Addition of numbers recursively in Scala

Question

In this Scala code I'm trying to analyze a string that contains a sum (such as 12+3+5) and return the result (20). I'm using regex to extract the first digit and parse the trail to be added recursively. My issue is that since the regex returns a String, I cannot add up the numbers. Any ideas?

object TestRecursive extends App { val plus = """(\w*)\+(\w*)""".r println(parse("12+3+5")) def parse(str: String) : String = str match { // sum case plus(head, trail) => parse(head) + parse(trail) case _ => str } }

I'm trying to build a calculator for the user enters a string and the program calculates the result — ps0604
– ps0604, Commented Mar 6, 2016 at 12:16
@csharpfolk that doesn't work as the parse function returns String — ps0604
– ps0604, Commented Mar 6, 2016 at 12:19
@ps0604 that was just the idea, you need to change parse signature to return Int of course — csharpfolk
– csharpfolk, Commented Mar 6, 2016 at 12:27
@csharpfolk that's the point of the question, how to make the function return String and Int, depending on what is returned — ps0604
– ps0604, Commented Mar 6, 2016 at 12:29

Andreas Neumann · Accepted Answer · 2016-03-06 15:15:49Z

You might want to use the parser combinators for an application like this.

"""(\w*)\+(\w*)""".r also matches "+" or "23+" or "4 +5" // but captures it only in the first group.

what you could do might be

scala> val numbers = "[+-]?\\d+" numbers: String = [+-]?\d+ ^ scala> numbers.r.findAllIn("1+2-3+42").map(_.toInt).reduce(_ + _) res4: Int = 42 scala> numbers.r.findAllIn("12+3+5").map(_.toInt).reduce(_ + _) res5: Int = 20

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