[racket] a small programming exercise

This works, assuming xs consists of positive integers:

(define (benford xs)
  (for-each
    (lambda (x) (printf "~a ~f~%" (car x) (/ (cdr x) (length xs))))
    (uniq-c = (sort < (map (compose car digits) xs)))))

Compose, digits and uniq-c are from my Standard
Prelude<http://programmingpraxis.com/standard-prelude>
.

Phil

On Wed, Oct 13, 2010 at 11:10 PM, Shriram Krishnamurthi <sk at cs.brown.edu>wrote:

> Given: A list of numbers.  Assume integers (though it isn't necessary
> for the underlying issue).
>
> Task: To determine how often the first digit is 1, the first digit is
> 2, ..., the first digit is 9, and present as a table.  You may exploit
> DrRacket's pretty-printer, e.g., by using a list of lists:
>
> '((#\1 22.51539138082674)
>  (#\2 16.44678979771328)
>  (#\3 15.567282321899736)
>  (#\4 12.401055408970976)
>  (#\5 9.058927000879507)
>  (#\6 7.651715039577836)
>  (#\7 6.420404573438875)
>  (#\8 5.804749340369393)
>  (#\9 4.133685136323659))
>
> I leave the precise format of the output unstated so you're free to
> choose a clever representation; your answer should be at least as
> visually clear as the above.
>
> You should not mutate the original list, since it may be necessary for
> other computations.
>
> Smallest/tightest/cleanest/best?
>
> Shriram
>
> PS: Of course, this is to explore Benford's Law:
>
>    http://en.wikipedia.org/wiki/Benford's_law
>
>    which is the subject of my lecture tomorrow.  The above
>    distribution is from the size of the littoral zone (in acres) of
>    the lakes of Minnesota.
>
> PPS: If you really want to you can assume the data are actually in a
>     CSV file (as mine are -- thanks, Neil!), in case you can come up
>     with something even cleverer.
> _________________________________________________
>  For list-related administrative tasks:
>  http://lists.racket-lang.org/listinfo/users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20101014/521054b5/attachment.html>