On Wednesday 08 July 2009, Kwankyu wrote:
> Hi,
>
> I was surprised to see
>
> sage: R.<x,y>=QQ[]
> sage: g=x+y
> sage: g.subs({x:x+1,y:x*y})
> x*y + x + y + 1
>
> So the order of substitution matters...unfortunately.
>
> sage: g.subs({x:x+1}).subs({y:x*y})
> x*y + x + 1
> sage: g.subs({y:x*y}).subs({x:x+1})
> x*y + x + y + 1
>
> So the order seems to be from right to left.
I am not sure what you mean by right to left, in the example above the
substitution is in the order of standard Python method chaining, where the
second subs() call is on whatever the first subs() call returns:
sage: gbar = g.subs({x:x+1})
x + y + 1
sage: gbar.subs({y:x*y})
x*y + x + 1
> This seems to me unnatural. Anyway this is undocumented.
This behaviour is documented in the subs() docstring:
.. note::
The evaluation is performed by evaluating every
``variable:value`` pair separately. This has side effects
if e.g. x=y, y=z is provided. If x=y is evaluated first,
all x variables will be replaced by z eventually.
> What should be done to this?
We could try to re-implement subs() such that it doesn't have these side-
effects but is still as fast.
Martin
--
name: Martin Albrecht
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