From octave-maintainers-request at bevo dot che dot wisc dot edu  Thu Sep  7 19:34:22 2000
Subject: Re: [PATCH] Add min/max tests. (4/4)
From: Paul Kienzle <pkienzle at kienzle dot powernet dot co dot uk>
To: ejr at EECS dot Berkeley dot EDU, jwe@bevo.che.wisc.edu
Cc: octave-maintainers at bevo dot che dot wisc dot edu
Date: Fri, 08 Sep 2000 01:26:46 +0100

John W. Eaton <jwe at bevo dot che dot wisc dot edu> writes:
>On  5-Sep-2000, I wrote
>
>| How about
>| 
>|   min (abs (x), abs (y))
>| 
>| ?
>
>as a suggestion for how to explicitly state that you want to find the
>numbers with the minimum magnitudes.  After sending it, I realized
>that this is not really what you want, since it returns the minimum
>magnitudes, not the NUMBERS with the minimum magnitudes.  So, I think
>what is really needed is some way to tell the min and max funtions
>what comparison operation (or function) to use.  That would allow the
>programmer to clearly state his intention instead of relying on some
>convention that can cause trouble when there are mixed types or the
>potential for automatic data type conversions to occur.

For the cases where you need a nonstandard comparison, you could easily
write a specialized function.  For example, for magnitude:

	function z=minmag(x,y)
	   z=x(:);
	   zi=find(abs(x)>abs(y));
	   do_fortran_indexing = 1;
	   z(zi) = y(zi);
	   z = reshape(z,size(x));
	endfunction

To make it completely proper is a bit more work.  You must protect the
do_fortran_indexing state.  You must handle 0, 1, 2, or many arguments,
and for one argument, you must handle row/column vectors separately from
matrices.  The following should do it (though more testing is required):

function [z, zi] = minmag(x, y, ...)
  if nargin == 0
    usage("[z,zi]=minmag(x) or z=minmag(x1,x2,...)");
  elseif nargin == 1
    [z, zi] = min(abs(x));
    [nr, nc] = size(x);
    if (nr == 1 || nc == 1)
      z(zi) = x(zi);
    else
      dfi = do_fortran_indexing;
      unwind_protect
	do_fortran_indexing = 1;
	z = x(zi + [0:nc-1]*nr);
      unwind_protect_cleanup
	do_fortran_indexing = dfi;
      end_unwind_protect
    endif
  else
    dfi = do_fortran_indexing;
    unwind_protect
      do_fortran_indexing = 1;
      z = x(:);
      idx = find(abs(x) > abs(y));
      z(idx) = y(idx);
      z = reshape(z, size(x));
    unwind_protect_cleanup
      do_fortran_indexing = dfi;
    end_unwind_protect
    if nargin > 2
      z = minmag(z, all_va_args);
    endif
  endif
endfunction

>
>Also, if it were up to me (or if I could do it over again) I would
>want to avoid the problem that results from min, max, and other
>functions working on columns of a matrix except when the matrix
>happens to have only one row.

No arguments there.

>
>jwe
>
>
>