Re: walk through list and add all n'th item

On 2024-08-30, B. Pym <Nobody447095@here-nor-there.org> wrote:

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)

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1> (flow '(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)
     (tuples 8)
     transpose
     (mapcar sum))
(4 8 12 16 20 24 28 32)

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