[Maxima] Maxima: lambdas and sublist_indices behaving strangely when acting on symbols
robert.dodier at gmail.com
Mon May 14 09:20:49 CDT 2012
On 2012-05-14, Andy L <buggi22 at hotmail.com> wrote:
> I'm having trouble using sublist_indices on lists of symbols.
> Basically, I'm trying to implement a function that will find the
> indices at which a given symbol appears within a list of symbols.
> Can anyone explain why the following code should yield  instead of ?
> f: lambda([l,x], sublist_indices(l, lambda([z], z = x)))$ f([x,y,z], a); /* this yields  */
Well, I was going to go on about lexical versus dynamic scope and
Maxima's evaluation policy, but I think the origin of this problem is
that there is a bug in a function named 'maybe', which is called by
sublist_indices to evaluate the predicate. 'maybe' explicitly evaluates
its argument (incidentally contravening Maxima's general one-time
evaluation policy) and that causes the confusion you observed.
Fixing 'maybe' isn't so easy because (short answer) logical predicates
are not simplifying functions.
A simple workaround in this case is, I think, to name the argument x% or
something else which is unlikely to cause a name collision. Yes, that is
a fragile solution.
> The same function behaves as expected when the arguments are strings:
> f(["x","y","z"], "a"); /* this yields , as expected */
It works as expected with strings since a string can only evaluate to itself.
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