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Rock Paper Scissors

I had a test for my comp sci class and one of the questions was to make a rock paper scissors game, if winner 1 won, it would return -1, if player won 2, it would return 1. and if it was a draw it would return 0. I did my program and ran and it worked, but according to my professor, he said it wasn't.

def rps(x,y):
    player1 = -1
    player2 = 1
    tie = 0
    'R'>'S'
    'P'>'R'
    'S'>'P'
    if x>y:
        return player1
    if x<y:
        return player2
    else:
        return tie

I don't see what's wrong with that? If you do rps ("R", "P") then it will return -1 because x = player1 and because Rock hits Paper. Can anyone help me find out if my code is wrong?

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5 answers


There are several problems in your code:

1. The following lines do nothing. You cannot set a character R

greater than a character S

:

'R' > 'S'
'P' > 'R'
'S' > 'P'

2. Reason for section 1, yours if x>y:

doesn't do what you think. It just checks if the content is x

before the content y

in the alphabet. (Assume content x

and y

are characters)



Your code is difficult to fix, I would recommend that you approach this problem from a completely different angle.

The solution here is completely different from yours, but it works. It just uses a lot of checks if

to get the result.

def rps(p1, p2):
    if p1 == p2:
        return 0
    elif (p1 == "R" and p2 == "S")\
    or (p1 == "S" and p2 == "P")\
    or (p1 == "P" and p2 == "R"):
        return -1
    else:
        return 1

I replaced x, y

with p1, p2

as they represent the best player options, but if you need to use x

and y

, just change them.

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You can follow these steps:



def rps(p1,p2):
    retval= {
        'R':{'R': 0, 'S':-1, 'P': 1},
        'S':{'R': 1, 'S': 0, 'P':-1},
        'P':{'R':-1, 'S': 1, 'P': 0}
    }
    return retval[p1][p2]
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You asked:

Can someone help me see if my code is wrong?

Yes, it’s wrong. That's why.

If you run rps('R','S')

, you should get 1

it because rock beats paper. Likewise, I rps('R','P')

have to give -1

, because paper hits rock. They both work in your code.

However, if you run rps('S','P')

, you should get 1

because the scissors hit the paper, but you don't - your code returns -1

, which is wrong.

As Emumi pointed out in the comments, three lines

'R'>'S'
'P'>'R'
'S'>'P'

which I assume you think is the defining application to use, is not really doing anything.

+3


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def rps(x,y):
    return [0, -1, 1]['RPS'.index(x) - 'RPS'.index(y)]

Or if you want an interactive program:

from random import randint
['Tie', 'I win', 'You win'][randint(0, 2) - 'RPS'.index(raw_input("Enter R, P, or S: "))]
+1


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def rps(x,y):
    d = {'R': 1, 'S': 2, 'P': 3}
    return ((d[x] - d[y]) + 1) % 3 - 1


for p1 in 'RPS':
    for p2 in 'RPS':
        print p1, p2, rps(p1, p2)

prints

R R 0
R P 1
R S -1
P R -1
P P 0
P S 1
S R 1
S P -1
S S 0
0


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