A quick conversation with ChatGPT

[robve]

On the subject of RPN/Forth, here is an interesting conversation (not mine):

ChatGPT and Forth

Question

write a prime test program. The program should take a number from the stack and return 1 on the stack if it is prime and 0 if it is not. The program should be written in Forth.

ChatGPT

Here is a simple prime test program in Forth:
sql: prime? ( n -- f )
    2 dup >r
    begin dup * dup >r
        dup 2 < over 0= until
        r> drop
    r> swap 2 <
    if drop 0 exit then
    1 ;

Explanation:

• The : declares a new word named prime?.
• ( n -- f ) is the stack effect comment, which indicates that the word takes one number n from the stack and returns one number f on the stack.
• 2 dup >r duplicates the input number n and stores it on the return stack.
• begin starts a loop that checks if n is divisible by any number between 2 and the square root of n.
• dup * dup >r calculates the square of the current divisor and stores it on the return stack.
• dup 2 < over 0= compares the current divisor with 2 and checks if it is divisible by n.
• until repeats the loop as long as the current divisor is less than the square root of n and is not a divisor of n.
• r> drop discards the square of the current divisor from the return stack.
• r> swap 2 < checks if the original input number n is less than 2.
• if drop 0 exit then if n is less than 2, it is not prime, so 0 is returned.
• 1 ; If n is prime, 1 is returned.
  

Note that this is a simple implementation and there are other more efficient ways to test for prime numbers.

The full conversation is on reddit: https://www.reddit.com/r/Forth/comments/...and_forth/

Not much of a threat to Forth programmers

- Rob