Re: palindromic number

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))]))

"Each new term in the Fibonacci sequence is generated by adding
the previous two terms. By starting with 1 and 2, the first 10
terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... By
considering the terms in the Fibonacci sequence whose values do
not exceed four million, find the sum of the even-valued terms."

(do ((a 1 b)
     (b 2 (+ a b))
     (s 0 (if (even? a) (+ a s) s)))
  ((> a 4000000)  s))

 ===>
4613732