Question

(define l '(* - + 4))

(define (operator? x)
    (or (equal? '+ x) (equal? '- x) (equal? '* x) (equal? '/ x)))

(define (tokes list)
  (if (null? list)(write "empty")
  (if (operator? (car list))

       ((write "operator")
        (tokes (cdr list)))

      (write "other"))))

The code works just fine til (tokes (cdr list))) reaches the end of file. Can someone give me a tip in how I can prevent that. I’m new at Scheme so I’m forgive me if the question is absurd.

Answer 1

You must make sure of advancing the recursion on each case (except the base case, when the list is null). In your code you’re not making a recursive call for the (write "other") case. Also, you should use cond when there are several conditions to test, Let me explain with an example – instead of this:

(if condition1
    exp1
    (if condition2
        exp2
        (if condition3
            exp3
            exp4)))

Better write this, is much more readable and has the added benefit that you can write more than one expression after each condition without the need to use a begin form:

(cond (condition1 exp1) ; you can write additional expressions after exp1
      (condition2 exp2) ; you can write additional expressions after exp2
      (condition3 exp3) ; you can write additional expressions after exp3
      (else exp4))      ; you can write additional expressions after exp4

… Which leads me to the next point, be aware that you can write only one expression for each branch of an if, if more than one expression is needed for a given condition in an if form then you must surround them with a begin, for example:

(if condition
    ; if the condition is true
    (begin  ; if more than one expression is needed 
      exp1  ; surround them with a begin
      exp2) 
    ; if the condition is false
    (begin  ; if more than one expression is needed 
      exp3  ; surround them with a begin
      exp4))

Going back to your question – here’s the general idea, fill-in the blanks:

(define (tokes list)
  (cond ((null? list)
         (write "empty"))
        ((operator? (car list))
         (write "operator")
         <???>)   ; advance on the recursion
        (else
         (write "other")
         <???>))) ; advance on the recursion