Homework 4


Type the following into the terminal to copy the template file to the current directory (note the period at the end):

cp ~cs61as/autograder/templates/hw4.rkt .

Or you can download the template here.


If you are working on the lab computers, the grader command will run the autograder. If you are working on your own personal machine, you should download grader.rkt and the HW 4 tests.


Try and predict what the following expressions will return, then check your answers with the Racket interpreter:

  • (define x (cons 4 5))
  • (car x)
  • (cdr x)
  • (define y (cons 'hello 'goodbye))
  • (define z (cons x y))
  • (car (cdr z))
  • (cdr (cdr z))
  • (cdr (car z))
  • (car (cons 8 3))
  • (car z)
  • (car 3)

Exercise 1

The first few exercises refer to SICP 2.1.4. See the text for details about interval arithmetic.

SICP 2.7

Alyssa's program is incomplete because she has not specified the implementation of the interval abstraction. Here is a definition of the interval constructor:

(define (make-interval a b) (cons a b))

Define selectors upper-bound and lower-bound to complete the implementation.

SICP 2.8

Using reasoning analogous to Alyssa's, describe how the difference of two intervals may be computed. Define a corresponding subtraction procedure, called sub-interval.

SICP 2.10

Ben Bitdiddle, an expert systems programmer, looks over Alyssa's shoulder and comments that it is not clear what it means to divide by an interval that spans zero. Modify Alyssa's code to check for this condition and to signal an error if it occurs.

Note: Spans zero means that one bound is <= zero and the other is >= zero!

SICP 2.12

Define a constructor make-center-percent that takes a center and a percentage tolerance and produces the desired interval. You must also define a selector percent that produces the percentage tolerance for a given interval. The center selector is the same as the one shown above.

SICP 2.17

Define a procedure last-pair that returns the list that contains only the last element of a given (nonempty) list:

-> (last-pair (list 23 72 149 34))

SICP 2.20

The procedures +, *, and list take arbitrary numbers of arguments. One way to define such procedures is to use define with dotted-tail notation. In a procedure definition, a parameter list that has a dot before the last parameter name indicates that, when the procedure is called, the initial parameters (if any) will have as values the initial arguments, as usual, but the final parameter's value will be a list of any remaining arguments. For instance, given the definition

(define (f x y . z) <body>)

the procedure f can be called with two or more arguments. If we evaluate

(f 1 2 3 4 5 6)

then in the body of f, x will be 1, y will be 2, and z will be the list '(3 4 5 6). Given the definition

(define (g . w) <body>)

the procedure g can be called with zero or more arguments. If we evaluate

(g 1 2 3 4 5 6)

then in the body of g, w will be the list '(1 2 3 4 5 6).

Use this notation to write a procedure same-parity that takes one or more integers and returns a list of all the arguments that have the same even-odd parity as the first argument. For example,

-> (same-parity 1 2 3 4 5 6 7)
(1 3 5 7)

-> (same-parity 2 3 4 5 6 7)
(2 4 6)

SICP 2.22

Louis Reasoner tries to rewrite the first square-list procedure of Exercise 2.21 so that it evolves an iterative process:

(define (square-list items)
  (define (iter things answer)
    (if (null? things)
        (iter (cdr things) 
              (cons (square (car things))
  (iter items nil))

Unfortunately, defining square-list this way produces the answer list in the reverse order of the one desired. Why?

Louis then tries to fix his bug by interchanging the arguments to cons:

(define (square-list items)
  (define (iter things answer)
    (if (null? things)
        (iter (cdr things)
              (cons answer
                    (square (car things))))))
  (iter items nil))

This doesn't work either. Explain.

Exercise 2

Write a procedure my-substitute that takes three arguments: a list, an old word, and a new word. It should return a copy of the list, but with every occurrence of the old word replaced by the new word, even in sublists. For example:

-> (my-substitute '((lead guitar) (bass guitar) (rhythm guitar) drums)
((lead axe) (bass axe) (rhythm axe) drums)

You might find the procedure list? useful:

-> (list? (list 1 2 3))
-> (list? 'apple)
-> (list? 4)

Exercise 3

Now write my-substitute2 that takes a list, a list of old words, and a list of new words; the last two lists should be the same length. It should return a copy of the first argument, but with each word that occurs in the second argument replaced by the corresponding word of the third argument:

-> (my-substitute2 '((4 calling birds) (3 french hens) (2 turtle doves))
                   '(1 2 3 4)
                   '(one two three four))
((four calling birds) (three french hens) (two turtle doves))

Extra for Experts

Do these if you want an extra challenge. These are not for credit.

Exercise 4

Write the procedure cxr-function that takes as its argument a word starting with c, ending with r, and having a string of letters a and/or d in between, such as cdddadaadar. It should return the corresponding function.

Exercise 5

SICP Ex. 2.6. Besides addition, invent multiplication and exponentiation of nonnegative integers. If you're really enthusiastic, see if you can invent subtraction. (Remember, the rule of this game is that you have only lambda as a starting point.) Read ~cs61as/lib/church-hint for some suggestions.

Exercise 6

SICP Ex. 2.18; this should take some thought, and you should make sure you get it right, but don't get stuck on it for a whole hour. Note: Your solution should reverse lists, not sentences! That is, you should be using cons, car, and cdr, not first, sentence, etc.

Submit Your Homework!

For instructions, see this guide. It covers basic terminal commands and assignment submission.

If you have any trouble submitting, do not hesitate to ask a TA!