On 12/19/2012 12:28 AM, Charles Bouillaguet wrote:
Hi all,
There seems to be an inconsistency between the docstrings and the actual
behavior of some module-related methods. For instance, I am surprised by the
behavior of the coordinates() method :
sage: A = ZZ^3
sage: A.coordinates?
…
Definition: A.coordinates(self, v, check=True)
Docstring:
Write v in terms of the basis for self.
….
"check" - bool (default: True); if True, also verify that v is really in
self.
….
If v is not in self, raises an "ArithmeticError" exception.
sage: v = vector( [1/2, 1/3, 1/4] )
sage: v in A
False
sage: A.coordinates( v )
[1/2, 1/3, 1/4] # BIZARRE
Shouldn't this raise the ArithmeticError exception, as advertised ?
Looking at the code of this method reveals that it does:
return self.coordinate_vector(v, check=check).list()
A.coordinate_vector is presumably doing:
Write v in terms of the standard basis for self and return the
resulting coefficients in a vector over the fraction field of the
base ring.
...
If v is not in self, raises an ArithmeticError exception.
And, presumably, it does not really implement this specification.
Unless I am missing something, of course.
Cheers,
---
Charles Bouillaguet
http://www.lifl.fr/~bouillaguet/
It is consistent with whatever A.ambient_vector_space() is doing. There
is even a doctest:
sage: M = ZZ^3;
sage: V = M.ambient_vector_space(); V
Vector space of dimension 3 over Rational Field
Since the "ambient vector space" is the Rational field, A.coordinates
returns the rational numbers. I don't know what is meant by the term
"ambient vector space" in this context.
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