mazari - in the line of code
neighbour_n(k)= [neighbour_n(k) j];
you are trying to access neighbour_n(k) (for k equals one) before it has been created. I think that you are trying to compensate for this with your line of code
neighbour_n(k)= [];
which isn't quite correct in terms of syntax. As the code only updates the kth row of neighbour_n if the distance condition is met, I suspect that you want neighbour_n to be a cell array as each row can have a different number of columns (neighbours). Try initializing this array outside the for loop as
neighbour_n = cell(N,1);
and then update the kth row as
neighbour_n{k}= [neighbour_n{k} j];
Note the use of the curly braces as neighbour_n is a cell array. Try implementing the above and see what happens!
