Scheme代写 | Homework Assignment 2

本次北美CS代写之Scheme代写主要是使用Scheme完成内存单元模拟程序。

  1. (15 points) Your goal is to implement a memory cell encoded with functions as data. A memory cell is a function that takes a list as its only argument. The list may have zero or one elements.

    • If the list has one element, then we are setting the value of the memory cell. Setting the value of a memory cell returns a new memory cell.

    • If the list is empty, then we are getting the value contained in the memory cell. The following definitions abstract these notions:

      (define (cell-get c) (c (list)))
      (define (cell-set c x) (c (list x)))
    
    1. (a)  (7.5 points) Implement a read-write cell. Function rw-cell takes a number that initializes the memory cell. Operation set returns a new cell with the value given. Operation get returns the contents of the cell.
      (define w1 (rw-cell 10))

            (check-equal? 10 (cell-get w1))
            (define w2 (cell-set w1 20))
            (check-equal? 20 (cell-get w2))
      
    2. (b)  (7.5 points) Implement a read-only cell. Function ro-cell takes a number and return a read-only cell. Putting a value in a read-only cell should not change the stored value. Getting a value should always return the initial value.
      (define r1 (ro-cell 10))

            (check-equal? 10 (cell-get r1))
            (define r2 (cell-set r1 20))
            (check-equal? 10 (cell-get r2))
      
  2. (15 points) Implement a tail-recursive function intersperse that takes a list l and an element e and returns a list with the elements in list l interspersed with element e. The implementation must only use the list constructors and selectors that we covered in our class. That is, return a list where we add element e between each pair of elements in l. For instance,
      (check-equal? (list 1 0 2 0 3) (intersperse (list 1 2 3) 0))
    
  3. (15 points) Implement a tail-recursive function find that takes as arguments a function predicate and a list l and returns either a pair index-element or #f. The implementation must only use the list constructors and selectors that we covered in our class. The objective of the function is to find a index- element in a list given some predicate. Function find calls function predicate over each element of the list until the predicate returns true. If predicate returns true, then function find returns a pair with the zero-based index of the element and the element.

    Function predicate takes an integer (the zero based index in the list) and the element we are trying to find.

    • (a) (10 points) Implement function find.(check-equal? (cons 0 10) (find (lambda (idx elem) #t) (list 10 20 30))) (check-equal? #f (find (lambda (idx elem) #f) (list 10 20 30)))
    • (b)  (2.5 points) Implement function member in terms of function find. Function member takes an element x and a list l and returns #t if the element x is in list l, otherwise it returns #f. (check-true (member 20 (list 10 20 30)))
      (check-false (member 40 (list 10 20 30)))
    • (c)  (2.5 points) Implement function index-of in terms of function find. Function index-of takes a list l and an element x and returns the index of the first occurrence of element x in list l, otherwise it returns #f.
      (check-equal? 1 (index-of (list 10 20 30) 20))
  1. (10 points) Implement function uncurry which takes as argument a curried function f and returns a new function which takes as parameter a list of arguments which are then applied to f. The implementation must only use the list constructors and selectors that we covered in our class.
      (define (f x y z w)
        (+ x y z w))
    
      (define g (uncurry (curry f)))
      (check-equal? 10 (g (list 1 2 3 4)))
    
  2. (45 points) Recall the AST we defined in Lecture 5. Implement function parse-ast that takes a da- tum and yields an element of the AST. You will need as auxiliary functions real? and symbol? from Racket’s standard library and functions lambda?, define-basic?, and define-func? from Homework Assignment 1 (Part II).

    The function takes a datum that is a valid term. Expression do not include conditional nor booleans, only functions declarations, definitions, variables, and numbers.

      (check-equal? (parse-ast ’x) (r:variable ’x))
      (check-equal? (parse-ast ’10) (r:number 10))
      (check-equal?
    
        (parse-ast ’(lambda (x) x))
    
        (r:lambda (list (r:variable ’x)) (list (r:variable ’x))))
      (check-equal?
    
        (parse-ast ’(define (f y) (+ y 10)))
        (r:define
    
          (r:variable ’f)
          (r:lambda
    
            (list (r:variable ’y))
            (list (r:apply (r:variable ’+) (list (r:variable ’y) (r:number 10)))))))
    

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