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

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

get user input – getuser()
get computer choice – getcomp()
determine winner – whowins()

Part #3: Refine algorithm

We can now put this into Python (see rps-1.py):


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", end=" ")
print(score["computer"], "game(s), and you two tied", score["tie"], "game(s)")

Step #3: Figure out who wins

Represent (object_1, object_2) where object_1 beats object_2 as a list of pairs called winlist. To see if user won, check if the user-chosen object is the same as the computer-chosen object; if so, there is no winner. Otherwise, check whether (user-chosen object, computer-chosen object) pair is in winlist; if so, the user wins. Otherwise the user loses.

This leads to rps-2.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-3.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 (usually control-D) or an interrupt (usually 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-4.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.

Step #6: Make it human-friendly

The program now works correctly, but it’s rather unfriendly— the “game(s)” should be “game” or “games” as appropriate, and it should tell the user who wins each round. So we need to add something to the while True loop in the main routine, and change the print statements at the end.

Telling the user who wins is straightforward. Simply put in an if statement at the end of the loop. One tricky point is that there are actually four conditions: winner can take on three known values (“user”, “computer”, and “tie”), and any other unknown value. It should never do the latter, but just in case, we program defensively and put a special case in to catch that. The resulting code is:


if winner == "user":
	print("You win")
elif winner == "computer":
	print("Computer wins")
elif winner == "tie":
	print("Tie")
else:
	print( "*** INTERNAL ERROR *** winner is", winner)
	break

Next, the program should distinguish between 1 “game” and any other number of “games” (note you say “0 games” in English). Again, we use an if statement to handle it. Both the computer’s number of games, the user’s number of game, and the number of tie games have to be handled.


print("You won", end=”)
if score["user"] == 1:
	print("1 game, the computer won",end=’ ’)
else:
	print(score["user"], "games, the computer won", end=’ ’)
    if score["computer"] == 1:
        print("1 game, and you two tied", end=’ ’)
    else:
        print(score["computer"], "games, and you two tied", end=’ ’)
    if score["tie"] == 1:
        print("1 game.")
    else:
        print(score["tie"], "games.")

The resulting program is rps-5.py.

Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: mabishop@ucdavis.edu

ECS 235A, Computer and Information Security
Version of October 29, 2023 at 1:45PM

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