CODEWITHSUNDEEP
functionsyntaxcompiler-errorsfunctional-programmingscheme
Jodast
Jodast

Reputation: 1299

Why is the parameter for my procedure getting a 'not a function' error?

I am going through the book Structure and Interpretation of Computer Programming, which uses Scheme, and I just got through a portion on recursion. I wrote a program for exercise 1.11:

A function f is defined by the rule that f(n) = n if n<3 and f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n> 3. Write a procedure that computes f by means of a recursive process. Write a procedure that computes f by means of an iterative process.

I wrote the code at repl.it and when I run the procdure with the input 2, it gives me the error: Error: 2 is not a function [fRecurse, (anon)]. Can somebody explain to me what this means and how I can fix it? Why is it expecting my input to be a function?

Code: 

(define (fRecurse n)(
(cond ((< n 3) n)
      ((>= n 3) 
        (+ (procRecurse (- n 1))
           (* 2 (f (- fRecurse 2)))
           (* 3 (f (- fRecurse 3))))))))
(fRecurse 2)

Upvotes: 2

Views: 59

Answers (1)

Christian Santos
Christian Santos

Reputation: 5456

The error is due to an extra pair of parentheses before (cond...). To fix the issue, we simply remove the extra pair of parentheses:

(define (fRecurse n)
  (cond ((< n 3) n)
      ((>= n 3) 
        (+ (fRecurse (- n 1))
           (* 2 (fRecurse (- n 2)))
           (* 3 (fRecurse (- n 3)))))))

(fRecurse 2)

And some additonal fixes I made to your example to make it work correctly:

  • Change f to fRecurse
  • Change procRecurse to fRecurse
  • Change (* 2 (f (- fRecurse 2))) to (* 2 (fRecurse (- n 2)))
  • Change (* 3 (f (- fRecurse 3))) to (* 3 (fRecurse (- n 3)))

See the updated repl.it

Upvotes: 2

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