How to throw exception in Scala pattern matching?

Question

I have a string "-3 + 4 - 1 + 1 + 12 - 5 + 6" and I want to find solution to this equation. And secure it from unwanted characters in it (like abc, or #).

Solution of this equation is correct but I can't handle exception when other signs in string occure. I'm using Scala and pattern matching, it's a new topic for me and I'm not sure why it doesnt work.

object Main extends App {

  val numberString = "-3 + 4 - 1 + 1 + 12 - 5  + 6";

  val abc:  List[String] = numberString.split("\\s+").toList

  var temp = abc.head.toInt


  for (i <- 0 until abc.length) {
    abc(i) match {
      case "+" => temp += abc(i+1).toInt
      case "-" => temp -= abc(i+1).toInt
      case x if -100 to 100 contains x.toInt=> println("im a number ")

      case _ => throw new Exception("wrong opperator")

     }

}

println(temp);

Output when

numberString = "-3 + 4 # - 1 + 1 + 12 - abc 5 + 6";

should be throwing wrong operator Exception but I have:

Exception in thread "main" im a number 
im a number 
java.lang.NumberFormatException: For input string: "#"

the problem is you are trying to convert # into int which is causing the exception what you can do is check for isDigit and in the else party raise an Exception you want — Raman Mishra
– Raman Mishra, Commented Jan 6, 2019 at 14:09

Raman Mishra · Accepted Answer · 2019-01-06 14:31:38Z

4

you just need to assign 0 to the temp as you are trying to convert - into digit it's giving you the NumberFormatException.

you just need to keep that in mind after every operator ("-", "+") there should be a space.

object Solution extends App {
  val numberString = "- 3 + 4 - 1 + 1 + 12 - 5  + 6"

  val abc: List[String] = numberString.split("\\s+").toList

  var temp = 0


  for (i <- abc.indices) {
    abc(i) match {
      case "+" => temp += abc(i + 1).toInt
      case "-" => temp -= abc(i + 1).toInt
      case x if x.forall(_.isDigit) => println("im a number ")

      case _ => throw new Exception("wrong opperator")
    }
  }

  print(temp)
}

answered Jan 6, 2019 at 14:31

Raman Mishra

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Comments

Phasmid · Accepted Answer · 2019-01-07 02:20:48Z

1

Correcting Dima's answer:

val num = """(\d+)""".r // Regex to parse numbers
def compute(in: List[String], result: Int = 0): Int = in match {
  case Nil => result
  case "+" :: num(x) :: tail => compute(tail, result + x.toInt)
  case "-" :: num(x) :: tail => compute(tail, result - x.toInt)
  case ("+" | "-") :: x :: _ => throw new Exception(s"Bad number $x")
  case x :: Nil => throw new Exception(s"Invalid syntax: operator expected, but $x found.")
  case op :: _  => throw new Exception(s"Invalid operator $op")
}

answered Jan 7, 2019 at 2:20

Phasmid

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Comments

Dima · Accepted Answer · 2019-01-07 14:12:36Z

0

Don't use mutable state it's evil ...

   val num = """(\d+)""".r // Regex to parse numbers
   @tailrec
   def compute(in: List[String], result: Int = 0): Int = in match {
      case Nil => result
      case "+" :: num(x) :: tail => compute(tail, result + num.toInt)
      case "-" :: num(x) :: tail => compute(tail, result - num.toInt)
      case ("+"|"-") :: x :: _ => throw new Exception(s"Bad number $x")
      case x :: Nil => throw new Exception(s"Invalid syntax: operator expected, but $x found.")
      case op :: _  => throw new Exception(s"Invalid operator $op")

  }

edited Jan 7, 2019 at 14:12

answered Jan 6, 2019 at 15:21

Dima

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3 Comments

codenoodle Over a year ago

OP mentioned they're new to pattern matching. I like your solution, but it might be useful to add an explanation of how you used cons in your match cases.

Dima Over a year ago

@codenoodle feel free to edit and add an explanation where you feel it is necessary.

Travis Brown Over a year ago

The OP specifically said "it's a new topic" for them, and not only is this vague and unclear to me (and it's not a new topic to me), it isn't even working code. Sorry, but this is just a bad answer and isn't worth editing.

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