How to return a list from callable symbolic expression

answered 2013-08-17 14:32:49 +0200

Luca
870 ●5 ●21

updated 2013-08-17 15:32:07 +0200

If you type

sage: f??

You will see that the method .__call__ (responsible for the behaviour of evaluating f) is the one-liner

return vector([e(*args, **kwargs) for e in self])

This has been designed so that symbolic functions can safely be used in symbolic expressions in a meaningful way. For example, you can combine f with another symbolic function using the vector product operator *:

sage: f(x) = [x,x]
sage: f
x |--> (x, x)
sage: f * f
x |--> 2*x^2

This would not work with ordinary python lists.

There is no way around it. If you really only need to return a list, then an ordinary python function (like (2) or (3)) is probably enough. Or you can convert the output of f to a list like so:

sage: list(f(x))

edit flag offensive delete link

Comments

Can you provide me with an example of what you mean by 'unsafe' or 'not meaningful' if the callable symbolic expression would actually return what I have asked for, namely a list?

Mark ( 2013-08-17 14:54:52 +0200 )edit

Edited my answer to account for your comment.

Luca ( 2013-08-17 15:34:10 +0200 )edit

(Maybe) that helps. Let me ask one for clarification. If I get you right, kind of your point is, that callable symbolic expression should allow for meaningful usage with symbolic operations like multiplication, addition a.s.o. But while a scalar product, as f*f may make sense (even though I wasn't even looking for that) the division f(x)/f(x) is also a symbolic operation, which does not make sense for me in case of vectors - although sage simply return f(x)/f(x) = 1 with my definition (1)

Mark ( 2013-08-17 16:19:34 +0200 )edit

This is due to the semantics of symbolic vectors in Sage, as it has been discussed in <http://ask.sagemath.org/question/2917/vector-division>. I agree that this kind of inverse of multiplication by a scalar is somewhat surprising, and that it does not seem to be documented anywhere.

Luca ( 2013-08-18 08:40:32 +0200 )edit

add a comment

How to return a list from callable symbolic expression

1 Answer

Comments

Your Answer

Question Tools

Stats

Related questions

How to return a list from callable symbolic expression edit

1 Answer

Comments

Your Answer

Question Tools

Stats

Related questions

How to return a list from callable symbolic expression