# [racket] spirit of Racket?

 From: Joe Gilray (jgilray at gmail.com) Date: Sun Mar 11 17:37:56 EDT 2012 Previous message: [racket] spirit of Racket? Next message: [racket] spirit of Racket? Messages sorted by: [date] [thread] [subject] [author]

```Gorgeous!  Thanks Justin.

To speed it up, change the second "101" to "first"

To speed it up even a little more, you could do this, but it not quite as
pretty:

(define (euler4e)
(for*/fold ([greatest 0])
([first (in-range 101 1000)]
[second (in-range first 1000)])
(let ([prod (* first second)])
(if (palindromic? prod)
(max greatest prod)
greatest))))

Just out of curiosity, have you used for/fold with >1 accum?  It seems very
powerful.

thanks again,
-Joe

On Sun, Mar 11, 2012 at 1:33 PM, Justin Zamora <justin at zamora.com> wrote:

> 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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