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POBinary Search Tree Scheme
 

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    <p>My friend and I are currently working on creating a Binary Search Tree in Scheme. We cannot get it to save what we have inserted. My professor said we are to use set-car! (cdr ( for the left subtree somehow, but I don't know where exactly to put it in. We are supposed to use set-car! (cddr ( for the right subtree also. </p> <p>We have done all of this so far correctly, but we just need help making it save our inserted nodes. </p> <p>Code: </p> <pre><code>(define make-empty-BST '()) ;create function to see if BST is empty (define (emptyBST? BST) (null? BST)) ;make non-empty BST with explicit list implementation (define (make-BST root left-subtree right-subtree) (list root left-subtree right-subtree)) ;helper get root function (define (getRoot BST) (car BST)) ;helper get left subtree function (define (getLeftsubTree BST) (cadr BST)) ;(car (cdr tr)) ;helper get right subtree function (define (getRightsubTree BST) (caddr BST)) ;(car (cdr (cdr tr))) ;Checks to see if a leaf is empty (define (emptyleaf? BST) (and (null? (getLeftsubTree BST)) (null? (getRightsubTree BST)))) ;inserts an item into the BST (define (BST-insert BST item) (cond ((emptyBST? BST) ;if empty, create a new root with given item - use empty lists for left and right subtrees (make-BST item make-empty-BST make-empty-BST)) ((&lt; item (getRoot BST)) ;if item less than root, add to left subtree (make-BST (getRoot BST) (BST-insert (getLeftsubTree BST) item) ;recursion (getRightsubTree BST))) ((&gt; item (getRoot BST)) (make-BST (getRoot BST) (getLeftsubTree BST) (BST-insert (getRightsubTree BST) item))) (else BST))) ; it's already in BST, do nothing </code></pre>
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    1. USJared
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    1. COThis is using Dr. Racket and R5RS.
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      1. USJared
    2. COAt the moment, your BST-insert is returning a new tree that shares structure with the old tree, rather than updating the existing tree. Do you see why? You should only create a new tree node at the leaf, and should only be assigning the left or right child of its parent.
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      1. USJoshua Taylor
    3. COHave you had a look at [Insert node key and value in binary search tree using scheme](http://stackoverflow.com/q/12910579/1281433)? [using set! to change the value of a variable in drscheme](http://stackoverflow.com/q/4416862/1281433) is sort of relevant, too (though not high quality).
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