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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: palindromic number
Date: Sat, 14 Sep 2024 23:18:36 -0000 (UTC)
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> "A palindromic number reads the same both ways. The largest
> palindrome made from the product of two 2-digit numbers is 9009
> = 91 x 99. Find the largest palindrome made from the product of
> two 3-digit numbers."


Gauche Scheme

(use srfi-13)  ;; string-reverse

(define (divisor? n m) (= 0 (mod m n)))

"We don't need no stinkin' loops!"

(define (prod-of-3-dig-nums? n)
  (let1 sq (exact-integer-sqrt n)
    (any (is divisor? n) (lrange sq 999))))

(define (good? n)
  (let1 s (number->string n)
    (and (equal? s (string-reverse s))
         (prod-of-3-dig-nums? n))))

(find good? (lrange 998001 0 -1))
  ===>
906609

Given:

(define-syntax is
  (syntax-rules ()
    [(is x)
     (lambda (y) (equal? y x))]
    [(is compare x)
     (lambda (y) (compare y x))]
    [(is key compare x)
     (lambda (y) (compare (key y) x))]))