Matrix multiplication in a for-loop

I'm currently trying to transform this expression:

     k = 1:m;
     t1 = sum((theta(1) + theta(2) .* X(k,2)) - y(k)); 
     t2 = sum(((theta(1) + theta(2) .* X(k,2)) - y(k)) .* X(k,2));
     
     theta(1) = theta(1) - (alpha/m) * (t1);
     theta(2) = theta(2) - (alpha/m) * (t2);

into a for loop expression, so that it will work for longer theta vectors. So far I came up with this:

     for j=1,length(theta)
          t = sum(((X * theta)- y) .* X(:,j));
          theta(j) = theta(j) - (alpha/m) * t;
     end

But the values for theta(2) are not updated. What am I doing wrong?

Answer 1

[JESUS DAVID ARIZA ROYETH]

you have 1,length(theta), change it by 1:length(theta) :

  for j=1:length(theta)
          t = sum(((X * theta)- y) .* X(:,j));
          theta(j) = theta(j) - (alpha/m) * t;
  end