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## manipulating sparse matrices

**From**: |
Mike Miller |

**Subject**: |
manipulating sparse matrices |

**Date**: |
Fri, 14 Oct 2005 18:27:36 -0500 (CDT) |

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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**manipulating sparse matrices**,
*Mike Miller* **<=**