function l = collatz_length ( n )
%% collatz_length.m
%
  l = 1;
  while ( n ~= 1 )
    if ( n % 2 == 0 )
      n = n / 2;
    else
      n = 3 * n + 1;
    end
    l = l + 1;
  end
  return
end
