# Getting gradient descent to work in octave.(Andrew ng's machine learn course, excersise 1)

So i am trying to implement/solve the first programming excersise from Andrew ng`s machine learn cours on coursera. I have trouble implementing linear gradient descent(for one variable) in octave. I don't get the same paramters values back like in the solution but my parameters goes in the same direction(at least i think so). So i may have somewhere in my code a bug. Maybe someone who has more experience than me can enlighten me.

``````function [theta, J_history] = gradientDescent(X, y, theta, alpha, num_iters)
%GRADIENTDESCENT Performs gradient descent to learn theta
%   theta = GRADIENTDESCENT(X, y, theta, alpha, num_iters) updates theta by
%   taking num_iters gradient steps with learning rate alpha

% Initialize some useful values
m = length(y); % number of training examples
J_history = zeros(num_iters, 1);

theta1 = theta(1);
theta2 = theta(2);

temp0 = 0;
temp1 = 0;

h = X * theta;
for iter = 1:(num_iters)

% ====================== YOUR CODE HERE ======================
% Instructions: Perform a single gradient step on the parameter vector
%               theta.
%
% Hint: While debugging, it can be useful to print out the values
%       of the cost function (computeCost) and gradient here.
%
temp0 = 0;
temp1 = 0;
for i=1:m
error = (h(i) - y(i));
temp0 = temp0 + error * X(i, 1));;
temp1 = temp1 + error * X(i, 2));
end
theta1 = theta1 - ((alpha/m) * temp0);
theta2 = theta2 - ((alpha/m) * temp1);
theta = [theta1;theta2];

% ============================================================

% Save the cost J in every iteration
J_history(iter) = computeCost(X, y, theta);

end
end
``````

My exspected results for excersise 1 with theta initialized with [0;0] should be for theta1: -3.6303 and for theta2: 1.1664

But i become as output theta1 is 0.095420 and thetha2 is 0.51890

This is the formula i use for linear gradient descent.

EDIT1: Edited code. Now i got for theta1:

87.587

and for theta2

979.93

• In the inner for loop, you are replacing `temp0` and `temp1` `m` times, and then just using the last value – Ander Biguri Apr 11 at 14:00
• thank`s i think this might be the bug. I totally didn't see that i am so stupid. Thank you very much. – Yuto Apr 11 at 14:04

## 1 Answer

I now know what my problem was. I am going to describe it quick for anbody who might be intrested in it. So i accidently calulated the avriable `h` outside of my loop. So every time in the loop it calulated with the same value.

Here is the fixed code:

``````function [theta, J_history] = gradientDescent(X, y, theta, alpha, num_iters)
%GRADIENTDESCENT Performs gradient descent to learn theta
%   theta = GRADIENTDESCENT(X, y, theta, alpha, num_iters) updates theta by
%   taking num_iters gradient steps with learning rate alpha

% Initialize some useful values
m = length(y); % number of training examples
J_history = zeros(num_iters, 1);

theta1 = theta(1);
theta2 = theta(2);

temp0 = 0;
temp1 = 0;
error = 0;

for iter = 1:(num_iters)
% ====================== YOUR CODE HERE ======================
% Instructions: Perform a single gradient step on the parameter vector
%               theta.
%
% Hint: While debugging, it can be useful to print out the values
%       of the cost function (computeCost) and gradient here.
%

h = X * theta; %heres the variable i moved into the loop

temp0 = 0;
temp1 = 0;
for i=1:m
error = (h(i) - y(i));
temp0 = temp0 + (error * X(i, 1));
temp1 = temp1 + (error * X(i, 2));
%disp(error);
end
theta1 = theta1 - ((alpha/m) * temp0);
theta2 = theta2 - ((alpha/m) * temp1);
theta = [theta1;theta2];

% ============================================================

% Save the cost J in every iteration
J_history(iter) = computeCost(X, y, theta);

end
end
``````