Question

I can’t seem to figure out how to delete an element from the BST. This is my code

(define remove (lambda (x t)
       (if  (< x (car t)) (list (car t) (remove x (cadr t)) (caddr t))
             (if (> x (car t)) (list (car t) (cadr t) (remove x (caddr t)))
                   (if (not(and (null? (cadr t)) (null? (caddr t)))) 
                       (let ((r (minimum (caddr t)))) ((remove r t) (set-car! t r))) 
                       (list '() (cadr t) (caddr t)))))))

Minimum returns the minimum value in the tree.
If I try to delete an element that’s not a leaf, it goes into an infinite loop. How can I fix it?

Answer 1

I think your biggest problem lies in this section:

((remove r t) (set-car! t r))

You are removing r from t, but you should really be removing r from the right subtree of t. I’m not sure why you are using mutability/setting, either; I think it complicates things in what could easily be a non-side effecting function. I would try something like:

(list r (cadr t) (remove r (caddr t)))

I also have to admit that I’m a bit confused about your intent with the last line. What are you using the empty list for?