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POexpression tree evaluation
 

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  1. POexpression tree evaluation
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    <p>I'm trying to evaluate an expression tree. Here is my code:</p> <pre><code>(define (eval-tree expr-tree) (eval-treeaux eval-tree expr-tree)) (define (eval-treeaux f expr-tree) (if (null? expr-tree) 0 (cond ((equal? '+ (operator expr-tree)) (+ (left-op expr-tree) (right-op expr-tree))) ((equal? '- (operator expr-tree)) (- (left-op expr-tree) (right-op expr-tree))) ((equal? '* (operator expr-tree)) (* (left-op expr-tree) (right-op expr-tree))) ((equal? '/ (operator expr-tree)) (/ (left-op expr-tree) (right-op expr-tree))) (else (f eval-treeaux expr-tree))))) </code></pre> <p>When I run this <code>(eval-tree '((6 * 3) + (4 - 2)))</code>, it gives me this error:</p> <pre><code>+: expects type &lt;number&gt; as 1st argument, given: (6 * 3); other arguments were: (4 - 2) </code></pre> <p>Can anyone tell me what the problem is and how I can fix it?</p> <hr> <p>Here is my new code:</p> <pre><code>(define (eval-tree expr-tree) (if (null? expr-tree) 0 ((eval-treeaux eval-tree (car expr-tree)) (eval-tree (cdr expr-tree))))) (define (eval-treeaux f expr-tree) (cond ((null? expr-tree) '()) ((not (isExpression expr-tree)) (list expr-tree)) (else (cond ((equal? '+ (operator expr-tree)) (+ (left-op expr-tree) (right-op expr-tree))) ((equal? '- (operator expr-tree)) (- (left-op expr-tree) (right-op expr-tree))) ((equal? '* (operator expr-tree)) (* (left-op expr-tree) (right-op expr-tree))) ((equal? '/ (operator expr-tree)) (/ (left-op expr-tree) (right-op expr-tree))) (else (f eval-treeaux expr-tree)))))) </code></pre> <p>But, when I run this <code>(eval-tree '((6 * 3) + (4 - 2)))</code>, I get this error:</p> <pre><code>procedure application: expected procedure, given: 2; arguments were: 0 </code></pre> <p>I'm really confused. Can anyone help me please? Thanks</p>
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    1. PTQuestion
    1. USRobert Harvey
    1. USprgrammer
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    1. PO
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      1. PTAnswer
    1. This table or related slice is empty.
    1. COI concur with the answer below. A separate issue: why are you returning zero on empty lists? Also, why have you separated this function into eval-tree and eval-treeaux? Also, why are you passing eval-tree to eval-treeaux? This code needs a lot of cleanup!
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      1. POexpression tree evaluation
      1. USJohn Clements
 

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