cs1120: Notes 8

Assignments Due

• Monday, 14 September: Problem Set 2

Notes and Questions

All of our List procedures depend on this definition, so it is important to know and understand it well.

List Procedures

(define (is-list? p)
    (if (null? p) 
        true
        (if (pair? p) 
             (is-list? (cdr p)) 
             false)))

(define (list-sum p) 
    (if (null? p)
        0
        (+ (car p) (list-sum (cdr p)))))

(define (deep-list-sum p) 
    (if (null? p)
        0 
        (+ (if (list? (car p))
               (deep-list-sum (car p))
               (car p)) 
           (deep-list-sum (cdr p)))))

(define (list-length p) 
    (if (null? p)
        0
        (+ 1 (list-length (cdr p)))))

Tracing

A helpful way to see what is going on when an expression is evaluated is to use the tracing. See Section 4.5.2 of the coursebook for details.

   (require (lib "trace.ss"))
   (trace deep-list-sum)

Closest Number

Define find-closest-number, a procedure that takes two parameters, a goal and a list of numbers, and produces the number in the list numbers list that is closest to goal.