manipulating sparse matrices

[Mike Miller]

I'm using Octave 2.1.71 on Linux with Octave-Forge 2005.06.13.

I have two questions about sparse matrices...

If I do this...

a = speye(1000);
a(1,1) = 2;

...the resulting 'a' matrix is no longer of class "sparse" and it takes up 
8 MB of RAM.  That's not good for me.  Is there a way to reassign an 
element of a sparse matrix without changing its class?  I believe this 
just works in 2.9, but is there a way to do it in 2.1.71?

How can I find out how much memory is being used by a sparse matrix?  If I 
do this...

octave:33> a=speye(1000);
octave:34> whos a

...I am told "Total is 1000000 elements using 0 bytes," but is it really 
zero bytes?  That's an impressive feat.  There seems to be a function 
called nzmax that will tell me the memory allocation, but I can't figure 
out how to get that function...

http://octave.sourceforge.net/index/f/nzmax.html

Best,

Mike



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