[racket] a small programming exercise

Oh yeah, duh.


On Thu, Oct 14, 2010 at 12:48 PM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>
> Because (require racket) provides everything you'd ever want.
>
>
> On Oct 14, 2010, at 12:39 PM, Nadeem Abdul Hamid wrote:
>
>>> Thanks for setting my head straight!
>>
>>
>> Oh, I wasn't meaning to do that! :)
>>
>> Incidentally, why does your solution run even when the language level
>> is set to "Intermediate Student"? In fact, even mine runs in ISL while
>> I use lambda. I'm running DrRacket, version 5.0.1.7--2010-10-06.
>>
>> --- nadeem
>>
>>
>> On Thu, Oct 14, 2010 at 12:29 PM, Matthias Felleisen
>> <matthias at ccs.neu.edu> wrote:
>>>
>>> I ran this stress test for your 5 line solution and mine:
>>>
>>>> (define *the-list* (build-list 10000000 (lambda (i) (random (+ i 1)))))
>>>> (time (______ *the-list*))
>>>
>>> For small benchmarks, your solution may take twice as long as my 8 line ASL solution,
>>> but for something of the above size -- there can't be that many lakes in MN -- it takes
>>> almost the same time.
>>>
>>> In short, don't be afraid of 10 iterations over long lists, and don't bother with
>>> hash tables and fancy algorithmics until you know you need it -- by measuring.
>>>
>>> I used to believe this but have temporarily forgotten this lesson due to external influences.
>>>
>>> -- Matthias
>>>
>>>
>>>
>>>
>>> On Oct 14, 2010, at 12:09 PM, Nadeem Abdul Hamid wrote:
>>>
>>>> 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
>>>>>
>>>>
>>>>
>
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