Fundamentals of Computer Science I (CSC-151.02 2000F)

Naming Values with Local Bindings


Exercise 0: Preparation

If you haven't done so already (or if you had problem understanding the short lecture), you may want to scan through the reading on local bindings.

Exercise 1: Evaluating let

What are the values of the following let-expressions? You may use DrScheme to help you answer these questions, but be sure you can explain how it arrived at its answers.


(let ((tone "fa")
      (call-me "al"))
  (list call-me tone "l" tone))


;; Solutions to the quadratic equation x^2 - 5x + 4:
(let ((discriminant (- (* -5 -5) (* 4 1 4))))
  (list (/ (+ (- -5) (sqrt discriminant)) (* 2 1))
        (/ (- (- -5) (sqrt discriminant)) (* 2 1))))


(let ((total (+ 8 3 4 2 7)))
  (let ((mean (/ total 5)))
    (* mean mean)))


(let ((val (read)))
  (display "The square of ")
  (display val)
  (display " is ")
  (display (* val val))

Exercise 2: Matching Likes

Suppose we are recording student information for Vivek's Dating Service in the following form:

(first-name last-name gender list-of-likes)

Write a procedure (likes thing list-of-students) that extract all the students that like a particular thing. Use let to simplify the process.

Exercise 3: Nesting Lets

Write a nested let-expression that binds a total of five names, a, b, c, d, and e, with a bound to 9387 and each subsequent name bound to a value twice as large as the one before it -- b should be twice as large as a, c twice as large as b, and so on. The body of the innermost let-expression should compute the sum of the values of the five names.

Exercise 4: Simplifying Nested Lets

Write a let*-expression equivalent to the let-expression in the previous exercise.

Exercise 5: Finding the Longest List

Here is a procedure that takes a non-empty list of lists as argument and returns the longest list on the list (or one of the longest lists, if there is a tie).

(define longest-list-in-list
  (lambda (ls)
    (if (null? (cdr ls))
        (car ls)
        (longer-list (car ls) (longest-list-in-list (cdr ls))))))

This definition of the longest-string-in-list procedure includes a call to the longer-string procedure, which returns the longer of two given strings:

(define longer-string
  (lambda (left right)
    (if (<= (string-length right) (string-length left))

Revise the definition of longest-list-in-list so that the name longer-list is bound to the procedure that it denotes only locally, in a let-expression.

Exercise 6: Alternate Techniques

Note that there are two possible ways to do the previous exercise: The definiens of longest-list-in-list can be a lambda-expression with a let-expression as its body, or it can be a let-expression with a lambda-expression as its body.

a. Define longest-list-in-list in whichever way that you did not define it for the previous exercise.

b. Does the order of nesting affect what happens when the procedure is invoked? If so, which arrangement is better? Why?



Monday, 2 October 2000

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