Rock-Paper-Scissor

-- Step #1: Goal and General Algorithm Idea

Goal: write a game to play “rock, paper, scissors” The user chooses one of these, the computer chooses the other

• If the pair is “rock, paper”, the paper wins
• If the pair is “scissors, paper”, the scissors wins
• If the pair is “scissors, rock”, the rock wins Specification: user enters selection of rock, paper, scissors Program prints computer’s selection, who wins At end, computer prints number of games human won and it won

High-level design:

initialize score
loop
    ask user for choice
    if quit, exit loop
    computer selects one
    select winner and increment win count
endloop
print number of games user won, computer won, ties

-- Step #2: Data Representation and Program Structure

Part #1: Data

Represent the rock, paper, scissors using strings: “rock”, “paper”, “scissors” (sequence things) Represent commands as strings as above, plus “quit” (sequence cmdlist) Store the scores in a dictionary with keys “user”, “computer”, “tie” and integer values (initially set to 0)

Part #2: Functions

1. get user input – getuser()
2. get computer choice – getcomp()
3. determine winner – whowins()

Part #3: Refine algorithm

while True:
    userchoice = getuser();
    if (userchoice == quit):
        break
    compchoice = getcomp();
    winner = whowins(userchoice, compchoice)
    score[winner] += 1
print You won, score[“user”], game(s), the computer won, score[“computer”], game(s)
print and you tied, score[“tie”], game(s)

-- Step #3: Figure out who wins

Represent (object1, object2) where object1 beats object2 as list of tuples, winlist. To see if user won, see if the (user-chosen object, computer-chosen object) tuple is in that list.

def whowins(user, comp):
    if user == comp:
        win = "tie"
    elif (user, comp) in winlist:
        win = "user"
    else:
        win = "computer"
    return win

-- Step #4: Get computer choice

Given the three objects in the sequence things, choose randomly.

def getcomp():
    pick = random.choice(things)
    print("Computer picks", pick)
    return pick

-- Step #5: Get user input

Loop until you get a valid input. If the user types an end of file (control-d) or an interrupt (control-c), act as though the user typed “quit”; report any other exceptions and then act as though the user typed “quit”.

def getuser():
    while True:
        try:
            n = input("Human: enter rock, paper, scissors, quit: ")
        except (EOFError, KeyboardInterrupt):
            n = "quit"
            break
        except Exception as msg:
            print("Unknown exception:", msg, "-- quitting")
            n = "quit"
            break
        *** check input ***
    return n

To check input, we need to be sure it’s a valid command, so see if it’s in cmdlist:

    if n not in cmdlist:
        print("Bad input; try again")
    else:
        break

Put these together to get the user input routine.