Re: Matrix index, Please e-mail back.

[Paul Kienzle]

On Dec 16, 2006, at 6:37 AM, Joan Picanyol i Puig wrote:

> * Paul Kienzle <address@hidden> [20061206 03:36]:
> > On Dec 5, 2006, at 6:39 PM, Jordi Gutierrez Hermoso wrote:
> >
> > > There are cute little indexing hacks that can be accomplished if you
> > > remember that Octave stores matrices in Fortran column-major order. 
> > > In
> > > other words, a matrix can be indexed by a single index as if it were
> > > one long vector.
> >
> > The functions sub2ind and ind2sub hide the details, letting you
> > access an array element by element rather than by slices.
>
> Could you please share some hints as how are they to be used? The docs
> are a bit too concise for my taste: what is "DIMS" supposed to be? What
> about "S1, ..., SN"? I've started to use indexing heavily as of late,
> but have been unable to use sub2ind and ind2sub, I tipically get an
> "index out of range" error (see
> http://www.cae.wisc.edu/pipermail/help-octave/2006-October/001983.html 
> )

 octave> z=reshape(1:3*4*2,[3,4,2])
 z =

 ans(:,:,1) =

   1   4   7  10
   2   5   8  11
   3   6   9  12

 ans(:,:,2) =

  13  16  19  22
  14  17  20  23
  15  18  21  24


Let's pick out elements (1,3,1)=7 and (2,2,2)=17

 octave> z(sub2ind(size(z), [1 2],[3 2],[1 2]))
 ans =

   7  17

Let's find the elements containing 11<z<15

 octave> [i,j,k] = ind2sub(size(z),find(z>11 & z<15))
 i =

  3
  1
  2

 j =

  4
  1
  1

 k =

  1
  2
  2

- Paul