Re: Another code review perhaps?

On 2024-09-12, B. Pym <Nobody447095@here-nor-there.org> wrote:

Arthur Lemmens wrote:
>

Define iterative and recursive versions of a function that takes an
object x and a vector v, and returns a list of all the objects that
immediately precede x in v.

 
Graham doesn't like LOOP, but I do. So here's a LOOP-version:
 
(defun precedes (object vector)
  (loop for x across vector
        and i from 0
        when (and (equal x object) (> i 0))
        collect (elt vector (1-i))))

>
>
Look at that:
>
(1-i)
>
Don't you think that that should be:
>
(1- i)
>
or
>
(- i 1)
>
?
>
It's shorter when you use a Lispy language instead of CL.
>
Gauche Scheme
>
(use srfi-42) ; list-ec
>
(define (precedes obj vec)
  (list-ec (: x (index i) vec)
    (if (and (> i 0) (equal? x obj)))
    (ref vec (- i 1))))
>
(precedes 5 #(5 0 4 5 8 9 5))
 ===>
(4 9)
>
Another way:
>
(define (precedes o v)
  (let ((l (vector->list vec)))
    (filter-map
      (^(a b) (and (equal? o a) b))
      (cdr l)
      l)))

1> (match @(scan-all (@x 5 . @nil)) '(5 0 4 5 8 9 5) x)
(4 9)
2> (window-mappend 1 nil (do if (and @1 (= @2 5)) (list @1)) #(5 0 4 5 8 9 5))
#(4 9)


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