def collatz_dict ( n, C ):

## collatz_dict() adds the Collatz sequence for n to an existing dict.
#
  from collatz import collatz

  if ( not C ):
    C = {}

  while ( not ( n in C ) ):
    Tn = collatz ( n )
    C[n] = Tn
    n = Tn
#
#  Doing this extra step makes the dictionary appear sorted in the 
#  first item.
#
  C = dict ( sorted ( C.items(), key = lambda item : item[0] ) ) 

  return C

def collatz_dict_test ( ):

  C = {}

  n = 341
  C = collatz_dict ( n, C )
  print ( '  Start with n = 341' )
  print ( C )

  n = 84
  C = collatz_dict ( n, C )
  print ( '' )
  print ( '  Add n = 84' )
  print ( C )

  return

if ( __name__ == "__main__" ):
  collatz_dict_test ( )