You have not assigned a value to x. Maybe you meant:
for s=2:m
x = tridiagonal(n); % assign x!
for t=1:n
w(s,t)=x(t);
end
end
But you can save one of the for loops:
for s=2:m
x = tridiagonal(n); % assign x!
w(s, :) = x; % or x', depending on dimension of x
end
