[plt-scheme] List comprehension style programming

This one-liner is what an HtDP student with full knowledge of ISL  
would write:

;; myfunc ::  list of list of numbers -> number
;; returns the number of numbers in the list which has the most non- 
zero numbers

(check-expect (myfunc '((0 0 1 0) (1 2 0 1) (0 1 1 0))) 3)
(check-expect (myfunc '((0 0 1 0) (1 2 0 1) (0 1 1 0 8 2))) 4)

(define (myfunc ll)
   (apply max (map (lambda (l) (length (filter (compose not zero?)  
l))) ll)))

-- Matthias





On May 7, 2009, at 6:10 PM, Sam TH wrote:

> On Thu, May 7, 2009 at 6:00 PM, wooks <wookiz at hotmail.com> wrote:
>> General question. How deep should you layer invocations of HOF's.
>
> I would write this with the `for' set of list comprehensions/loops.
> Here's your example, with no extra traversals:
>
> (define (f l)
>   (for/fold ([k 0])
>     ([e l])
>     (max k
>          (for/fold ([k 0]) ([i e] #:when (not (zero? i))) (add1 k)))))
>
> (f '((0 0 1 0) (1 2 0 1) (0 1 1 0)))
> (f '((0 0 1 0) (1 2 0 1) (0 1 1 0 8 2)))
>
> -- 
> sam th
> samth at ccs.neu.edu
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