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Rock, Paper, Scissors Game in Common Lisp

Om Rawal

Jul 16, 2025

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1. Code Analysis

Let’s walk through how the code works, step by step:

a. Setup

(defun rock-paper-scissors ()

(let ((choices '("rock" "paper" "scissors"))

(random-state (make-random-state t)))

I defined a function called rock-paper-scissors.
Inside, I created a list of choices: "rock", "paper", and "scissors".
I also made a random state to generate random numbers.

b. Welcome Message

(format t "~%--- Welcome to the Rock, Paper, Scissors Game! ---~%")

(format t "Instructions: Type 'rock', 'paper', or 'scissors' to play.~%")

(format t "You can also type 'quit' anytime to exit the game.~%~%")

Prints a welcome message and tells the player how to play.

c. The Game Loop

(loop

(format t "Your move: ")

(let ((player-move (string-downcase (read-line))))

...

Starts an infinite loop for continuous gameplay.
Asks the player to type their move.
Converts the input to lowercase so it works even if the player types “Rock” or “ROCK”.

d. Handling “quit”

(if (string= player-move "quit")

(progn

(format t "Thanks for playing! Goodbye.~%")

(return))

If the user types quit, the program says goodbye and exits the loop.

e. Validating Input

(if (not (member player-move choices :test #'string=))

(format t "Oops! Please enter only 'rock', 'paper', or 'scissors'.~%")

Checks if the user typed a valid move.
If not, it prints a warning and loops back to ask again.

f. Computer’s Move

(let* ((computer-move (nth (random 3 random-state) choices)))

(format t "Computer chose: ~A~%" computer-move)

Picks a random move from the list of choices.
Prints out what the computer chose.

g. Determining the Winner

(cond

((string= player-move computer-move)

(format t "It's a tie! We both chose ~A.~%" player-move))

((or (and (string= player-move "rock") (string= computer-move "scissors"))

(and (string= player-move "scissors") (string= computer-move "paper"))

(and (string= player-move "paper") (string= computer-move "rock")))

(format t "You win! ~A beats ~A.~%" player-move computer-move))

(format t "You lose! ~A beats ~A.~%" computer-move player-move))))

Checks for a tie first.
Then checks if the player wins.
Otherwise, prints that the player lost.

2. Output

Here’s what a sample run looks like: