Racket Error: expects a non-empty list but given empty

Question

I'm writing a program where it asks me to convert all the uppercase letters in str to lowercase and lowercase to uppercase, and all other characters remain the same. Below is my code:

(define (switch-case str)
  (list->string (switch-char (string->list str))))

(define (switch-char loc)
   (cons
    (cond
      [(empty? (first loc)) empty]
      [(char-lower-case? (first loc)) (char-upcase (first loc))]
      [(char-upper-case? (first loc)) (char-downcase (first loc))]
      [else (first loc)]) (switch-char (rest loc))))

And the error message for (switch-case "ABC") is:

first: expects a non-empty list; given: empty

Can someone help me with this? I don't know which part of the code is wrong:(

Óscar López Óscar López · Accepted Answer · 2019-10-12T23:22:50

There are several syntax errors in your code. I'd suggest you spend more time studying the basic syntax of Scheme, and how to construct a recursive procedure. Notice that:

That cons at the beginning shouldn't be there.
The base case is wrong, you should ask for (empty? loc) instead.
The last case is incorrect, that's not how else is used.
And the most serious mistake: you forgot to call the recursion in all cases. That's the point where cons comes into play!

This version addresses all the issues mentioned above:

(define (switch-char loc)
  (cond
    [(empty? loc) empty]
    [(char-lower-case? (first loc))
     (cons (char-upcase (first loc)) (switch-char (rest loc)))]
    [(char-upper-case? (first loc))
     (cons (char-downcase (first loc)) (switch-char (rest loc)))]
    [else (cons (first loc) (switch-char (rest loc)))]))

Racket Error: expects a non-empty list but given empty

1 Answers