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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: rec.puzzles
Subject: Re: The "Strand" puzzle --- ( Continued Fractions using Lisp orPython? )
Date: Fri, 2 Aug 2024 12:06:09 -0000 (UTC)
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B. Pym wrote:

> HenHanna 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))
>

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))
          (else  #f))))