sicp每日一题[2.53-2.55]

2.53不用写代码，2.54和2.55属于一道题，所以就放到一起吧

Exercise2.53

What would the interpreter print in response to evaluating each of the following expressions?

(list 'a 'b 'c)
(list (list 'george))
(cdr '((x1 x2) (y1 y2)))
(cadr '((x1 x2) (y1 y2)))
(pair? (car '(a short list)))
(memq 'red '((red shoes) (blue socks)))
(memq 'red '(red shoes blue socks))

---

结果如下：

'(a b c)
'((george))
'((y1 y2))
'(y1 y2)
#f
#f
'(red shoes blue socks)

Exercise 2.54

Two lists are said to be equal? if they contain equal elements arranged in the same order. For example,

(equal? '(this is a list) '(this is a list))

is true, but

(equal? '(this is a list) '(this (is a) list))

is false. To be more precise, we can define equal? recursively in terms of the basic eq? equality of symbols by saying that a and b are equal? if they are both symbols and
the symbols are eq?, or if they are both lists such that (car a) is equal? to (car b) and (cdr a) is equal? to (cdr b). Using this idea, implement equal? as a procedure.

---

这道题挺简单的，依次比较就行了。

(define (equal? a b)
  (cond ((and (null? a) (null? b)) true)
        ((or (null? a) (null? b)) false)
        ((eq? (car a) (car b)) (equal? (cdr a) (cdr b)))
        (else false)))


(equal? '(this is a list) '(this is a list))
(equal? '(this is a list) '(this (is a) list))

; 执行结果
#t
#f

Exercise 2.55

Eva Lu Ator types to the interpreter the expression

(car ''abracadabra)

To her surprise, the interpreter prints back quote. Explain.

---

因为 (car ''abracadabra) 被解释器理解为 (car (quote (quote abracadabra)))，第一个 quote 引用了后面的内容 (quote abracadabra)，
这实际上是一个有2个元素的 list，对这个list 调用 car 就取出了第一个元素 quote。也就是第二个 quote 没有被当作函数使用，而是被当作字符串了。