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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