CS 115, Fall 2002
Homework Assignment 6
Due: 11 November 2002

  • Hand Evaluation  

    Using hand-evaluation, determine the meaning of these expressions. Show your steps.

    For each step, copy and paste all of the definitions, changing any that are set!d, to show how they change.

    (define f 1)
(begin
  (set! f (+ f 1))
  f)
(begin
  (set! f (+ f 1))
  f)

    Solution 

    (define f 2)
(begin 
  f)
(begin
  (set! f (+ f 1))
  f)

(define f 2)
2
(begin
  (set! f (+ f 1))
  f)

(define f 3)
2
(begin
  f)

(define f 3)
2
3


    (define (inside)
  (let ([f 1])
    (begin
      (set! f (+ f 1))
      f)))
(inside)
(inside)

    Solution 

    (let ([f 1])
  (begin
    (set! f (+ f 1))
    f))
(inside)

(define f^ 1)
(begin
  (set! f^ (+ f^ 1))
  f^)
(inside)


(define f^ 2)
(begin
  f^)
(inside)

(define f^ 2)
2
(inside)


(define f^ 2)
2
(let ([f 1])
  (begin
    (set! f (+ f 1))
    f))


(define f^ 2)
(define f^^ 1)
2
(begin
  (set! f^^ (+ f^^ 1))
  f^^)

(define f^ 2)
(define f^^ 2)
2
(begin
  f^^)

(define f^ 2)
(define f^^ 2)
2
2

  • Add to phone book  

    A phone-book is either:

    • '(), or

    • (cons (make-entry name phone-number) phone-book)

    where name is a symbol and phone-number is a number. 

    (define-struct entry (name phone-number))

    This is the template for phone-books: 

    (define (phone-book-template a-pb)
  (cond
    [(null? a-pb) ...]
    [else (entry-name (car a-pb))
          (entry-phone-number (car a-pb))
          (phone-book-template (cdr a-pb))]))

    Write this function: 

    ;; add-to-phone-book : phone-book symbol number  phone-book
;; adds name and number to a-pb unless the
;; name is already in the phone book
(define (add-to-phone-book a-pb name number)
  ...)

    Do not forget to design test cases before designing the function.

    Solution 

    (define (add-to-phone-book a-pb name number)
  (cond
    [(null? a-pb) (cons (make-entry name number) '())]
    [else (if (eq? name (entry-name (car a-pb)))
              a-pb
              (cons (car a-pb)
                    (add-to-phone-book (cdr a-pb)
                                       name
                                       number)))]))


(add-to-phone-book '() 'me 5551212)
(cons (make-entry 'me 5551212) '())

(add-to-phone-book (cons (make-entry 'me 5551212) '()) 'me 5551212)
(cons (make-entry 'me 5551212) '())

(add-to-phone-book (cons (make-entry 'me 5551212) '()) 'you 5551234)
(cons (make-entry 'me 5551212) 
      (cons (make-entry 'you 5551234)
            '()))

  • Lookup in phone book  

    Write this function: 

    ;; lookup-in-phone-book : phone-book symbol  number or #f
;; looks up name in a-pb. Returns the name's
;; number or \#f if the name isn't in the phone book.
(define (lookup-in-phone-book a-pb name)
  ...)

    Do not forget to design test cases before designing the function.

    Solution 

    (define (lookup-in-phone-book a-pb name)
  (cond
    [(null? a-pb) #f]
    [else (if (eq? name (entry-name (car a-pb)))
              (entry-phone-number (car a-pb))
              (lookup-in-phone-book (cdr a-pb) name))]))

(lookup-in-phone-book '() 'me) 
#f

(lookup-in-phone-book (cons (make-entry 'me 5551212) '()) 'me)
5551212

(lookup-in-phone-book (cons (make-entry 'me 5551212) '()) 'you)
#f

  • Adding State  

    Add a single definiton for the current phone book, initially an empty phone book. Then, define the functions add and lookup: 

    ;; add : number symbol  void
;; adds name and number to the current phone book.
(define (add name number) ...)

;; lookup : symbol  number or #f
;; looks up name in the phone book
(define (lookup name) ...)

    Solution 

    (define current-phone-book '())
(define (add name number)
  (set! current-phone-book (add-to-phone-book current-phone-book name number)))

(define (lookup name)
  (lookup-in-phone-book current-phone-book name))

    Here are some tests for those functions: 

    (lookup 'me)
;; should be
#f

(add 'me 5551212)
(lookup 'me)
;; should be
5551212

Last modified: Mon, Nov 11, 2002, 5:28 pm
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