[sage-support] Re: side-effect when evaluating a function

On Sat, Jul 25, 2009 at 12:31 PM, pca<[EMAIL PROTECTED]> wrote:
>
> Dear all,
>
> I have been puzzled, and annoyed, by an unexpected side-effect.  It
> can be demonstrated in the notebook by the following example.  It
> models the speed as distance / time, then solve for the distance, then
> evaluate the distance with different arguments:
>
> var('v d t')
> equation  = v == d / t
> solution = solve(equation, d, solution_dict = True)
> d_   = solution[0][d]; print d_ #-> t*v
> constants= {v: 2}
> print d_(constants, t=3) #--> 6
> # BEWARE: the previous call has the side effect that t=3 in later
> evaluation !

This is *definitely* a bug in Sage.  I've made this trac #6622:

   http://trac.sagemath.org/sage_trac/ticket/6622

Here's how to workaround it (namely, put "dict(constants)" instead of
"constants"):

sage: var('v t')
sage: f = v*t
sage: s = {v: 2}
sage: f(dict(s), t=3)
6
sage: s
{v: 2}

William


> print d_(constants) #--> 6 : the side-effect is apparent here !
> print d_ #--> t*v
>
> I would expect d_(constants) to result in t*2, but instead its using a
> previous assignment of t to yield 6 !
>
> I'm using Sage for an engineering pblm, and I find it useful to
> separate constants from arguments.  The side effect is thus annoying
> to me: is there any other way to evaluate a function with constants
> and arguments, without any side-effect with the arguments ?  I'm new
> to this forum, so not sure if it has been raised before. Sorry if
> there is an easy answer.
>
> Thanks in advance.
> PC
>
> >
>



-- 
William Stein
Associate Professor of Mathematics
University of Washington
http://wstein.org

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