Gradient descent Search implemented in matlab theta1 incorrect - matlab

Question

I studied the Machine learning course taught by Prof. Andrew Ng. This is the link
I try to implement the 1st assignment of this course. Exercise 2: Linear Regression based upon Supervised learning problem
1.Implement gradient descent using a learning rate of alpha=0.07.Since Matlab/Octave and Octave index vectors starting from 1 rather than 0, you'll probably use theta(1) and theta(2) in Matlab/Octave to represent theta0 and theta1.
I write down a matlab code to solve this problem:
clc
clear
close all
x = load('ex2x.dat');
y = load('ex2y.dat');
figure % open a new figure window
plot(x, y, '*');
ylabel('Height in meters')
xlabel('Age in years')
m = length(y); % store the number of training examples
x = [ones(m, 1), x]; % Add a column of ones to x
theta = [0 0];
temp=0,temp2=0;
h=[];
alpha=0.07;n=2; %alpha=learning rate
for i=1:m
temp1=0;
for j=1:n
h(j)=theta(j)*x(i,j);
temp1=temp1+h(j);
end
temp=temp+(temp1-y(i));
temp2=temp2+((temp1-y(i))*(x(i,1)+x(i,2)));
end
theta(1)=theta(1)-(alpha*(1/m)*temp);
theta(2)=theta(2)-(alpha*(1/m)*temp2);
I get the answer :
>> theta
theta =
0.0745 0.4545
Here, 0.0745 is exact answer but 2nd one is not accurate.
Actual answer
theta =
0.0745 0.3800
The data set is provided in the link. Can any one help me to fix the problem?

Answer 1

You get wrong results because you write long unnecessary code that is easily prone to bugs, that is exactly why we have matlab:
clear
x = load('d:/ex2x.dat');
y = load('d:/ex2y.dat');
figure(1), clf, plot(x, y, '*'), xlabel('Age in years'), ylabel('Height in meters')
m = length(y); % store the number of training examples
x = [ones(m, 1), x]; % Add a column of ones to x
theta=[0,0]; alpha=0.07;
residuals = x*theta' - y ; %same as: sum(x.*theta,2)-y
theta = theta - alpha*mean(residuals.*x);
disp(theta)