# HG changeset patch
# User Francesco Potortì <address@hidden>
# Date 1229133593 -3600
# Node ID 1235e389bbd99059b370ddbfbc4e1c6e24268f34
# Parent  87cca636a6c61e0cc8c3ab88a2652782203aff26
nchoosek checks, warning, corner cases and tests

diff -r 87cca636a6c6 -r 1235e389bbd9 scripts/ChangeLog
--- a/scripts/ChangeLog	Fri Dec 12 23:25:54 2008 +0100
+++ b/scripts/ChangeLog	Sat Dec 13 02:59:53 2008 +0100
@@ -1,3 +1,8 @@ 2008-12-11  Jaroslav Hajek  <address@hidden
+2008-12-13  Francesco Potortì  <address@hidden>
+
+	* specfun/nchoosek.m: Check for input arguments, signal loss of
+	precision, correctly handle k==0 and k==n cases, add proper tests.
+
 2008-12-11  Jaroslav Hajek  <address@hidden>
 
 	* optimization/fsolve.m: Optionally allow pivoted qr factorization.
diff -r 87cca636a6c6 -r 1235e389bbd9 scripts/specfun/nchoosek.m
--- a/scripts/specfun/nchoosek.m	Fri Dec 12 23:25:54 2008 +0100
+++ b/scripts/specfun/nchoosek.m	Sat Dec 13 02:59:53 2008 +0100
@@ -49,31 +49,44 @@
 ## of @var{n}, taken @var{k} at a time, one row per combination. The 
 ## resulting @var{c} has size @code{[nchoosek (length (@var{n}), 
 ## @var{k}), @var{k}]}.
+##
+## @code{nchoosek} works only for nonnegative integer arguments; use
+## @code{bincoeff} for non-integer scalar arguments and for using vector
+## arguments to compute many coefficients at once.
 ##
 ## @seealso{bincoeff}
 ## @end deftypefn
 
 ## Author: Rolf Fabian  <address@hidden>
 ## Author: Paul Kienzle <address@hidden>
-
-## FIXME -- This function is identical to bincoeff for scalar
-## values, and so should probably be combined with bincoeff.
+## Author: Jaroslav Hajek
 
 function A = nchoosek (v, k)
 
-  if (nargin != 2)
+  if (nargin != 2
+      || !isnumeric(k) || !isnumeric(v)
+      || !isscalar(k) || (!isscalar(v) && !isvector(v)))
     print_usage ();
+  endif
+  if ((isscalar(v) && v < k) || k < 0
+      || k != round(k) || any (v < 0 || v != round(v)))
+    error ("nchoosek: args are nonnegative integers with V not less than K");
   endif
 
   n = length (v);
 
   if (n == 1)
-    if (k > v/2)
-      k = v - k;
+    k = min (k, v-k);		# improve precision at next step
+    A = round (prod ((v-k+1:v)./(1:k)));
+    if (A*2*k*eps >= 0.5)
+      warning ("nchoosek", "nchoosek: possible loss of precision");
     endif
-    A = round (prod ((v-k+1:v)./(1:k)));
+  elseif (k == 0)
+    A = [];
   elseif (k == 1)
     A = v(:);
+  elseif (k == n)
+    A = v(:).';
   elseif (k > n)
     A = zeros (0, k, class (v));
   else
@@ -110,6 +123,8 @@ function A = nchoosek (v, k)
   endif
 endfunction
 
-# nchoosek seems to be slightly more accurate (but only allowing scalar inputs)
-%!assert (nchoosek(100,45), bincoeff(100,45), -1e2*eps)
+%!warning (nchoosek(100,45));
+%!error (nchoosek(100,45.5));
+%!error (nchoosek(100,145));
+%!assert (nchoosek(80,10), bincoeff(80,10))
 %!assert (nchoosek(1:5,3),[1:3;1,2,4;1,2,5;1,3,4;1,3,5;1,4,5;2:4;2,3,5;2,4,5;3:5])