[The Little Schemer]member?

「The Little Schemer」P.22 ～

 

• The First Commandment
  • (preliminary)
  • Always ask null? as the first question in expressing any function.
  • 関数を評価する際は、null?手続きが常に初めの問いとなるでしょう。

 

(member? 'tea '(coffee tea or milk)) ;; => #t

• because, one of the atoms of the '(coffee tea or milk), is the same as the atom tea.
• なぜなら、'(coffee tea or milk)のatomの一つと、atomであるteaは同じであるから。

 

(member? 'poached '(fried eggs and scrambled eggs)) ;; => #f

 

自前

(define member?   (lambda (a lat)     (cond ((null? lat) #f)                ((eq? (car lat) a) #t)                (else (member? a (cdr lat))))))

 

member? ;; =>  #<closure member?>   (member? 'tea '(coffee tea or milk)) ;; => #t   (member? 't '(coffee tea or milk)) ;; => #f   (member? 'meat '(mashed potatoses and meat gravy)) ;; => #t

 

回答

(define member?   (lambda (a lat)     (cond      ((null? lat) #f)      (else (or (eq? (car lat) a)                (member? a (cdr lat)))))))

 

(member? 'tea '(coffee tea or milk)) ;; => #t   (member? 'coke '(cofee tea or milk)) ;; => #f   (member? 'meat '(mashed potatoses and meat gravy)) ;; => #t

 

 

• cond
  • else
    • Is else really a question?
    • Yes, else is a question whoose value is always true.

 

 

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