Linear Regression exercise (Coursera course: ex1_multi)

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PaulineJV
PaulineJV 2018-7-9
Hi there,
I am taking Andrew Ng's Coursera class on machine learning. After implementing gradient descent in the first exercise (goal is to predict the price of a 1650 sq-ft, 3 br house), the J_history shows me a list of the same value (2.0433e+09). So when plotting the results, I am left with a straight line giving me that single value of J.
  • Here is my code:
function [theta, J_history] = gradientDescentMulti(X, y, theta, alpha, num_iters)
m = length(y);
J_history = zeros(num_iters, 1);
for iter = 1:num_iters
delta = zeros(size(X,2),1);
for i=1:size(X,2)
delta(i,1)=sum((theta'*X'-y').*(X(:,i))')*alpha/m;
end
theta = theta - delta;
J_history(iter) = computeCostMulti(X, y, theta);
end
end
  • What I entered in the Command Window:
data = load('ex1data2.txt');
X = data(:, 1:2);
y = data(:, 3);
m = length(y);
[X, mu, sigma] = featureNormalize(X)
X = [ones(m, 1) X];
std(X(:,2))
alpha = 0.01;
num_iters = 400;
theta = zeros(3, 1);
computeCostMulti(X,y,theta)
[theta, J_history] = gradientDescentMulti(X, y, theta, alpha, num_iters);
figure;
plot(1:numel(J_history), J_history, '-b', 'LineWidth', 2);
xlabel('Number of iterations');
ylabel('Cost J');
fprintf('Theta computed from gradient descent: \n');
fprintf(' %f \n', theta);
fprintf('\n');
I get the following result for theta: 340412.659574 110631.050279 -6649.474271
price = 0;
parameter = [1650,3];
parameter = (parameter-mu)./sigma;
parameter = [1,parameter];
price = theta'*parameter';
fprintf(['Predicted price of a 1650 sq-ft, 3 br house ' ... '(using gradient descent):\n $%f\n'], price);
I get the following result for the price: $293081.464335
  • Here is the result for J_history:
I'm not sure where the issue comes from. I've attached the exercise and functions to this post.
Thanks for your help!
Pauline

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