关于机器学习的问题，吴恩达的机器学习WEEK2编程任务中为什么总是报错：y没有被定义？

在week2的computeCost和gradientDescent这两个任务中，总是显示y没有被定义，如何解决这个问题？

computeCost部分的代码：

%COMPUTECOST Compute cost for linear regression
%   J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
%   parameter for linear regression to fit the data points in X and y

% Initialize some useful values
m = length(y); % number of training examples

% You need to return the following variables correctly 
J = 0;

% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
%               You should set J to the cost.



J = sum((X*theta-y).^2)/(2*m);


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

end

gradientDescent部分的代码：

%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);

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.
    %

theta(1) = theta(1) - alpha*sum(X*theta_s-y)/m;
theta(2) = theta(2) - alpha*sum((X*theta_s-y).*X(:,2))/m;
theta_s = theta;






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

Octave输出的结果：

error: 'y' undefined near line 7, column 7
error: called from
    computeCost at line 7 column 3

error: 'y' undefined near line 7, column 7
error: called from
    gradientDescent at line 7 column 3

m = length(y); y没定义阿哥哥