Question

I am currently trying to write a function that finds a value in racket using DrRacket. With some help I came up with this below. However I need someone to explain to me what the difference is between cadr and caddr? Also in DrRacket how would I create a BST? Is it similar to making a list?

(define (find-val bst elt)
    (cond ((null? bst) #f)
          ((< elt (car bst))
           (find-val (cadr bst) elt))
          ((> elt (car bst))
           (bst (caddr bst) elt))
          ((equal? elt (car bst))
           #t)))

Answer 1

For the first part of your question,

(cadr x)

is equivalent to:

(car (cdr x))

and

(caddr x)

is equivalent to:

(car (cdr (cdr x)))

Did you notice the pattern? each d between the c and r is a shorthand for a cdr, and each a between the c and r is a shorthand for a car, in left-to-right order.

Regarding the second part of your question, there’s a very detailed explanation of how to represent a BST in the book SICP, section §2.3.3 under the title sets as binary trees. There you’ll find the required procedures for creating and manipulating a tree.