# [racket] spirit of Racket?

 From: Justin Zamora (justin at zamora.com) Date: Sun Mar 11 16:33:14 EDT 2012 Previous message: [racket] spirit of Racket? Next message: [racket] spirit of Racket? Messages sorted by: [date] [thread] [subject] [author]

```This is a great situation to use Racket's advanced list
comprehensions.  There is no need to use set!.  You can keep track of
the maximum as you loop.  Here is my solution.

; Problem 4
; Find the largest palindrome made from the product of two 3-digit numbers.
; 3-digit numbers are the numbers 100-999
(define (pe-004)
(for*/fold ([greatest 0])
([first (in-range 101 1000)]
[second (in-range 101 1000)]
#:when (palindrome? (* first second)))
(max greatest (* first second))))

Justin

On Sun, Mar 11, 2012 at 4:12 PM, Joe Gilray <jgilray at gmail.com> wrote:
> Hi,  I'm redoing the ProjectEuler problems to learn Racket (great fun!).
>  Below are three solutions to PE#4.  Which is most in the spirit of Racket?
>  Of course, I'd love to see solutions that are more idiomatic too.  Is there
> a better way that doesn't use "for" at all?
>
> Thanks!
> -Joe
>
> ; function that turns a positive integer into a list of digits (in reverse
> order)
> ; invoke as (listize 234511) or (set-count (list->set (listize 112342)))
> (define (listize num)
>   (if (= num 0)
>       '()
>       (cons (remainder num 10) (listize (quotient num 10)))))
>
> ; function that returns #t if the passed number is palindromic
> ; invoke as (palindromic? n)
> (define (palindromic? n)
>   (if (equal? (listize n) (reverse (listize n))) #t #f))
> )
>
> ; ProjectEuler problem #4
> (define (euler4a)
>   (let ([max 100000])
>     (for ([i (build-list 899 (lambda (x) (+ 100 x)))])
>       (for ([j (build-list (- 999 i) (lambda (y) (+ i y)))])
>         (let ([prod (* i j)])
>           (when (palindromic? prod)
>             (begin
>               (when (> prod max) (set! max prod))
>               (printf "~a * ~a = ~a~n" i j prod))))))
>     (printf "Max palindromic product is ~a~n" max)))
>
> ; ProjectEuler problem #4 using for*
> (define (euler4b)
>   (let ([max 100000])
>     (for* ([i (build-list 899 (lambda (x) (+ 100 x)))]
>            [j (build-list 899 (lambda (y) (+ 100 y)))]
>            #:when (and (>= j i) (palindromic? (* i j))))
>       (let ([prod (* i j)])
>         (when (> prod max) (set! max prod))
>         (printf "~a * ~a = ~a~n" i j prod)))
>     (printf "Max palindromic product is ~a~n" max)))
>
> ; ProjectEuler problem #4 - a mix of 4a and 4b
> (define (euler4c)
>   (let ([max 100000])
>     (for* ([i (build-list 899 (lambda (x) (+ 100 x)))]
>            [j (build-list (- 999 i) (lambda (y) (+ i y)))]
>            #:when (palindromic? (* i j)))
>       (let ([prod (* i j)])
>         (when (> prod max) (set! max prod))
>         (printf "~a * ~a = ~a~n" i j prod)))
>     (printf "Max palindromic product is ~a~n" max)))
>
> ____________________
>  Racket Users list:
>  http://lists.racket-lang.org/users
>

```

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