ActiveState Code

Recipe 181064: Using a Dictionary in place of a 'switch' statement

Description: Rock, Scissors, Paper Game. Shows a clean way of implementing a 'switch' statement in Python via a dictionary container. The dictionary is made up of known 'named states' that are tested in sequence for their current 'state'.

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#! /usr/local/bin/python

# /////////////////////////////////////////////////////////////////////////
# /**
# * Title: rochambeau.py
# *
# * Description: Rock, Scissors, Paper Game. 
# *              Shows a clean way of implementing a 'switch'
# *              statement in Python via a dictionary container. 
# *              The dictionary is made up of known 'named states' that
# *              are tested in sequence for their current 'state'.
# *
# * Copyright: Copyright (c) 2003
# *            This file is distributed as EXAMPLE SOURCE CODE ONLY!
# *            The following code is considered 'Freeware' and can be 
# *            freely copied/reused/distributed as needed. 
# *
# * Company: None
# * @author: Alan Haffner
# * @version 1.0
# */
# 
# /////////////////////////////////////////////////////////////////////////
# Date: 02/16/03


import os, sys
import string, random
import types

def cli():

   c = '?'
   while c not in 'rps':

      try:
         print
         # tailing index '[0]' picks only first char from input
         c = raw_input('\tPlease enter (r)ock, (p)aper or (s)cissors to play... ')[0]
      except IndexError:
         # bad input, so like get another...
         pass

      c = c.lower()

      #   x, q...   --> quit
      if c in ('x', 'q' ):
         raise 'USER_QUIT_ERROR'

   return c

if __name__=='__main__':

   errorCode = 0

   stateList = ['r', 'p', 's']

   validStates = { 'User Wins'      : (('p','r'), ('r','s'), ('s','p')),
                   'No One Wins'    : (('p','p'), ('r','r'), ('s','s')),
                   'Computer Wins'  : (('r','p'), ('s','r'), ('p','s')),
   }

   try:
      while 1:
         testTuple     = (None, None)
         userInput     =        None
         computerInput =       '?'

         userInput     = cli()
         computerInput = ( stateList[random.randint(0,2)] )
   
         testTuple = (userInput, computerInput)

         for select in validStates:
            if testTuple in validStates[select]:
               print
               print "You chose:         ", userInput
               print "The computer chose:", computerInput
               print " ****", select, " ****" 
               print

   # Note: By convention, all local exception 'constants' end 
   # in '_ERROR' regaurdless of their intended use. 
   except KeyboardInterrupt:
      print '\n' * 3
      print '[interrupted by user]'
      print '\n' * 3
   except 'USER_QUIT_ERROR':
      print '\n' * 3
      print '[interrupted by user]'
      print '\n' * 3
   except:
      # unexpected error
      print '\n' * 3
      traceback.print_exc()
      print '\n' * 3

      errorCode = 2

   sys.exit(errorCode)

Discussion

Shows a nice clean way of working around Python's lack of a 'switch' or 'case' statement. Dictionaries turn out to be a very clean way of doing what you only thought a true 'switch' statement could do.

Cheers,

Yaipa

Comments

  1. 1. At 11:52 a.m. on 11 may 2003, Ville Tirronen said:

    Doing this Right. This recipe does not in any way use dictionary, just lists. For doing this as it should be, one could do this:

       stateList = ['r', 'p', 's']

#   validStates = { 'User Wins'      : (('p','r'), ('r','s'), ('s','p')),
#                   'No One Wins'    : (('p','p'), ('r','r'), ('s','s')),
#                   'Computer Wins'  : (('r','p'), ('s','r'), ('p','s')),
#   }
# This is not wise. Just the same as using a list.

# The following is better:

# First some handy aliases to save typing.
win = "User wins"
draw ="No one wins"
lose = " Computer wins"
validStates = { ('p','r') : win,
            ('r','s') : win,
            ('s','p'): win,
                        ('p','p'): draw,
            ('r','r'):  draw,
            ('s','s'): draw,
                        ('r','p'): lose,
            ('s','r'): lose,
            ('p','s'): lose
   }

   try:
      while 1:
 #        testTuple     = (None, None)
 #        userInput     =        None
 #        computerInput =       '?'
# These can safely be commented out. Pythons power is conciseness after all

         userInput     = cli()
         computerInput = ( stateList[random.randint(0,2)] )

         testTuple = (userInput, computerInput)

 #        for select in validStates:
 #           if testTuple in validStates[select]:
 #              print
 #              print "You chose:         ", userInput
 #              print "The computer chose:", computerInput
 #              print " ****", select, " ****"
 #              print
# Iterating over a dictionary makes really no sense. After all, it IS a dictionary
# Instead we can:

print validStates[testTuple]

# For the exact same effects.

    Less to type, faster to execute, easier to extend.

  2. 2. At 3:30 p.m. on 17 feb 2004, Runsun Pan said:

    A more concise way.

    import random
userPick=''
while userPick not in ['r', 'p', 's']:
     userPick = raw_input('\tPlease enter (r)ock, (p)aper or (s)cissors to play... ')[0]

computerPick= random.choice(['r','p','s'])
pair = (userPick, computerPick)

result= { pair==('r','r') or pair==('p','p') or pair==('s','s'): 'draw!',
          pair==('p','r') or pair==('s','p') or pair==('r','s'): 'You won!',
          pair==('r','p') or pair==('p','s') or pair==('s','r'): 'Computer won!'}[1]

print 'You entered: ', userPick, ', Computer entered: ', computerPick
print result
  3. 3. At 3:30 p.m. on 17 feb 2004, Runsun Pan said:

    A more concise way.

    import random
userPick=''
while userPick not in ['r', 'p', 's']:
     userPick = raw_input('\tPlease enter (r)ock, (p)aper or (s)cissors to play... ')[0]

computerPick= random.choice(['r','p','s'])
pair = (userPick, computerPick)

result= { pair==('r','r') or pair==('p','p') or pair==('s','s'): 'draw!',
          pair==('p','r') or pair==('s','p') or pair==('r','s'): 'You won!',
          pair==('r','p') or pair==('p','s') or pair==('s','r'): 'Computer won!'}[1]

print 'You entered: ', userPick, ', Computer entered: ', computerPick
print result
  4. 4. At 9:35 p.m. on 25 apr 2005, Josiah Carlson said:

    Your example is more concise than the original poster, but not significantly more concise than Ville Tirronen's.

    Also, your method is slower over the course of long-term execution because the dictionary needs to be computed in every pass. Ville Tirronen's example mechanism is far faster in practice.

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