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   % OUTPUT:
  17   %
  18   %   - ``A`` -- an integerN-by-integerN square matrix.
  19   %
  20   if (integerN < 0)
  21     A = NA;
  22     return;
  23   end
  24
  25   sub_diag = repmat(x, 1, integerN-1);
  26   on_diag = repmat(y, 1, integerN);
  27   super_diag = repmat(z, 1, integerN-1);
  28   A = diag(on_diag) + diag(super_diag, 1) + diag(sub_diag, -1);
  29 end