On 27/05/15 22:27, Johan S. R. Nielsen wrote:
The behavior is bad but I would not call it a bug. You modified wrongly
the vector.
I disagree: from a user-point of view, a superficially perfectly reasonable
sequence of steps were taken, at the end of which Sage prints a result
which is mathematically incorrect, without any error or warning. That
should never happen! Even for an *unreasonable* sequence of steps.
If it was me I would set the parent to be the ambient space and not the
subspace itself (but it might break a lot of code).
Is the best/fastest/safest way to do this to cast this to the full vector
space? I.e. something like
y = VS(c)
What I suggesteded is to modify the code of vector subspace in such way
that the elements have parent the ambient space and not the subspace.
Which is precisely related to the end of your e-mail.
For your sided question, the fastest way is to use Element._set_parent()
sage: V = VectorSpace(QQ,3)
sage: U = V.subspace([V((1,1,1))])
sage: v = U.an_element()
sage: v._set_parent(V)
sage: v
(1, 1, 1)
sage: v.parent()
Vector space of dimension 3 over Rational Field
but I would not recommend it.
On Wednesday, May 27, 2015 at 10:02:24 PM UTC+2, Volker Braun wrote:
IMHO the bug here is a lack of validation. Its perfectly fine to have
mutable elements of a vector subspace, but y.__setitem__ should then check
that the vector really is in the subspace or raise a ValueError. Of course
it would almost always raise, so its not a particularly useful operation.
I agree, that's a possibility which is general and makes sense.
On a more general note, what is the rationale for defining the subspace -
and not the full vector space - as the parent of the vectors? The vector is
in many subspaces, and the user just happened to construct it from one of
them; but it's just as much part of the other subspaces. I find it somehow
slightly arbitrary that this type of sub-structure -- vector space
subspaces -- become parents like this. Are there other examples? Perhaps
I'm not seeing the bigger picture here.
In coding theory there's an echo of the same thing: a codeword of a linear
code is a vector which is in the code (the code being a vector subspace).
Currently, when you ask for a codeword of the code, you'll get a vector
whose parent is the subspace (and not the LinearCode object). This actually
surprised me, as I expected the parent to be the full vector space. To be
consistent, should we change it so that the LinearCode is the parent of the
codewords?
Not necessarily. There is a well defined concept of facade parent in
Sage. It is a parent who does not own its elements. Like Primes:
sage: Primes() in Sets().Facade()
True
sage: Primes().an_element().parent()
Integer Ring
The question in your case is rather do *you* *want* that the elements
have parent the linear code?
Vincent
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