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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: palindromic number
Date: Sun, 15 Sep 2024 05:02:46 -0000 (UTC)
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B. Pym wrote:

> > "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))]))


> Let's start with Project Euler Problem 1 : "If we list all the natural
> numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
> The sum of these multiples is 23. Find the sum of all the multiples of
> 3 or 5 below 1000."

Gauche Scheme

(use srfi-1)  ;; lset-union

(fold + 0 (lset-union = (lrange 3 1000 3) (lrange 5 1000 5)))

  ===>
233168