# [Pkg-octave-devel] Bug#706376: Bug#706376: Bug#706376: octave: sparse matrix n*2^16

Jordi Gutiérrez Hermoso jordigh at octave.org
Tue Apr 30 13:37:36 UTC 2013

```On 29 April 2013 17:51, Ed Meyer <eem2314 at gmail.com> wrote:

> I'm not saying numel() is to blame or should be changed, only that I
> see no reason to ever use numel when handling sparse matrices unless
> you are converting it to full in which case the current behavior is
> ok.

Well, look at the current implementation of trace (), in which numel
is a perfectly reasonable function to call. If you don't want to ever
call numel () for sparse matrices, then you would need to rewrite this
function to check for sparsity as well as any other function that
calls numel ().

> Here again, why would you ever want A(idx) for a sparse matrix?

Because this is the only way to do arbitrary indexing, say, indexing
with a logical index:

n = 2^16;
A = sprandsym (n, 1e-7);
idx = A > 0.5;
A(idx) *= -1;

The alternative to arbitrary indexing is looping.

> I agree that a special index type is the wrong approach; what I'm
> saying is that with sparse matrices you should never run into this
> problem in the first place if you don't try to treat them the same
> as full. Otherwise why have sparse matrices at all?

It is desirable to have sparse matrices behave like ordinary matrices
under most circumstances, such as indexing and when writing the trace
function. Otherwise, you would have to write special code all over the
place to make sure that if the matrix is sparse, you don't linearly
index it nor do you call numel or who knows what other functions that