On Wed, May 20, 2009 at 13:55, Ondrej Certik <ond...@certik.cz> wrote:
>
> Hi Pablo!
>
> On Wed, May 20, 2009 at 4:40 AM, Pablo W. <pablo.win...@gmail.com> wrote:
>>
>> Hello,
>>
>> I have been using sympy a lot recently (economical modelling) and
>> (while it's a great software) I found a strange behavior when doing
>> substitution : it is not respecting operators priority. I think it's
>> because it works sequentially with all its arguments, but it leads to
>> somewhat surprising results.
>>
>> In the following session, I was expecting to get the square root of 4,
>> i.e. 2, but instead the program is taking to fractional exponents of a
>> negative number which has a non unique definition.
>>
>> In [1]: from sympy import *
>>
>> In [2]: var('a b')
>> Out[2]: (a, b)
>>
>> In [3]: eq = (a/b)**0.5
>>
>> In [4]: eq.subs({a:-2,b:-0.5})
>> Out[4]: -2.00000000000000
>>
>> As a workaround, I found myself lambdifying expressions, but I know
>> it's a bad workaround working only when substituion is made with
>> respect to all arguments. What do you suggest ?
>
> Indeed, that's a bug that has to be fixed:
>
> http://code.google.com/p/sympy/issues/detail?id=1427
>
> thanks for discovering it and reporting it.
>
> The sqrt() (or rational power) simplifications has to be fixed in
> sympy, as pointed out by Sebastian and Aaron. I didn't realize there
> is such a big problem in there. One way to fix it is check if "a" and
> "b" is positive, only then split sqrt(a/b) to sqrt(a)/sqrt(b).Hmm, I thought I fixed that already: http://code.google.com/p/sympy/issues/detail?id=1263 -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sympy@googlegroups.com To unsubscribe from this group, send email to sympy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sympy?hl=en -~----------~----~----~----~------~----~------~--~---
