use the vectorization to accelerate the calculation

[comeonsunny]

% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
% You should set J to the cost.
sum = 0;
temp_cost = X * theta - y;
for i = 1:m
sum = sum + temp_cost(i)^2;
end

J = (1/(2*m))*sum;