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

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
    ask user for choice
    if quit, exit loop
    computer selects one
    select winner and increment win count
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

Part #3: Refine algorithm

while True:
    userchoice = getuser();
    if (userchoice == quit):
    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:

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

Step #4: Get computer choice

Given the three objects in the sequence things, choose randomly. This leads to:

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

Step #5: Get user input

Loop until you get a valid input. On end of file, act as though the user typed “quit”; report any other exceptions. This leads to:

def getuser():
    while True:
            n = input ("Human: enter rock, paper, scissors, quit: ")
        except EOFError:
            n = "quit"
        except Exception as msg:
            print ("Exception:", msg)
        *** 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")

Put these together to get the user input routine.

A PDF version is available here.
ECS 10, Basic Concepts of Computing
Winter Quarter 2012