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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: map and reduce
Date: Fri, 19 Jul 2024 15:48:02 -0000 (UTC)
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> > Consider n vectors (or lists) v1, ..., vn of equal length, an n-
> > variate function f, and a bivariate function g.
> >
> > I want to calculate (reduce g (map 'vector f v1 v2 ... vn)), eg
> >
> > (reduce #'+ (map 'vector #'* '(1 2 3) '(4 5 6)))  ; => 32
> >
> > but without the intermediate vector (my vectors are long).  I can of
> > course write a function to do this, but I am wondering if there is a
> > clever way to do it with CL library functions.
> 
> I don't think there is a way to do this sort of thing directly.  It's
> not difficult (or even particularly ugly) to do using LOOP, though.  For
> the case G = #'+, it's particularly nice:
> 
>   (loop for x across first-vector
>         for y across second-vector
>         sum (funcall f x y))

That cannot handle any number of vectors.


> 
> but for general G, you need
> 
>   (loop for x across first-vector
>         for y across second-vector
>         for temp = (funcall f x y)
>         for result = temp then (funcall result temp)

Wrong. Try (funcall G result temp)

>         finally (return result))

Gauche Scheme

(use srfi-43) ;; vector ops.

(define (vec-map-reduce red-func seed map-func . vectors)
  (apply vector-fold
    (lambda (i accum . elements)
      (red-func accum (reduce map-func #f elements)))
    seed
    vectors))

(vec-map-reduce + 0 *
  #(9 2 3 4)
  #(5 6 7 8)
  #(20 22 23 24))

  ===>
2415