From bug-octave-request at bevo dot che dot wisc dot edu Sun Dec 21 12:54:25 2003
Subject: Re: Missing ND array functionality
From: Quentin Spencer <qspencer at ieee dot org>
To: bug-octave at bevo dot che dot wisc dot edu
Date: Sun, 21 Dec 2003 12:53:42 -0600

John W. Eaton wrote:

>On 20-Dec-2003, Quentin Spencer <qspencer at ieee dot org> wrote:
>
>| If you have a 3 dimensional array of size (x,y,z), there is not 
>| currently a function to "rotate" the dimensions to convert the array to 
>| an array of size (y,z,x) (such a conversion in two dimensions would be 
>| the same as a transpose). Matlab provides the function "shiftdim" to 
>| accomplish this. I think this may be possible using reshape as well.
>
>We have permute and ipermute.  Can shiftdim be based on that?
>
>  
>
Thanks for pointing those out. Yes, shiftdim can be based partially on
that. For positive arguments, shiftdim would look like this:

function out = shiftdim(in,N)

ndims = length(size(in));
out = permute(in,[N:ndims,1:N-1]);

However, the Matlab function shiftdim also takes a negative argument,
and adds singleton dimensions (sort of the opposite of the "squeeze"
function). For example if A is a 1xN vector, shiftdim(A,-1) is a 1x1xN
array. I think that part could be handled using reshape, but it's not
obvious to me exactly how.

Quentin





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