Top-Down Programming Example: Rock, Paper, Scissors

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

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

• get user input – getuser()
• get computer choice – getcomp()
• 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.

This leads to rps-prog1.py:

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.

This leads to rps-prog2.py:

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”.

This leads to rps-prog3.py:

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.

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