Re: Which is the best way to collect lists?

[Matt Wette]

Guile queues will do what you want.

On Aug 10, 2013, at 5:45 AM, 康桥 wrote:

> We often process a list into another one in LISP.
> For example:
> (define (filter-a l)
>   (if (null? l) '()
>       (let ((n (car l))
>         (r (filter-a (cdr l))))
>     (if (> n 10) r (cons n r)))))
> The procedure above can choose the items in 'l' which is
> less than or equal ten, then put them in a new list,
> and return the new list.
> Lists in LISP are very simple structures and they have a
> big disadvantage - it's not easy to append a new element
> to the tail of the list.Some programmers would push them
> into the top of the list, then reverse the list to get
> the final result.But I like recursing very much, so I
> usually use recursion to collect the result list.
> But in my opinion, both of the two ways are not good.
> I think my way is wasting stack - local variables use 
> stack, but only the last pair of the list being
> collected is useful(we use it to connect to next pair).
> To reverse the list after collecting it is wasting time
> - why not collect it at once but do a twice processing?
> I have invented a way to solve this problem.My algorithm
> is similar to the implement of queue in SLIB.
> Here is my new way to do filtering:
> (define (filter-a l)
>   (letrec ((result '())
>        (last-pair '())
>        (grow (lambda (x)
>            (let ((tmp (list x)))
>              (set! result tmp)
>              (set! last-pair tmp))
>            (set! grow (lambda (x)
>                 (let ((tmp (list x)))
>                   (set-cdr! last-pair tmp)
>                   (set! last-pair tmp)))))))
>     (let growing ()
>       (if (null? l) result
>       (let ((first (car l)))
>         (if (<= first 10)
>         (grow first))
>         (set! l (cdr l))
>         (growing)))))) 
> It's a little complex, but it can save a lot of space
> especially when collecting much data.
> But maybe my way could slow the collecting, because
> to make a closure need some time.
> Which way is good?
> 
> I am a schoolboy from China.I have a lot of friends
> who would say "Microsoft has monopolized all." while
> using Dev-C++.They would prefer Python or C# rather 
> than LISP or other old languages.So I need your help.
> 
> Sorry for some mistakes in my mail due to my terrible
> English.
>