Question

I’m trying to write a function that checks if the given input holds any characters other than digits . However , I get contract violation in DrRacket under Scheme .

Here’s the code :

A method that separating a string into characters :

(define (breaking str) (list->vector (string->list str)))

Creating a vector for the number :

(define myNumber (breaking "123498765")) 

Here I check the given number :

(define (vectorFunc myVector)
(define i 0)                      
(do ()                             
  ((= i (vector-length myVector))) ; run until the end of the vector
  (cond ((< (vector-ref myVector i) #\0) 'incorrect)
        ((> (vector-ref myVector i) #\9)  'also-incorrect))
  (set! i (+ i 1))    ; inc "i+ by 1
); end of do
)

And the output of (vectorFunc myNumber) is :

. . >: contract violation
  expected: real?
  given: #\1
  argument position: 1st
  other arguments...:
   #\0
> 

What’s wrong with it ?

Answer 1

If you need to determine if a value is a character between #\0 and #\9, better use the char-numeric? predicate:

(char-numeric? a-char)

From the documentation:

Returns #t if char has the Unicode “Numeric” property.

In the case of your code, it looks like this:

(cond ((not (char-numeric? (vector-ref myVector i)))
       'incorrect)
      (else ...))

I agree with @LudwigMeier’s comment – your code doesn’t look Scheme-like. You’re trying to use Scheme as if it were a C-like language, and it looks a bit weird. In Scheme things are done … different.