X-Git-Url: http://gitweb.michael.orlitzky.com/?p=octave.git;a=blobdiff_plain;f=divided_difference.m;h=4705f0a3d8d1ff3cf699cca2dd21c04d30a3c2fa;hp=7b8020e95c3b145fe637502802a09113126b329e;hb=13374cc93fd93143fb57601c8251bd07a8a53031;hpb=1a5bceb9de6bd3a511e7edcb6ab08f84dae63948

diff --git a/divided_difference.m b/divided_difference.m
index 7b8020e..4705f0a 100644
--- a/divided_difference.m
+++ b/divided_difference.m
@@ -3,23 +3,31 @@ function dd = divided_difference(f, xs)
   ## is assumed to be a vector containing at least one element. If it
   ## contains n elements, the (n-1)st divided difference will be
   ## calculated.
-  order = length(xs);
-  
-  if (order < 1)
+  ##
+  ## INPUTS:
+  ##
+  ##   * ``f`` - The function whose divided differences we want.
+  ##
+  ##   * ``xs`` - A vector containing x-coordinates. The length of `xs`
+  ##   determines the order of the divided difference.
+  ##
+  ##
+  ## OUTPUTS:
+  ##
+  ##   * ``dd`` - The divided difference f[xs(1), xs(2),...]
+  ##
+
+  order = length(xs) - 1;
+
+  if (order < 0)
     ## Can't do anything here. Return nothing.
     dd = NA;
-  elseif (order == 1)
+  elseif (order == 0)
     ## Our base case.
-    dd = f(xs(1))
+    dd = f(xs(1));
   else
-    ## Order > 1, recurse.
-
-    ## f[x0,...,x_n-1]
-    f0 = divided_difference(f, xs(1:end-1));
-    ## f[x1,...,x_n]
-    f1 = divided_difference(f, xs(2:end));
-
-    # http://mathworld.wolfram.com/DividedDifference.html
-    dd = (f0 - f1)/(xs(1) - xs(end))
+    ## Order >= 1.
+    cs = divided_difference_coefficients(xs);
+    dd = dot(cs, f(xs));
   end
 end