From Newsgroup: comp.lang.lisp
B. Pym 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)))
Shorter:
(use srfi-42) ;; list-ec
(use gauche.sequence) ;; subseq
(define (precedes obj vec)
(list-ec (:parallel (: b (subseq vec 1)) (: a vec))
(if (equal? b obj))
a))
(precedes '! #(! 0 ! 3 4 !))
===>
(0 4)
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