function prc = prcdiff(a, b)
%PRCDIFF Percent difference.
%
%   PRCDIFF(A, B) computes the percent difference between A and B.  That is,
%   the difference in percent between A and B relative to A.
%
%   Author:      Peter John Acklam
%   Time-stamp:  2003-10-13 14:28:03 +0200
%   E-mail:      pjacklam@online.no
%   URL:         http://home.online.no/~pjacklam

   % check number of input arguments
   error(nargchk(2, 2, nargin));

   i = ~a;          % true when a is zero
   j = ~b;          % true when b is zero

   % use an adjusted denominator to avoid "division by zero" error
   c = a;
   c(i) = 1;

   prc = 100 * (b - a) ./ c;

   prc(i &  j) = NaN;                           % when a = 0 and b = 0
   prc(i & ~j) = sign(b(i & ~j)) * Inf;         % when a = 0 and b ~= 0