hw06： https://inst.eecs.berkeley.edu/~cs61a/sp21/hw/hw06/#scheme-editor
CS 61A Scheme Specification： https://inst.eecs.berkeley.edu/~cs61a/sp21/articles/scheme-spec/
Scheme Built-In Procedure Reference： https://inst.eecs.berkeley.edu/~cs61a/sp21/articles/scheme-builtins/
Q2: Thane of Cadr
返回列表的第2、3個元素，參考題中給出的cddr即可。
(define (cddr s)
(cdr (cdr s)))
(define (cadr s)
'YOUR-CODE-HERE
)
(define (caddr s)
'YOUR-CODE-HERE
)
(define (cddr s) (cdr (cdr s)))
(define (cadr s) (car (cdr s)))
(define (caddr s) (car (cdr (cdr s))))
Q3: Sign
用cond進行條件判斷。
(define (sign val)
(cond
((< val 0) -1)
((= val 0) 0)
(else 1)))
Q4: Pow
用cond和even? 進行冪的運算，可以先寫python版，再按照scheme語法來寫。
(define (square x) (* x x))
(define (pow base exp)
(cond
((= base 1) 1)
((= exp 0) 1)
((even? exp) (square (pow base (/ exp 2))))
(else (* base (square (pow base (/ (- exp 1) 2)))))))
python版：
def squares(x): return x * x
def pows(base, exp):
if base == 1: return 1
if exp == 0: return 1
if exp % 2 == 0:
return squares(pows(base, exp // 2))
else:
return base * squares(pow(base, exp // 2))