Rock-Paper-Scissors¶

In this chapter you will:¶

area	topic
🚀	write a rock-paper-scissors game
⚙	use the conditional statement `if`
⚙	use comparison operators
💡	use the data type `bool`
🔀	use a state variable
🐞	recognize indentation errors

An important structural element in program is making decisions. In Python, the instruction if decides which instruction to execute next.

Exercise 1: Decision¶

With the instruction if, you can make a decision inside your program. Test the following program with different inputs:

player = input("Please enter R, P or S (for [R]ock, [P]aper and [S]cissors) ")
computer = "P"

if player == computer:
    print("it's a draw")

Exercise 2: Alternative decisions¶

Insert the words elif, else and if into the gaps in the code so that it runs:

import random

player = input("Please enter R, P or S (for [R]ock, [P]aper and [S]cissors) ")
computer = random.choice('RPS')

___ player == 'R' and computer == 'P':
    print("Computer wins")
___ player == 'R' and computer == 'S':
    print("Player wins")
___:
    print("it's a draw")

Exercise 3: Paper¶

Extend the program, so that it also works if the player chooses paper.

Exercise 4: Debugging¶

Fix one bug in each if-statement:

elif player.upper() not in 'RPS':
    print('Invalid input. Please enter R,P or S.')

elif player == computer
    print('You chose the same as I did')

if player = 'S':
    print('You chose "scissors".')

else:
print('You chose something else than "scissors"')

Exercise 5: Expressions¶

Which comparisons in the following if statements result in True:

a = 3
b = 4
c = 7

if a + b < c:
    print(True)

if a + b == 5 + 2:
    print(True)

if a * b == 12 and b * c == 28:
    print(True)

if a + b * c >= 28:
    print(True)

if a + b == "7":
    print(True)

Exercise 6: State variables¶

The following program saves the result of a comparison in a variable of the data type bool. Complete the code:

player_wins = (
               (player == "R" and computer == "S") or
               (player == "P" and ___) or
               (___)
              )

if player_wins:
    print('You won!')

Exercise 7: Nested if statements¶

Complete the program, so that it covers all possible situations:

winner = 'draw'

if player == "S":
    if computer == "P":
        winner = "player"
    elif computer == "T":
        winner = "computer"

elif player == "P":
    ___

print("The winner is:", winner)

Hint

A nested if is an if inside another if block.

Exercise 8: Rock-Paper-Scissors¶

Complete the Rock-Paper-Scissors game.

Optional goals:¶

take draws into account as a possibility
inputs should be valid in upper and lower case
use a single if..elif..else block
extend the game by lizard and Spock
use bool variables, so that only one or two if statements without elif or else) remain

Reflection Questions¶

in which order do the parts of an if statement have to be?
which parts of an if statement are optional?
what is indentation?
which comparison operators do you know so far?