From help-octave-request at bevo dot che dot wisc dot edu Thu Dec 11 20:32:55 2003
Subject: Re: Arguments out?
From: Paul Kienzle <pkienzle at users dot sourceforge dot net>
To: Vic Norton <vic at norton dot name>
Cc: help-octave at bevo dot che dot wisc dot edu, Thorsten Meyer <thorsten.meyier@gmx.de>
Date: Thu, 11 Dec 2003 21:31:56 -0500


On Dec 11, 2003, at 6:05 PM, Vic Norton wrote:

> Yeah, I thought about that later, Thorsten. Thanks for pointing it out.
>
> So when you write something like
>
>    function   [U, S, V] = svd(A)
>
> and A comes in as an m x n, you might do the computations to get
>
>    U - orthogonal m x m
>    S - diagonal m x n
>    V - orthogonal n x n
>
> satisfying
>
>    A = U * S * V'
>
> Then you look at nargout.
>
> You will only accept nargout = 0, 1, or 3. If nargout = 0 or 1, you 
> want to return diag(S). If nargout = 3, you want to return the triple 
> [U, S, V]. Apparently your final lines should read something like this
>
>    if (nargout < 2)
>       U = diag(S);
>    elseif (nargout != 3)
>       error("you don't know what you are doing, dummy!");
>    endif
>
> This is what bothers me. U and diag(S) are completely different kinds 
> of objects. Writing U = diag(S) simply goes against my grain.

So write it using varargout:

function varargout = svd(A)
     ...
     switch nargout
       {0,1}, varargout = { diag(s) };
       3, varargout = { u, s, v };
       otherwise, usage("sigma=svd(A) or [u,s,v]=svd(A)");
    end
end

or equivalently using meaningless placeholders
for the returned values:

function [r1,r2,r3] = mysvd(A)
   ...
   if nargout<=1, r1=diag(s);
   else r1=u; r2=s; r3=v; end
end

Paul Kienzle
pkienzle at users dot sf dot net



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