[2021 Spring] CS61A Discussion 10: Scheme, Scheme Lists

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Discussion 10： https://inst.eecs.berkeley.edu/~cs61a/sp21/disc/disc10/#introduction

• Q1: Factorial
• Q2: (Tutorial) Fibonacci
• Q3: List Concatenation
• Q4: (Tutorial) Warm-up
• Q5: (Tutorial) List Duplicator
• Q6: (Tutorial) List Insert

Q1: Factorial

x的阶乘

# python
def factorial(x):
    if x <= 1:
        return 1
    else:
        return x * factorial(x-1)

# scheme
(define (factorial x)
  (if (<= x 1) 1 (* x (factorial (- x 1))))
)

Q2: (Tutorial) Fibonacci

斐波那契数列

# python
def fib(n):
    if n < 2:
        return n
    else:
        return fib(n-1) + fib(n-2)

# scheme
(define (fib n)
    (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2)))))

Q3: List Concatenation

# scheme
(define (list-concat a b)
    (if (null? a)
        b
        (cons (car a) 
              (list-concat (cdr a) b)))
)

Q4: (Tutorial) Warm-up

# scheme
(car (cdr (cdr (cdr s))))

Q5: (Tutorial) List Duplicator

# scheme
(define (duplicate lst)
    (if (null? lst)
        lst
        (cons (car lst) (cons (car lst) (duplicate (cdr lst)))))
)

Q6: (Tutorial) List Insert

# python
def inserte(element, lst, index):
    if index == 0:
        return [element] + lst
    else:
        return [lst[0]] + inserte(element, lst[1:], index - 1)

# scheme
(define (insert element lst index)
    (if (= index 0)
        (cons element lst)
        (cons (car lst) (insert element (cdr lst) (- index 1))))
)

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