Calculate simple arithmetic string in Scala

Question

Problem

Given an arithmetic string like 2+2*3 calculate result 8.

Assumptions you can make

• string will be always well formed (no error checking needs to be done)
• only operators passed in string will be + and *
• only single digit numbers allowed

Example

input = 2*2*2+2*1*5
output = 18

input = 2+2*3
output = 8

Hers is scala code for the same

object Expression extends App {
  implicit class Converter(char: Char) {
    def toNum: Int = char - '0'
  }

  def evaluateExpression(s :List[Char]): Int = {
    s match {
      case left::op::List(right) =>  // a+b or a*b
        op match {
          case '+' =>  left.toNum + right.toNum
          case  _ =>   left.toNum * right.toNum
        }
      case left::op::right => // a+b*c or a*b+c
         op match {
           case '+' =>
             left.toNum + evaluateExpression(right)
           case '*' =>
             val nextAdd = right indexOf '+'
             val (multiplication ,pendingExpression) = if (nextAdd == -1) {
                                                         (right,List[Char]())
                                                       } else {
                                                         (right.take(nextAdd), right.drop(nextAdd + 1))
                                                       }
             val product = (multiplication.filter( _ != '*') map (_.toNum)).foldLeft(left.toNum) (_ * _)
             if (pendingExpression.isEmpty) product
             else product + evaluateExpression(pendingExpression)
          }
     }
   }
  List("3*3*3", "2+2*3", "0*0*1", "0+0*1", "0*0*1", "2+2*3*3", "2+2+2*3+2+2") foreach { expr =>
    println(expr + " = " + evaluateExpression(expr.toList))
  }
}

Output for above code

3*3*3 = 27
2+2*3 = 8
0*0*1 = 0
0+0*1 = 0
0*0*1 = 0
2+2*3*3 = 20
2+2+2*3+2+2 = 14

Answer 1

You might want to spend some time becoming more familiar with the Standard Library. You're doing a few things that are already provided for.

implicit class Converter(char: Char) {
  def toNum: Int = char - '0'
}

To convert a digit character into the value it represents: char.asDigit. If you have one or more digit characters in a string: digits.toInt

Your solution also fails for expressions like "2*2+3". It looks like the evaluateExpression() code doesn't anticipate that, in the recursive call evaluateExpression(pendingExpression), the value of pendingExpression might be a single digit.

But mostly, with the input under so many tight restrictions, you should consider what shortcuts might be available to you.

def evaluateExpression(s :String) :Int =
  s.split("\\+")
   .map(_.split("\\*")
         .map(_.toInt)
         .product)
   .sum