[The Little Schemer]rember

「The Little Schemer」P.33 ～
3. Cons the Magnificent

• magnificent
• 壮大な
• 堂々とした
• 気高い
• すばらしい
• 飛び切り上等

今回のは「すばらしい」かな？わがんね。

• The Second Commandment
  
  • Use cons to build lists.

(rember 'mint '(lamb chops and mint jelly)) ;; => (lamb chops and jelly)

• Rember
  
  • stands for remove a member.

(rember 'mint '(lamb chops and mint flavored mint jelly) ;; => (lamb shops and flavored mint jelly)

• 最初のmintだけなくなってる

(rember 'toast '(bacon lettuce and tomato)) ;; => (bacon lettuce and tomato)

(rember 'cup '(coffee  cup tea cup and hick cup)) ;; => (coffee  tea cup and hick cup)

• なくなったのは最初のcupだけ

自前のrember

(define rember   (lambda (a lat)     (cond ((null? lat) lat)           ((eq? (car lat) a) (cdr lat))           (else (cons (car lat) (rember a (cdr lat)))))))

(rember 'a '(a b c d)) ;; => (b c d) (rember 'b '(a b c d)) ;; => (a c d) (rember 'a '(a b c d a b c d)) ;; => (b c d a b c d)

回答１：不完全なrember

(define rember   (lambda (a lat)     (cond      ((null? lat) (quote ()))      (else (cond             ((eq? (car lat) a) (cdr lat))             (else (rember a (cdr lat))))))))

(rember 'bacon '(bacon lettuce and tomato)) ;; => (lettuce and tomato) (rember 'and '(bacon lettuce and tomato)) ;; => (tomato)

• What did we do wrong?
  
  • We dropped "and", but we also lost all the atoms preceding "and".
• How can we keep from losing the atoms "bacon" and "lettuce".
  
  • We use Cons the Magnificent. Remember cons, from chapter 1 ?

回答２

(define rember   (lambda (a lat)     (cond      ((null? lat) (quote ()))      (else (cond             ((eq? (car lat) a) (cdr lat))             (else (cons (car lat)                         (rember a (cdr lat)))))))))

回答３：少し短くなってる

(define rember   (lambda (a lat)     (cond      ((null? lat) (quote ()))      ((eq? (car lat) a) (cdr lat))      (else (cons (car lat)                  (rember a (cdr lat)))))))

再度、自前

(define rember   (lambda (a lat)     (cond ((null? lat) lat)           ((eq? (car lat) a) (cdr lat))           (else (cons (car lat) (rember a (cdr lat)))))))

改行位置と、最初のnull?の戻り値が違う。

(rember 'sauce '(soy souce and tomato sauce) ;; => (soy and tomato sauce)