You can generally do this sort of thing using replace(). Unfortunately, the pattern matcher doesn't recognize rational numbers as a/b, so you have to do a more manual check. This should work:
e.replace(lambda i: i.is_Pow and i.base == x and i.exp.is_Rational, lambda i: real_root(-1, i.exp.as_numer_denom()[1])**i.exp.as_numer_denom()[0]) (replace -1 with whatever value you want to replace). A "cleaner" solution would be to create a custom Pow subclass that evaluates like you want, and replace instances of Pow with your class before doing a substitution. Aaron Meurer On Wed, Nov 4, 2015 at 1:19 PM, Paul Royik <distantjob...@gmail.com> wrote: > So, there is no way to do it using subs and/or some manipulations? > > real_root(-1, 3) is of no help, because I can have arbitrary expression. > > On Wednesday, November 4, 2015 at 6:29:29 PM UTC+2, Aaron Meurer wrote: >> >> You need to use real_root, like >> >> In [3]: real_root(-1, 3) >> Out[3]: -1 >> >> In [4]: real_root(-1, 3)**2 >> Out[4]: 1 >> >> SymPy, like most math libraries, uses complex roots (i.e., principal >> roots) because they have nicer mathematical properties. >> >> Aaron Meurer >> >> On Wed, Nov 4, 2015 at 3:30 AM, Paul Royik <distan...@gmail.com> wrote: >> > I have the following expresssion: >> > >> > f=x**(Rational(2,3)) >> > >> > How can I get 1, when substituting (-1) instead of complex number? >> > For now, I got complex number when run f.subs(x,-1).evalf() >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "sympy" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> > an >> > email to sympy+un...@googlegroups.com. >> > To post to this group, send email to sy...@googlegroups.com. >> > Visit this group at http://groups.google.com/group/sympy. >> > To view this discussion on the web visit >> > >> > https://groups.google.com/d/msgid/sympy/662a2085-1030-42d5-bbde-feb524fd246c%40googlegroups.com. >> > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sympy+unsubscr...@googlegroups.com. > To post to this group, send email to sympy@googlegroups.com. > Visit this group at http://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/c88261c0-3449-4d7a-abf8-8d28ebdca59d%40googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sympy+unsubscr...@googlegroups.com. To post to this group, send email to sympy@googlegroups.com. Visit this group at http://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6%2Bk-X%2BVqRVc2eWs0mDZmRe4Wf9kRtsM0RS8C6H21K-Pzw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
