On 05/04/2010 10:32 AM, janwillem wrote:
> I need some explanation on the workings of SymPy. As an example the
> following script:
> import sympy
> X, F, B = sympy.symbols('XFB')
> Y = X / F - B #eqn 1
> DY = sympy.Matrix(sympy.diff(Y, (X, F, B))).T
>
Do sympy.diff(Y, (X, F, B), evaluate=True) instead for now. But you're
right - this shouldn't be necessary. Somehow the vectorization of the
diff method seems to fail and set evaluate to False.
> print DY
>
> I had expected (eqn 2): [1/F, -X/F**2, -1]
> But get: [D(-B + X/F, X), D(-B + X/F, F), D(-B + X/F, B)]
>
> Not after doing a trick of which I cannot remember why I tried it, I
> get the desired result
> DY = DY.subs({X:X, F:F, B:B})
>
This works because subs evaluates the expression in the end (which
wasn't done before).
> From the doc I had thought that DY.doin() would work but that gives
> "raise AttributeError()".
>
First, the name of the method is doit. Second, DY is a python tuple
which doesn't have this method.
> So there is obviously something I do not understand, please some help
> Janwillem
>
>
Sebastian
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