Re: [Pkg-octave-devel] Bug#394982: octave2.9: Crashes on "x(:,

[John W. Eaton]

On 26-Oct-2006, David Bateman wrote:

| Ok then use the attached patch in addition to the previous patch... It
| essentially removes the previous patch and does it differently.
| 
| Note that this is still a very bad idea as it reallocates the memory to
| the sparse matrix and every assignment, even if you size the initial
| matrix correctly. This is due to the fact that only enough space for the
| nonzeros is ever created... I'd therefore highly recommended forming
| vectors of row, col and value and use the sparse function instead. This
| is discussed in the sparse section of the manual....

I applied the patch and now I see

  octave:5> x = sparse (2,0)
  x = Compressed Column Sparse (rows = 2, cols = 0, nnz = 0)

  octave:6> x(:,1:2) = speye (2)
  x =

     1   0
     0   1

  octave:7> x(:,3:4) = speye (2)
  x =

     1   0   1   0
     0   1   0   1


so it seems OK, but what is happening here:

  octave:1> x = sparse (2,0)
  x = Compressed Column Sparse (rows = 2, cols = 0, nnz = 0)

  octave:2> x(:,2,3) = speye (2)
  error: A(I, J) = X: can only have 1 or 2 indexes for sparse matrices
  error: assignment failed, or no method for `sparse matrix = sparse matrix'
  error: evaluating assignment expression near line 2, column 10
  octave:2> x
  x = Compressed Column Sparse (rows = 2, cols = 0, nnz = 0)

  octave:3> x(:,1:2) = speye (2)
  error: A(I, J) = X: can only have 1 or 2 indexes for sparse matrices
  error: assignment failed, or no method for `sparse matrix = sparse matrix'
  error: evaluating assignment expression near line 3, column 10

The command on line 2 was a mistake, but it seems to have a persistent
effect.  Even though x still seems to be a 2x0 sparse matrix,
subsequent assignments fail after the first failure.  Clearing x
allows it to work again, but how is it that the error is causing the
later failures?

jwe