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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: walk through list and add all n'th item
Date: Fri, 30 Aug 2024 16:42:30 -0000 (UTC)
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> > I'm just wondering if there is a lispier way to scan once through a
> > list and add each n'th item with n+constant.
> > Like kind of apply a list through a vector..
> > In my approach i  just used the loop macro:
> > (defparameter vals '(1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1
> > 2 3 4 5 6 7 8))
> ...
> > CL-USER> (sum-values vals)
> > (4 8 12 16 20 24 28 32)
> >
> > Can you do better? (i hope you do and I am prepared to bear the shame)
> 
> I see that you ask to "scan once" but still feel awfully
> tempted to reblock the data
> 
> CL-USER> (defun batch (list size)
>            (if (endp list)
>                '()
>                (cons (subseq list 0 size)
>                      (batch (nthcdr size list) size))))
> BATCH
> 
> CL-USER> (batch vals 8)
> 
> ((1 2 3 4 5 6 7 8) (1 2 3 4 5 6 7 8) (1 2 3 4 5 6 7 8) (1 2 3 4 5 6 7 8))
> 
> CL-USER> (defun add (list)
>            (reduce (lambda(u v)(mapcar #'+ u v)) list))
> ADD
> 
> CL-USER> (add (batch vals 8))
> (4 8 12 16 20 24 28 32)

Mark Wooding wrote:

> Or you could just do it the easy way:
> 
> (defun sum-every-softcore (period list)
>   (loop with sums = (make-array period :initial-element 0)
>         for item in list
>         for i = 0 then (mod (1+ i) period)
>         do (incf (aref sums i) item)
>         finally (return (coerce sums 'list))))

Gauche Scheme

(use srfi-42)  ;; do-ec

(define (sum-every-nth n nlist)
  (rlet1 result (make-vector n 0)
    (do-ec (:list x (index i) nlist)
      (inc! (vector-ref result (mod i n)) x))))

(define vals '(1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1
   2 3 4 5 6 7 8))

(sum-every-nth 8 vals)

#(4 8 12 16 20 24 28 32)