Path: ...!news.nobody.at!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: The "Strand" puzzle --- ( Continued Fractions using Lisp orPython? )
Date: Sat, 3 Aug 2024 02:39:52 -0000 (UTC)
Organization: A noiseless patient Spider
Lines: 68
Message-ID: <v8k59k$382b5$1@dont-email.me>
References: <v7u7qd$2dgbs$1@dont-email.me> <6f90c2b4abed28c153dea46de3af408d@www.novabbs.com> <v88ood$jnp6$1@dont-email.me> <v8fkps$23nr5$1@dont-email.me> <v8ikg2$2qrts$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Injection-Date: Sat, 03 Aug 2024 04:39:52 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="a358947f45c9b99589af96dcca686d10";
	logging-data="3410277"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX199gXi9Br2gbfjCcexf/NKF"
User-Agent: XanaNews/1.18.1.6
Cancel-Lock: sha1:Zt7P8ANA+N4CnCChTJXH32k9O1c=
Bytes: 2858

B. Pym wrote:

> B. Pym wrote:
> 
> > > > > e.g. -------- For the (street)  Numbers (1,2,3,4,5,6,7,8)
> > > > > 
> > > > >        (1,2,3,4,5)  and  (7,8)  both add up to 15.
> > > > > 
> > > > > 
> > > > > 
> > > > > "In a given street of houses with consecutive numbers between
> > > > > 50 and 500, find the house number, for which, the sum of
> > > > > numbers on the left is equal to the sum of numbers on the
> > > > > right"
> > 
> > Gauche Scheme
> > 
> > (define (strand lst)
> >   (let go ((left-sum 0) (tail lst))
> >     (if (null? tail)
> >       #f
> >       (let ((right-sum (fold + 0 (cdr tail))))
> >         (cond ((< left-sum right-sum)
> >                (go (+ left-sum (car tail)) (cdr tail)))
> >               ((= left-sum right-sum) (car tail))
> >               (#t #f))))))
> > 
> > (strand '(1 2 3 4 5 6 7 8))
> >   ===>
> > 6
> > 
> > (lrange 2 5)
> >   ===>
> > (2 3 4)
> > 
> > (any
> >   (lambda (n)
> >     (if (strand (lrange 50 n))
> >       n
> >       #f))
> >   (lrange 500 50 -1))
> >   ===>
> > 352
> > 
> > (strand (lrange 50 352))
> >  ===>
> > 251
> 
> 
> Faster:
> 
> (define (strand lst)
>   (let go ((left-sum 0) (right-sum (fold + 0 (cdr lst))) (tail lst))
>     (cond ((< left-sum right-sum)
>            (go (+ left-sum (car tail))
>                (- right-sum (cadr tail))
>                (cdr tail)))
>           ((= left-sum right-sum) (car tail))
>          

Using "do":

(define (strand lst)
  (do ((tail lst (cdr tail))
       (left-sum 0 (+ left-sum (car tail)))
       (right-sum (fold + 0 (cdr lst)) (- right-sum (cadr tail))))
    ((>= left-sum right-sum)
     (if (= left-sum right-sum) (car tail) #f))))