Using Try and Catch for Exception in Scala

Question

Can anyone teach me where to insert try/catch into this code for making sure it won't crash if user enters a string that isn’t “Quit” or also isn’t a number? The following code is used for calculating the average of some numbers. My professor didn't tell us which exception we shall use. Also I am not quite familiar with them.

import io.StdIn._

def sumAndCount() : (Int, Int) = {
    println("Please enter a number; or enter Quit:")
    val input = readLine.toLowerCase.trim
    input match {
        case "quit" => (0, 0)
        case _ => val (sum, count) =  sumAndCount()
                  (input.toInt+sum, count + 1)
    }
}

val (sum, count) = sumAndCount()
val aver = sum.toDouble/count

println(f"The average is $aver")

Hint: with your current code, what happens when something that is neither "Quit" nor a number is entered? (Have you tried it?) — Seth Tisue
– Seth Tisue, Commented Oct 3, 2015 at 18:18
when you call toInt on a string you can get NumberFormatException. I believe you should redesign this code, it doesn't even work. — Łukasz
– Łukasz, Commented Oct 3, 2015 at 18:25

Łukasz · Accepted Answer · 2015-10-03 18:43:11Z

1

import io.StdIn._

def sumAndCount(sum: Int, count: Int) : (Int, Int) = {
    println("Please enter a number; or enter Quit:")
    val input = readLine.toLowerCase.trim
    input match {
        case "quit" => (sum, count)
        case _ => try {
          val number = input.toInt
          sumAndCount(sum + number, count + 1)
        } catch {
          case _: NumberFormatException =>
            println("that's not a number, try again")
            sumAndCount(sum, count)
        }
    }
}

val (sum, count) = sumAndCount(0, 0)
val aver = sum.toDouble / count

println(f"The average is $aver")

You may also want to handle the case when user types only quit, atm result will be NaN.

Please take look at problems with your code. You want to call this function recursive like this, not like you did it.

answered Oct 3, 2015 at 18:43

Łukasz

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