[2021 Spring] CS61A Discussion 11: Interpreters

Disc 11：https://inst.eecs.berkeley.edu/~cs61a/sp21/disc/disc11/

目录

• Evaluation
• Scheme Lists
  • Q6: (Tutorial) Replicate
  • Q7: (Tutorial) Run Length Encoding
  • Q8: (Tutorial) Map
  • Q9: (Tutorial) Make Tree
  • Q10: (Tutorial) Tree Sum

Evaluation

跟Lab 11的思路是一样的：

Scheme Lists

Q6: (Tutorial) Replicate

递归

(define (replicate x n)
    (if (= n 0)
        nil
        (cons x (replicate x (- n 1))))
)

;;; Tests
(replicate 5 3)
; expect (5 5 5)

Q7: (Tutorial) Run Length Encoding

需要用到replicate和my-append。

(define (my-append a b)
    (if (null? a)
    b
    (cons (car a) (my-append (cdr a) b))))

(define (uncompress s)
    (if (null? s)
        nil
        (my-append (replicate (car (car s)) (car (cdr (car s)))) (uncompress (cdr s))))
)

;;; Tests
(uncompress '((a 1) (b 2) (c 3)))
; expect (a b b c c c)

Q8: (Tutorial) Map

(define (map fn lst)
    (if (null? lst)
        nil
        (cons (fn (car lst)) (map fn (cdr lst))))
)

;;; Tests
(map (lambda (x) (* x x)) '(1 2 3))
; expect (1 4 9)

Q9: (Tutorial) Make Tree

(define (make-tree label branches) (cons label branches))

(define (label tree)
    (car tree)
)

(define (branches tree)
    (cdr tree)
)

Q10: (Tutorial) Tree Sum

(define (tree-sum tree)
    (+ (label tree) (sum (map tree-sum (branches tree))))
)

(define (sum lst)
    (if (null? lst)
        0
        (+ (car lst) (sum (cdr lst)))))