, as you know, the implementation of this comparison in gmpxx.h is incorrect.
You could say that the interaction between gmpxx types and double is only guaranteed to work in the finite case and the bug is in the missing documentation of this feature. It feels unsatisfactory to be able to say z<d (for an infinite d) but have (z-d) crash the program... This would argue in favor of pushing the appropriate code to a library that augments the gmpxx types with +-Inf and NaN. Which doesn't mean this won't be done in gmp...
I don't know if there is any plan to add mpq_cmp_d.
Looks to me like it would make sense (but as I said in a previous email, I am not the one who needs convincing).