Simple Perceptron algorithm in matlab: cannot draw the classifier line

I am new to Matlab and I am trying to test my learning curve by writing the perceptron algorithm from scratch. I have seen this in R but I think it is fun to try out Matlab. It seems to me that I cannot draw the classifier line.

load data1.mat
w_0 = [1;-1];
for iteration = 1 : 100  %<- I do not know how to define a convergence or stopping criteria
  for ii = 1 : size(X,2)         %cycle through training set
    if sign(w'.*X(:,ii)) <=0 %wrong decision?
      w = w + X(:,ii) * y(ii);   %then add (or subtract) this point to w
    end
  end
end
x1 = [-1,b];
x2 = [1,d];
b = w(1)/w(2);
d = -w(1)/w(2);
scatter(X(:,1),X(:,2),50,y,'*');
hold on;plot(x1,x2);hold off

My data set ("data1.mat") has X as the feature matrix of 40x2 and y. I want to plot the boundary line i.e the line w^Tx = w_1*x_1 + w_2*x_2 = 0. I just choose 2 points p1 = (a,b) and p2 = (c,d). Since my data limits x_1 and x_2 between -1 and 1, I choose a = 1 and c = -1 and find out b and d.

But still, the syntax looks normal to me but I have an error:

Error using horzcat
Dimensions of matrices being concatenated are not consistent.

Error in Myperceptron (line 12)
x1 = [-1,b];
Thanks!

[Kaushik Lakshminarasimhan]

There is no reference to the variable b. I can only assume that it is loaded from data1.mat file. You need to make sure b is scalar.

[Hieu Nguyen]

I fixed the code. I moved the definitions of b and d before x1 and x2 but there is still not line drawn from the plot(x1,x2)