On Jul 20, 1:23 pm, Ondrej Certik <ondrej.cer...@gmail.com> wrote:
> On Wed, Jul 20, 2011 at 11:59 AM, Aaron Meurer <asmeu...@gmail.com> wrote:
> > First off, if you want an explicit order of substitution, you should
> > not use a dictionary. Subs also allows the [(old, new), ...] syntax,
> > which lets you define a specific order.
>
> > But even in this case, it will perform the substitution iteratively.
> > This is actually a useful feature. For example, you can take the
> > output of cse() and back-substituted it all in one step, even though
> > the various substitutions depend on each other.
For what it's worth, in Sage, the substitution is done simultaneously:
sage: var('x,y,a,b')
(x, y, a, b)
sage: (x**2+y**3).subs({x: a*x+b*y, y: b*x-a*y})
-(a*y - b*x)^3 + (a*x + b*y)^2
--Mike
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