I think you have found a bug with subs. Can you report it in the issues, with
the traceback?
Aaron Meurer
On Sep 19, 2010, at 11:19 AM, Nicholas Kinar wrote:
> Thanks for your response, smichr; that's greatly appreciated.
>
>>
>>> However, this does not seem to work, since the following error is produced:
>>>
>>> Traceback (most recent call last):
>>> File "test-1.py", line 17, in<module>
>>> print collect(expr.subs([(a**(n+1), u*a**(n+1)) for a in _]), u)
>>> NameError: name '_' is not defined
>>>
>> The underscore in an interactive session refers to the last output. In
>> a script it doesn't have that meaning; it's just a variable name which
>> you haven't defined yet.
>>
> Ah yes, I haven't been using iPython or something similar.
>
>> You are also missing some of what happened in the example I gave.
>> Atoms was used to find the bases of the powers of interest and then
>> those, raised to the n+1 power, where replaced with themselves*u. What
>> you wrote above should appear something like this in a script:
>>
>> ...
>> u = C.Dummy('u')
>> IE_atoms = expr1.atoms(IndexedElement)
>> print collect(expr.subs([(a**(n+1), u*a**(n+1)) for a in IE_atoms]),
>> u)
>>
>>
> I've tried changing some sections of my script, but I still don't know what I
> am doing wrong:
>
> # begin code
> from sympy import *
> from sympy.tensor import Indexed, Idx, IndexedElement
>
> M = Indexed('M')
> var('A B C D E')
> i,j,n = symbols('i j n', integer = True)
> i = 1
> j = 1
> expr = Eq( A*M(i,j)**n + B*M(i+1,j)**(n+1),
> C*M(i,j+1)**n + D*M(i,j)**(n+1) +
> E*M(i+1,j)**(n+1) )
> expr1 = expr.lhs - expr.rhs
> u = Symbol('u', dummy=True)
> IE_atoms = expr1.atoms(IndexedElement)
>
> print collect(expr1.subs([(a**(n+1), u*a**(n+1)) for a in IE_atoms]), u) #
> begin code
> # end code
>
> After a few seconds, this script generates a very long output, ending with
> the following error:
>
> RuntimeError: maximum recursion depth exceeded while calling a Python object
>
> So what would I have to change to make this script work as intended?
>
> Nicholas
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