Question

I’m having issues writing a function to walk through a binary tree, the function takes in a search_term, list and returns true or false. Here is what I have and it’s essentially the same thing I found googling how to implement binary search in Scheme.

(define (tree-lookup val tree)
  (if (empty-tree? tree) 
      #f
  (let ((curr-val (node-value tree))
    (left (node-left tree))
    (right (node-right tree)))
    (cond ((equal? val curr-val) #t)
      ((< val curr-val))
       (tree-lookup val left)
      (else
       (tree-lookup val right))))))

(define tree-test '(((1 2) 3)(4 (5 6)) 7 (8 9 10)))  ; Test tree

The problem comes when it tries to compare the “val” variable with the node. This means I am comparing a real number to a list, like (< 2 ‘((1 2) 3)). I tried testing for atom values only but then I am stuck on how to go back up the tree when I reach a leaf.

Here’s the error message:

  <: contract violation
  expected: real?
  given: '{{1 2} 3}
  argument position: 2nd
  other arguments...:
  8 

Answer 1

At first glance the procedure looks fine. I suspect that the problem is in the node-value, node-left and node-right procedures, or in the way you’re building the tree – for starters, the sample tree provided in the question doesn’t seem right to me.

Think about it, the error message signals that the < operator was applied to the list '{{1 2} 3}, meaning that curr-val is a list, but it should be a value.