Re: [Scheme] Few basics...

[Han-Wen Nienhuys]

On Fri, Nov 21, 2008 at 4:46 PM, Michael Käppler <address@hidden> wrote:
> Scheme hackers,
> following simple code extract:
>
> (define testlist (list 0 1 2 3 4 5 6 7))
>
> (define (addlist i nr)
>  (cons (list-ref testlist i) (if (= (+ i 1) nr) '() (addlist (+ i 1) nr)))
> )
>
> (display (addlist 0 3))
>
> The procedure addlist returns all list elements up to the element >before<
> that whose index is given through the second argument.

I think you should look into the srfi-1 library.  Also,

guile> (help list-head)
`list-head' is a primitive procedure in the (guile) module.

c snarfed from list.c:491
@deffn {Scheme Procedure} list-head lst k
Copy the first @var{k} elements from @var{lst} into a new list, and
return it.
@end deffn



Iwould probably write something like

(define (take lst n)
  (define (helper lst acc n)
     (if (or (=? 0 n) (null? lst))
         acc (helper (cdr lst) (cons lst acc) (1- n))))
  (reverse (helper lst '() n)))

-- 
Han-Wen Nienhuys - address@hidden - http://www.xs4all.nl/~hanwen