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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.lisp
Subject: Re: walk through list and add all n'th item
Date: Fri, 30 Aug 2024 17:02:59 -0000 (UTC)
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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)

This is the TXR Lisp interactive listener of TXR 296.
Quit with :quit or Ctrl-D on an empty line. Ctrl-X ? for cheatsheet.
TXR Lisp environments use genuine Saskatchewan sealskin bindings.
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)

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
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