; Strings
; you can think of strings as a special type of list of characters
; we can have a null string, ""
; we can pick out any character from a string, using STRING-REF

; strings are different from lists in that, you cannot use list operations on them
; and strings only consist of characters

; suppose that we want to write procedures to operate on strings as if
; they were lists, how can we write STRING-CAR, STRING-CDR, STRING-CONS?

; string-null? returns #t if input string is ""
(define string-null?
  (lambda (str)
    (equal? str "")))


; string-car returns the first character of a string
(define string-car
  (lambda (str)
    (string-ref str 0)))

; string-cdr returns the input string with the first character removed
(define string-cdr
  (lambda (str)
    (substring str 1 (string-length str))))
    
; string-cons takes in a character and a string and puts that character on the
; front of the string
(define string-cons
  (lambda (char str)
    (string-append (string char) str)))
    
; count-this-char takes in a character and a string and counts the number
; of occurences of that character in that string
(define count-this-char
  (lambda (char str)
    (if (string-null? str)
        0
        (if (equal? char (string-car str))
            (+ 1 (count-this-char char (string-cdr str)))
            (count-this-char char (string-cdr str))))))