[racket] a small programming exercise

Here's a solution in ISL:

;; benson : [listof positive-number] -> [listof (list number number)]
(define (benson a-lon)
  (let ([digs (map (compose
                    string->number first explode number->string
exact->inexact) a-lon)])
    (build-list 10
                (λ(i) (list i
                            (/ (length (filter (λ(d) (= i d)) digs))
                               (length digs)))))))


;; TESTS (matthias' and jay's, modified)

(require racket)   ; (only-in ... for/list)

(check-expect (filter (lambda (p) (not (zero? (second p))))
                      (benson '(123 124 125 126 23 24 31)))
              (list (list 1 4/7) (list 2 2/7) (list 3 1/7)))

(check-expect
 (benson (list 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.10
            11 12 13 14 15 16 17 18 19
            21 22 23 24 25 26 27 28
            31 32 33 34 35 36 37
            41 42 43 44 45 46
            51 52 54 54 55
            61 62 63 64
            71 72 73
            81 82
            91))
 (for/list ([i (in-range 10)])
   (list i
         (/ (- 10 i)
            (+ 10 9 8 7 6 5 4 3 2 1))))
 )




On Thu, Oct 14, 2010 at 11:30 AM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>
>
> I love this solution but let me supply a solution in ASL, the teaching language:
>
>
> ;; ASL
> (require racket) ; (only-in ... hash->list)
> ;; I could have used the key for sort to get around the second lambda
>
> ;; [Listof String] -> [Listof (cons Digit Nat)]
> ;; compute a frequency count of the leading digit in the number of lon
> ;; assume: the digits are positive
> (define (collect lon)
>  (local ((define loch (map (compose string->list number->string) lon))
>          (define (upd c H) (hash-set H c (+ (hash-ref H c 0) 1)))
>          (define hash (foldl (lambda (x H) (upd (first x) H)) #hash() loch))
>          (define loh# (hash->list hash))
>          (define srtd (sort loh# (lambda (l r) (> (cdr l) (cdr r))))))
>    (map (lambda (x) (cons (string->number (string (car x))) (cdr x))) srtd)))
>
> (check-expect (collect '(123 124 125 126 23 24 31))
>              (list (cons 1 4) (cons 2 2) (cons 3 1)))
>
>
> If actual I/O is required, I'd use batch-io to read CSV files and
> display the list above in a batch action.
> _________________________________________________
>  For list-related administrative tasks:
>  http://lists.racket-lang.org/listinfo/users
>



-- 
Nadeem Abdul Hamid
Associate Professor, Computer Science
Berry College
PO Box 5014
2277 Martha Berry Hwy NW
Mount Berry, GA 30149-5014
(706) 368-5632
http://cs.berry.edu/~nhamid/