From bug-octave-request at bevo dot che dot wisc dot edu Tue Jan  6 12:20:55 2004
Subject: Re: Missing ND array functionality
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Quentin Spencer <qspencer at ieee dot org>
Cc: bug-octave at bevo dot che dot wisc dot edu
Date: Tue, 6 Jan 2004 12:20:41 -0600

On 21-Dec-2003, Quentin Spencer <qspencer at ieee dot org> wrote:

| 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]);

I think that should be

  out = permute (in, [N+1:ndims, 1:N]);

otherwise if N is 1, it seems there won't be any shift.

| 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.

How about something like

  function out = shiftdim (in, N)
    if (N < 0)
      orig_dims = size (in);
      singleton_dims = ones (1, -N);
      out = reshape (in, [singleton_dims, orig_dims]);
    elseif (N > 0)
      ndims = length (size (in));
      out = permute (in, [N+1:ndims, 1:N]);
    else
      out = in;
    endif
  endfunction

Can you (or someone) write a complete implementation of this with
some error checking and a doc string?  You'll also need to handle the
case of

  [out, nls] = shiftdim (in)

which removes the leading singleton dimensions and returns the number
of leading singleton dimensions in nls.  I think you can also use
reshape for doing that.

Thanks,

jwe



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