Trouble with horner function in MATLAB

Question

I have the following homework question:

Apply linear least squares with the two models S1(A, B, C) = Ax^2 + Bx + C and S2(A, B, C, D) = Ax^3 + Bx^2 + Cx + D to the data set (0, 4), (1, −1), (2, 6), (3, 1), (4, −4), (5, −9). Solve in MATLAB using lspoly. Report the values of the parameters A, B, C and D clearly, and produce a plot showing the data and both fitting curves.

I am working with the following function in MATLAB:

Function 1

function y = horner(a,c)

n=length(a)-1;
y=a(n+1);

for k = n:-1:1
    y = a(k)+ c*y;
end

Function 2

function C = lspoly(x,y,M)

n = length(x);
F = zeros(n,M+1);
for k = 1:M+1
   F(:,k) = x'.^(k-1);
end
A = F'*F;
B = F'*y';
C = A\B;

And this is the code I wrote for the problem:

clc
clear all
close all

x = [0, 1, 2, 3, 4, 5];
y = [4, -1, 6, 1, -4, -9];

C = lspoly(x,y,2); % finds the parameters
xx = 0:0.01:5;
yy = zeros(1, length(xx));
for i=1:length(xx)
  yy(i) = horner(C,xx(i));
end

CC = lspoly(x,y,3); % finds the parameters
xxx = 0:0.1:5;
yyy = zeros(1, length(xxx));
for i=1:length(xxx)
  yyy(i) = horner(CC,xxx(i));
end

figure(1)
plot(x, y, 'o',  xx, yy, 'r-', xxx, yyy, 'b-')

I am encountering some issues with this code. When I try to run the program, I get the following error:

In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in HW7_1 (line 14) yy(i) = horner(C,xx(i));

I can't really wrap my head around what exactly I need to do to fix this issue. I tried breaking down my program piece by piece to determine results at different spots in the code, but so far nothing of note has been found.

Can someone help me fix this error?

Answer 1

It works just fine for me in Octave 3.8. The only thing I can think of is there a built-in function called horner in MATLAB (part of the Symbolic Math Toolbox), so maybe your code is calling that function instead of yours. Maybe try renaming it to something different, like my_horner or similar.