balanced-parentheses.rkt

;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-advanced-reader.ss" "lang")((modname balance) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f ())))

(check-expect 
 (balanced-parentheses? "") true)

(check-expect 
 (balanced-parentheses? "()") true)

(check-expect 
 (balanced-parentheses? "(moko)") true)

(check-expect 
 (balanced-parentheses? 
  "(if (zero? x) max (/ 1 x))") 
 true)

(check-expect 
 (balanced-parentheses? 
  "I told him (that it’s not (yet) done). (But he wasn’t listening)") 
 true)

(check-expect 
 (balanced-parentheses?  ")") false)

(check-expect 
 (balanced-parentheses?  "())( )") false)

(check-expect 
 (balanced-parentheses?  "())(") false)

(check-expect 
 (balanced-parentheses?  ":-)") false)

(define (balanced-parentheses? str) 
  (letrec 
      ([f 
        (lambda (char-list elements-in-stack)
          (cond ((empty? char-list) 
                 (= elements-in-stack 0)) 
                ((char=? (car char-list) #\() 
                 (f (cdr char-list) (+ elements-in-stack 1)))
                ((and (char=? (car char-list) #\)) 
                      (= elements-in-stack 0)) 
                 false)
                ((char=? (car char-list) #\)) 
                 (f (cdr char-list) (- elements-in-stack 1)))
                (else
                 (f (cdr char-list) elements-in-stack))))])
    
    (f (string->list str) 0)))