On Jun 10, 2008, at 10:58 AM, Stan Schymanski wrote:
>
> Dear all
>
> I have been using the .subs(locals()) functionality extensively, but
> now I found out that this does not work for piecewise defined
> functions.
>
> Example:
>
> sage: var('x a b')
> (x, a, b)
> sage: f1=a*sin(x)
> sage: f2=b*sin(x)
> sage: f = Piecewise([[(0,pi/2),f1],[(pi/2,pi),f2]])
> sage: a=1
> sage: b=-1
> sage: f.subs(locals())
> ----------------------------------------------------------------------
> -----
> AttributeError Traceback (most recent call
> last)
>
> ...
>
> AttributeError: PiecewisePolynomial instance has no attribute 'subs'
This is an (easily fixed) bug in PiecewisePolynomial.
> The python function approach also fails in this case:
>
> sage: def g(x):
> ....: return Piecewise([[(0,pi/2),f1],[(pi/2,pi),f2]])
> sage: a=1
> sage: b=-1
> sage: g(1)
> Piecewise defined function with 2 parts, [[(0, pi/2), a*sin(x)], [(pi/
> 2, pi), b*sin(x)]]
>
> Am I doing something wrong?
The python approach won't work recursively unless everything is a
python function (which may have performance impacts).
To better understand what's going on here, when you write "a=1" it
does not mean "the symbolic variable a is now 1 in every expression
it occurs" but rather "the global variable a is 1." In fact writing
"var('a,b,c')" simply binds the symbolic expression "a" to the global
variable "a." The difference is subtle but important.
- Robert
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