# 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

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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Version of February 23, 2019 at 2:37PM