From Newsgroup: comp.lang.lisp

B. Pym 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)

(define (sum-every-nth n nlist)
(let ((result (make-vector n 0))
(i (gmki 0 1 n)))
(dolist (x nlist) (inc! (~ result (i)) x))
result))

(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)

Given:

;; Make an index generator.
(define (gmki :optional (start 0) (step #f) (m #f))
(let ((i start)
(s step)
(m m))
(if m
(lambda () (begin0 (mod i m) (inc! i s)))
(if s
(lambda () (begin0 i (inc! i s)))
(lambda () (begin0 i (inc! i)))))))
--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2