make diagonal_matrix accept much more general arguments

[sagetrac-jkantor]

So I think this is a bug

sage: w=vector(RR,[1,2,3])
sage: d=diagonal_matrix(w)
UnboundLocalError: local variable 'v' referenced before assignment

The following fails as well

sage: d=diagonal_matrix(RR,w) 

the only thing that works is

sage: d=diagonal_matrix(RR,list(w))

A stupid but easy fix is to try to turn any argument to diagonal_matrix into a list before bailing out (its in matrix/constructor.py), but there should probably be logic actually expecting vectors and analyzing the parents?

Component: linear algebra

Issue created by migration from https://trac.sagemath.org/ticket/3704

[jasongrout]

comment:1

Would it be easy as taking the argument to diagonal_matrix and sending it through Sequence?

sage: v=vector(QQ,[1,2,3/4])
sage: v
(1, 2, 3/4)
sage: b=diagonal_matrix(Sequence(v)); b

[  1   0   0]
[  0   2   0]
[  0   0 3/4]
sage: b.parent()
Full MatrixSpace of 3 by 3 dense matrices over Rational Field

[jasongrout]

comment:2

The attached patch is a complete rewrite of diagonal_matrix which now does the following:

1. If the first argument is a ring, pop it off the argument list
2. If the next thing can be made into a Sequence, do so and use those elements as the diagonal elements. If the next thing cannot be made into a sequence, then take the remainder of the argument list and use those things as the diagonal elements.
3. Prepend the ring if one was specified and send everything to the matrix() function.

This changes the syntax slightly; in order to get a matrix of a specific size, you need to pass an nrows and/or ncols option. But it is very nice in that it allows diagonal_matrix(1,2,3) or, as mentioned in this ticket, diagonal_matrix(v) where v is a vector.

All doctests in sage/matrix/* pass and all doctests elsewhere that I found using diagonal_matrix also pass.

[jasongrout]

comment:4

I guess it really is a defect, as pointed about above; there is a bug that is fixed.