[regression] finished gradient section

regression/code/meanSquaredGradientCubic.m

@@ -0,0 +1,14 @@
+function dmsedc = meanSquaredGradientCubic(x, y, c)
+% The gradient of the mean squared error for a cubic relation.
+%
+% Arguments: x, vector of the x-data values
+%            y, vector of the corresponding y-data values
+%            c, the factor for the cubic relation. 
+%
+% Returns: the derivative of the mean squared error at c.
+
+  h = 1e-5;   % stepsize for derivatives
+  mse = meanSquaredErrorCubic(x, y, c);
+  mseh = meanSquaredErrorCubic(x, y, c+h);
+  dmsedc = (mseh - mse)/h;
+end

regression/code/plotcubicgradient.m

@@ -0,0 +1,9 @@
+cs = 2.0:0.1:8.0;
+mseg = zeros(length(cs));
+for i = 1:length(cs)
+    mseg(i) = meanSquaredGradientCubic(x, y, cs(i));
+end
+
+plot(cs, mseg)
+xlabel('c')
+ylabel('gradient')