help-octave
[Top][All Lists]

## Re: large sparse matrix

 From: Carlo de Falco Subject: Re: large sparse matrix Date: Tue, 1 Sep 2009 09:57:37 +0200

```
On 1 Sep 2009, at 09:14, Jaroslav Hajek wrote:

```
On Tue, Sep 1, 2009 at 1:12 AM, Carlo de Falco<address@hidden> wrote:
```
On 31 Aug 2009, at 21:53, David Bateman wrote:
```
```
Yes, but the configuration of SuiteSparse in terms of the pivot
threshold is the default that TIm Davis sets for Octave and whatever
mathworks chose to use for matlab. So don't expect exactly the
behavior.
```
```I see...

```
```In Octave what does

matrix_type(A)
give?
```
```

>> matrix_type (A)
ans = Positive Definite
>>

```
```If it doesn't flag your matrix as PD then there might be an issue
with the detection of the matrix and PD and the solver used in
Octave will be LU.
```
```
on the other hand what surprises me is that:

>> issparse (B)
ans =  1
>> matrix_type (B)
ans = Rectangular
>> issparse (A)
ans =  1
>> mat = [A B' spalloc(rows(A),1,0); B spalloc(rows(B),columns(A),1)
E'; spalloc(columns(A),1,1) E 0];
>> matrix_type (mat)
ans = Full
>> issparse (mat)
ans =  1

why is mat marked as full???

```
```
Full is what you get when nothing special about the matrix is
discovered. Did you expect something else? The matrix is obviously not
positive definite, as seen from its construction, and I can see no
apparent other structure (triangularity or bandedness), though it of
course depends on the values of A, B and E.

hth
```
```
Well, the matrix is of course symmetric by construction,
but I don't think there's much more...
But actually the reason I was surprised that it is marked as
"Full" is that I thought this mark had to do with its storage format.
```
Maybe another name like, e.g., "Generic" would have been less misleading...
```Thnaks for the info!
c.

```