Iterates through a string of characters and returns true if the input contains a balanced set of parentheses.

symbolbalancer.scala

package practice

object SymbolBalancer extends App {

  val data: List[String] = List(
    "(if (zero? x) max (/ 1 x))",
    "I told him (that it's not (yet) done). (But he wasn't listening)",
    ":-)",
    "())(",
    "((d)o(n)e)",
    ")(")

  /**
   * Iterates through a string of characters and returns true
   * if the input contains a balanced set of parentheses
   */
  def balance(chars: List[Char]): Boolean = {
    // This accumulates the result by or-ing the result. Once this evals to true, it will stay true
    def isMismatched(balanced: Boolean, total: Int, pred: Int => Boolean) = !balanced || pred(total)
    def isNeg(num: Int): Boolean = num < 0
    def findMatch(list: List[Char], isBalanced: Boolean = true, total: Int = 0): Boolean = {
      // If the total is positive and the string is balanced, hence no mismatch
      val balanced = !isMismatched(isBalanced, total, isNeg)
      if (list.isEmpty) balanced
      else if (list.head == '(') {
        findMatch(list.tail, balanced, total + 1)
      } else if (list.head == ')') {
        findMatch(list.tail, balanced, total - 1)
      } else {
        findMatch(list.tail, balanced, total)
      }
    }
    findMatch(chars)
  }

  println("Balancing Parentheses")
  println("---------------------")
  data.foreach(str => {
    println("isBalanced? " + balance(str.toList) + "\n\"" + str + "\"\n")
  })

}

/*
Balancing Parentheses
---------------------
isBalanced? true
"(if (zero? x) max (/ 1 x))"

isBalanced? true
"I told him (that it's not (yet) done). (But he wasn't listening)"

isBalanced? false
":-)"

isBalanced? false
"())("

isBalanced? true
"((d)o(n)e)"

isBalanced? false
")("
*/