[Octave-bug-tracker] [bug #32966] spdiags is slow and buggy, diag is not compatible

[Marco Caliari]

URL:
  <http://savannah.gnu.org/bugs/?32966>

                 Summary: spdiags is slow and buggy, diag is not compatible
                 Project: GNU Octave
            Submitted by: caliari
            Submitted on: Thu 31 Mar 2011 13:21:38 GMT
                Category: None
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 3.4.0
        Operating System: GNU/Linux

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Details:

Dear maintainers,

stimulated by the comment

## FIXME: we can do this without a loop if we are clever

in spdiags.m, I found the following:

1) diag is much faster then spdiags in extracting diagonals. I get

> v=sprand(1000,1000,0.5);
> tic,spdiags(v,0);,toc
Elapsed time is 0.072 seconds.
> tic,diag(v,0);,toc
Elapsed time is 0.0021 seconds.


2) spdiags fails in the following case:

> spdiags([0,1;0,0],2)
error: spdiags: A(I,J,...) = X: dimensions mismatch

whereas it works with [1,1;1,1].

3) diag is not compatible:

> diag(ones(2),2)
error: diag: requested diagonal out of range

whereas Matlab gives empty matrix 0-by-1.

If 3) is fixed, I can easily rewrite spdiags.m to avoid problems 2) and 1)
(without testing if the diagonal is out of range). However, I'm not clever and
I cannot do it without a loop :-(

4) there is another incompatible behaviour:

> spdiags(ones(2),-1)
ans =

   1
   0
> spdiags(ones(2,3),-1)
ans =

   1
   0

whereas in Matlab

>> spdiags(ones(2),-1)
ans =
     1
     0
>> spdiags(ones(2,3),-1)
ans =
     0
     1

I do not understand this Matlab (7.6.0) behaviour.

If we take a decision about 4) and someone fixes 3), I can work on 1) and 2).

Cheers,

Marco




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