--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/scripts/sparse/sprandsym.m	Thu Feb 09 09:12:03 2006 +0000
@@ -0,0 +1,75 @@
+## Copyright (C) 2004 David Bateman & Andy Adler
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 2 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; if not, write to the Free Software
+## Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+## 02110-1301  USA
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {} sprandsym (@var{n}, @var{d})
+## @deftypefnx {Function File} {} sprandsym (@var{s})
+## Generate a symmetric random sparse matrix. The size of the matrix will be
+## @var{n} by @var{n}, with a density of values given by @var{d}.
+## @var{d} should be between 0 and 1. Values will be normally
+## distributed with mean of zero and variance 1.
+##
+## Note: sometimes the actual density  may be a bit smaller than @var{d}. 
+## This is unlikely to happen for large really sparse matrices.
+##
+## If called with a single matrix argument, a random sparse matrix is
+## generated wherever the matrix @var{S} is non-zero in its lower
+## triangular part.
+## @end deftypefn
+## @seealso{sprand, sprandn}
+
+function S = sprandsym(n,d)
+  if nargin == 1
+    [i,j,v,nr,nc] = spfind(tril(n));
+    S = sparse(i,j,randn(size(v)),nr,nc);
+    S = S + tril(S,-1)';
+  elseif nargin == 2
+    m1 = floor(n/2);
+    n1 = m1 + 1;
+    mn1 = m1*n1;
+    k1 = round(d*mn1);
+    idx1=unique(fix(rand(min(k1*1.01,k1+10),1)*mn1))+1; 
+                # idx contains random numbers in [1,mn]
+  		# generate 1% or 10 more random values than necessary
+		# in order to reduce the probability that there are less than k
+		# distinct values;
+    		# maybe a better strategy could be used
+     		# but I don't think it's worth the price
+    k1 = min(length(idx1),k1);  # actual number of entries in S
+    j1 = floor((idx1(1:k1)-1)/m1);
+    i1 = idx1(1:k1) - j1*m1;
+
+    n2 = ceil(n/2);
+    nn2 = n2*n2;
+    k2 = round(d*nn2);
+    idx2=unique(fix(rand(min(k2*1.01,k1+10),1)*nn2))+1; 
+    k2 = min(length(idx2),k2);
+    j2 = floor((idx2(1:k2)-1)/n2);
+    i2 = idx2(1:k2) - j2*n2;
+
+    if isempty(i1) && isempty(i2)
+      S = sparse(n,n);
+    else
+      S1 = sparse(i1,j1+1,randn(k1,1),m1,n1);
+      S = [tril(S1), sparse(m1,m1); ...
+	   sparse(i2,j2+1,randn(k2,1),n2,n2), triu(S1,1)'];
+      S = S + tril(S,-1)';
+    endif
+  else
+    usage("sprandsym(n,density) OR sprandsym(S)");
+  endif
+endfunction