tridiagonal.m

   1 function A = tridiagonal(integerN, x, y, z)
   2   %
   3   % Construct the integerN x integerN (square) tridiagonal matrix
   4   % diag(x,y,z).
   5   %
   6   % INPUT:
   7   %
   8   %   - ``integerN`` -- The dimensions of the resulting matrix.
   9   %
  10   %   - ``x`` -- The value on the subdiagonal.
  11   %
  12   %   - ``y`` -- The value on the diagonal.
  13   %
  14   %   - ``z`` -- The value on the superdiagonal.
  15   %
  16   if (integerN < 0)
  17     A = NA;
  18     return;
  19   end
  20
  21   sub_diag = repmat(x, 1, integerN-1);
  22   on_diag = repmat(y, 1, integerN);
  23   super_diag = repmat(z, 1, integerN-1);
  24   A = diag(on_diag) + diag(super_diag, 1) + diag(sub_diag, -1);
  25 end