#10793: Matrices can be "constructed" from matrices of wrong dimensions
------------------------------+---------------------------------------------
Reporter: vbraun | Owner: jason, was
Type: defect | Status: positive_review
Priority: critical | Milestone: sage-4.7.2
Component: linear algebra | Keywords: sd31
Work_issues: | Upstream: N/A
Reviewer: Volker Braun | Author: Andrey Novoseltsev
Merged: | Dependencies: #11200
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Changes (by vbraun):
* status: needs_review => positive_review
* reviewer: => Volker Braun
* milestone: sage-4.7.1 => sage-4.7.2
Old description:
> Let's make a matrix and use it to define a morphism:
> {{{
> sage: projection = matrix(ZZ,[[1,0,0],[0,1,0]])
> sage: projection
> [1 0 0]
> [0 1 0]
> sage: H = Hom(ZZ^3, ZZ^2)
> sage: H(projection)
> Free module morphism defined by the matrix
> [1 0]
> [0 0]
> [1 0]
> Domain: Ambient free module of rank 3 over the principal ideal domain ...
> Codomain: Ambient free module of rank 2 over the principal ideal domain
> ...
> }}}
> As we see, the matrix of the morphism is very unlikely to be what it
> should be. Here is the source of the problem:
> {{{
> sage: projection.parent()
> Full MatrixSpace of 2 by 3 dense matrices over Integer Ring
> sage: M = MatrixSpace(ZZ, 3 , 2)
> sage: M
> Full MatrixSpace of 3 by 2 dense matrices over Integer Ring
> sage: M(projection)
> [1 0]
> [0 0]
> [1 0]
> }}}
> So the matrix space converts the input to the matrix no matter what (same
> with `matrix` command, but inside morphisms matrix spaces are used). I
> suppose this will work any time the number of entries in the original and
> in the destination is matching. I think that if one really wants to do
> it, then this one is very welcome to insert an explicit conversion of a
> matrix to a list and then back to a matrix, but the above should raise
> exceptions.
New description:
Let's make a matrix and use it to define a morphism:
{{{
sage: projection = matrix(ZZ,[[1,0,0],[0,1,0]])
sage: projection
[1 0 0]
[0 1 0]
sage: H = Hom(ZZ^3, ZZ^2)
sage: H(projection)
Free module morphism defined by the matrix
[1 0]
[0 0]
[1 0]
Domain: Ambient free module of rank 3 over the principal ideal domain ...
Codomain: Ambient free module of rank 2 over the principal ideal domain
...
}}}
As we see, the matrix of the morphism is very unlikely to be what it
should be. Here is the source of the problem:
{{{
sage: projection.parent()
Full MatrixSpace of 2 by 3 dense matrices over Integer Ring
sage: M = MatrixSpace(ZZ, 3 , 2)
sage: M
Full MatrixSpace of 3 by 2 dense matrices over Integer Ring
sage: M(projection)
[1 0]
[0 0]
[1 0]
}}}
So the matrix space converts the input to the matrix no matter what (same
with `matrix` command, but inside morphisms matrix spaces are used). I
suppose this will work any time the number of entries in the original and
in the destination is matching. I think that if one really wants to do it,
then this one is very welcome to insert an explicit conversion of a matrix
to a list and then back to a matrix, but the above should raise
exceptions.
Apply trac_10793_bug_in_matrix_construction.patch,
trac_10793_fixing_existing_bugs.patch
--
Comment:
I totally forgot that we haven't merged this ticket yet. Applies fine on
Sage-4.7.1.rc2. Positive review.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10793#comment:13>
Sage <http://www.sagemath.org>
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