I think what you are getting is expected behavior. Remember that when you
switch n and n + 1, you are switching the side of the equation that they will
end up on. So in the first one, you have all terms with n on the right-hand
side, and in the second one, you have all the terms with n + 1 on the
right-hand side. If you have something like p[1, 2, n]*p[1, 2, n + 1], the one
that you choose to collect with respect to in your script (Idx(n) or Idx(n +
1)) will determine the tie breaker.
Aaron Meurer
On Oct 8, 2010, at 1:34 PM, Nicholas Kinar wrote:
>>
>>>> Here's a short comment: using Idx(n+1) seems to work a little better for
>>>> me, since I've noticed that if I use Idx(n) with a very complicated
>>>> expression, there is a possibility of terms with (n+1) indices ending up
>>>> on the same side as the (n) indices.
>>> That sounds like a bug. Can you give a specific example of an expression
>>> that does that?
>>>
>>> Aaron Meurer
>>>
>>
>> Sure, I can attempt to give an example. I've been unable to reproduce this
>> behavior with a simple expression, so I am pasting below the full expression
>> that I've been using. Of course, it may be something that I'm doing wrong,
>> so I'm reluctant to cry wolf. But in a similar fashion to working with a
>> building, if the pipes are a bit leaky, perhaps a small patch job would be
>> beneficial if warranted.
>>
>
> Okay, here is a test program with a much simpler expression. The output of
> the program shows what I suspect to be right and wrong behavior. The
> questioned behavior seems to occur when two indexed terms are multiplied
> together:
>
> This appears to be wrong:
> (p[1, 2, 1 + n], -p[1, 2, n] + p[2, 2, n]*p[1, 2, 1 + n])
> This appears to be right:
> (-p[1, 2, n], p[2, 2, n]*p[1, 2, 1 + n] + p[1, 2, 1 + n])
>
>
> Nicholas
>
>
> #begin sample code
> from sympy import *
> from sympy.tensor import Indexed, Idx, IndexedBase
> from sympy.matrices import *
>
> p = IndexedBase('p')
> var('deltax deltay delta deltat A B')
> i,j,n = symbols('i j n', integer = True)
>
>
> def test_as_independent(expr):
> expr0 = expr.lhs - expr.rhs
> expr1 = expr0.as_independent(*filter(lambda t: t.args[-1] == Idx(n),
> expr0.atoms(Indexed) ) )
> print 'This appears to be wrong: '
> print expr1
> print 'This appears to be right: '
> expr2 = expr0.as_independent(*filter(lambda t: t.args[-1] == Idx(n+1),
> expr0.atoms(Indexed) ) )
> print expr2
>
>
> def run_scheme():
> expr1 = Eq( p[1,2,n+1] + p[2,2,n]*p[1,2,n+1], p[1,2,n])
> test_as_independent(expr1)
>
> def main():
> run_scheme()
>
> if __name__ == "__main__":
> main()
> # end sample code
>
>
>
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