--- title: Ninety-Nine Lisp Problems – P09 の解答 author: kazu634 date: 2010-11-22 url: /2010/11/22/_1628/ wordtwit_post_info: - 'O:8:"stdClass":13:{s:6:"manual";b:0;s:11:"tweet_times";i:1;s:5:"delay";i:0;s:7:"enabled";i:1;s:10:"separation";s:2:"60";s:7:"version";s:3:"3.7";s:14:"tweet_template";b:0;s:6:"status";i:2;s:6:"result";a:0:{}s:13:"tweet_counter";i:2;s:13:"tweet_log_ids";a:1:{i:0;i:5387;}s:9:"hash_tags";a:0:{}s:8:"accounts";a:1:{i:0;s:7:"kazu634";}}' categories: - gauche - Lisp ---

L-99: Ninety-Nine Lisp Problemsの問題を解答してみたよ。なんか、エレガントではないなぁ:

;; P09 (**) Pack consecutive duplicates of list elements into sublists.
;;     If a list contains repeated elements they should be placed in separate sublists.
;;
;;     Example:
;;     * (pack '(a a a a b c c a a d e e e e))
;;     ((A A A A) (B) (C C) (A A) (D) (E E E E))
(define (pack-seed front sublist result l)
(cond
[(null? l) (append result (list sublist))]
[(eq? front (car l)) (pack-seed front
(append sublist
(list front))
result
(cdr l))]
[else (pack-seed (car l)
(list (car l))
(if (null? result)
(if (null? sublist)
'()
(append result (list sublist)))
(append result (list sublist)))
(cdr l))]))
(define (pack l)
(pack-seed #f '() '() l))
(pack '(a a a a b c c a a d e e e e))