CODEWITHSUNDEEP
scheme
user1011861
user1011861

Reputation: 49

Reference to undefined identifier in Scheme

I'm trying to run this scheme program from my book, yet whenever I try to evaluate (minus 5 4). The error: "reference to undefined identifier: minus" appears. Running the function as (minus 5 4) obviously doesn't work. Did I call the evaluation wrong?

(define (eval-expr E)
  (cond
    ((number? E) E)
    ((eqv? (car E) 'plus) (apply + (eval-params (cdr E))))
    ((eqv? (car E) 'times) (apply * (eval-params (cdr E))))
    ((eqv? (car E) 'minus) (apply - (eval-params (cdr E))))      
    ((eqv? (car E) 'divide) (apply / (eval-params (cdr E))))
    (else '()) ; confused - return ()
    )
  )

(define (eval-params E)
  (if (null? E) '()
      (cons (eval-expr (car E)) (eval-params (cdr E)))
      )
)

Upvotes: 1

Views: 3066

Answers (1)

Greg Hewgill
Greg Hewgill

Reputation: 993125

It looks like you want to call

(eval-expr '(minus 5 4))

The eval-expr function takes data that represents an arithmetic expression. On the other hand, your example (minus 5 4) is code that's trying to call a function called minus.

Upvotes: 5

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