Why isn't this quicksort working?

#1 08-21-2008, 06:07 PM

frist why does this recurse endlessly?

(define (++ . lists)
(if (> (length lists) 0)
(flatten (cons (car lists) (++ (cdr lists))))
'()))

(define (quicksort lista)
(if (= (length lista) 0)
lista
(let ((pivot (car lista)))
(list (quicksort (filter (lambda (x)(< x pivot)) lista))
(list (pivot))
(quicksort (filter (lambda (x)(>= x pivot)) lista))))))

(quicksort '(22 3 23 12 3 4 2 12 22))
.. . procedure application: expected procedure, given: 2 (no arguments)
>

#2 08-21-2008, 08:26 PM

maestro <notnorwegian@yahoo.se> writes:

> frist why does this recurse endlessly?
>
> (define (++ . lists)
> (if (> (length lists) 0)
> (flatten (cons (car lists) (++ (cdr lists))))
> '()))

Because you always pass one argument to ++.

> (define (quicksort lista)
> (if (= (length lista) 0)
> lista
> (let ((pivot (car lista)))
> (list (quicksort (filter (lambda (x)(< x pivot)) lista))
> (list (pivot))
> (quicksort (filter (lambda (x)(>= x pivot)) lista))))))
>
>
> (quicksort '(22 3 23 12 3 4 2 12 22))
> . . procedure application: expected procedure, given: 2 (no arguments)

You are trying to call your pivot, and since it is 2, and not a
procedure, it cannot be called.

--
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