Also, what should the coefficient be when include=False? Aaron Meurer
On Sat, Jul 23, 2011 at 9:31 PM, Aaron Meurer <[email protected]> wrote: > So what do you think these should return? I'm assuming you think that > at least 0/q should return 0/1 regardless of what include is set to. > What about p/0? Should p.cancel(Poly(0, t)) work or raise some kind > of exception? > > Aaron Meurer > > On Tue, Jul 19, 2011 at 10:33 PM, Aaron Meurer <[email protected]> wrote: >> If you look at the code in dmp_cancel(), it is special-cased for f == >> 0 or g == 0, in which case it just returns f, g for include=True or >> K.one, K.one, f, g for include=False. So there is no bug, per se, >> just a design flaw. >> >> I think you wrote this code. Is there a reason you special-cased >> this? Perhaps for speed? Or maybe some stuff fails when g == 0 (I >> didn't try it)? >> >> Aaron Meurer >> >> On Tue, Jul 19, 2011 at 9:10 PM, Mateusz Paprocki <[email protected]> wrote: >>> Hi, >>> >>> On 20 July 2011 05:02, Aaron Meurer <[email protected]> wrote: >>>> >>>> Hi. >>>> >>>> This question is mainly for Mateusz, but I encourage others to chime >>>> in too if they have an opinion. >>>> >>>> I need the following behavior in the polys: >>>> >>>> >>> Poly(0, t).cancel(Poly(t, t), include=True) >>>> (Poly(0, t), Poly(1, t)) >>>> >>>> Whereas it currently does the following: >>>> >>>> >>> Poly(0, t).cancel(Poly(t, t), include=True) >>>> (Poly(0, t), Poly(t, t)) >>>> >>>> (notice that the denominator is reduced to 1 in the first case and >>>> left alone in the second). I need the former to make it easier to >>>> deal with zero rational functions in my Risch code. >>>> >>>> Now, this is one of four possibilities in cancel. They are: >>>> >>>> 1. Poly(0, t).cancel(Poly(t, t), include=True) # 0/q include=True; >>>> this is the one I use >>>> 2. Poly(0, t).cancel(Poly(t, t), include=False) # 0/q include=False >>>> 3. Poly(t, t).cancel(Poly(0, t), include=True) # p/0 include=True >>>> 4. Poly(t, t).cancel(Poly(0, t), include=False) # p/0 include=False >>>> >>>> I did not change 2-4 in my branch yet because I only ever use the >>>> first idiom in my code, but I think we should try to make it >>>> consistant. So my questions are: >>>> >>>> - Should 2 return a one denominator too? I don't actually care if it >>>> does, but it seems like it should to be consistant with 1. >>>> - What should the returned coefficient from 2 be? I never actually >>>> use include=False, so I don't really have a feel for this. >>>> - Should 3 and 4 even work? To me they should raise an exception, but >>>> maybe it isn't necessary to think of f.cancel(g) as f/g (though that >>>> is what the docstring says). >>> >>> I didn't yet analyze all the cases pointed out above, but it seems there is >>> a bug in cancel(): >>> In [1]: gcd(0, t) >>> Out[1]: t >>> In [2]: Poly(0, t).cancel(Poly(t, t)) >>> Out[2]: (1, Poly(0, t, domain='ZZ'), Poly(t, t, domain='ZZ')) >>> gcd(0, something) always returns something, thus no matter what `include` is >>> set to, cancel() whould give Poly(1, t) as the denominator. Note that >>> cofactors() works correctly: >>> In [3]: Poly(0, t).cofactors(Poly(t, t)) >>> Out[3]: (Poly(t, t, domain='ZZ'), Poly(0, t, domain='ZZ'), Poly(1, t, >>> domain='ZZ')) >>> (it gives GCD, cofactor(lhs), cofactor(rhs)). >>> >>>> >>>> Aaron Meurer >>>> >>>> -- >>>> You received this message because you are subscribed to the Google Groups >>>> "sympy" group. >>>> To post to this group, send email to [email protected]. >>>> To unsubscribe from this group, send email to >>>> [email protected]. >>>> For more options, visit this group at >>>> http://groups.google.com/group/sympy?hl=en. >>>> >>> >>> Mateusz >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "sympy" group. >>> To post to this group, send email to [email protected]. >>> To unsubscribe from this group, send email to >>> [email protected]. >>> For more options, visit this group at >>> http://groups.google.com/group/sympy?hl=en. >>> >> > -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
