Sum of list element in scala is not executable : error. How to fix?-CodePudding

I am not able to execute or **▶ button is not clickable in intelleiJ ** the sum function in following two ways. Please help me to identify the mistakes in both.

object Main{
  def sum1(xs : List[Int]): Int = {
    xs.filter(_ > 0).sum // am i missing return word here ?
  }
  def sum2(xs: List[Int]): Unit = {
    val mx = xs.filter(_ > 0).sum // is it ok to have return in a variable ?
    println(mx)
  }
  Main.sum1(List(1,2))
  Main.sum2(List(1,2,3,4))
}

CodePudding user response：

For Intellij, unless you code from within a Scala Worksheet where you can execute execute code without an entry point, in any normal application, you require an entry point to your code.

You can do this in 2 ways. Either define the main method yourself, which is the conventional entry point to any application:

object Main {
  def main(args: Array[String]): Unit = {
    // your code here
  }
}

Or - have your object extend the App trait, which provides the main method for you:

object Main extends App {
  // your code here
}

This is a bit cleaner, and one nested-levels less.

In Scala, statements are called expressions, because they generally return the value of the last expression, if the last expression is not an assignment. This code:

def sum1(xs: List[Int]): Int = 
  xs.filter(_ > 0).sum

It is a syntactic sugar for this:

def sum1(xs: List[Int]): Int = {
  return xs.filter(_ > 0).sum
}

Because braces are optional if the code has only 1 expression, and will return the value of the last expression by default. So even though you can write code both ways, you'll get the warning Return keyword is redundant, because you'll never need to use it explicitly.

Also, you don't need to write Main.sum1(...) from within Main. You can omit the object:

object Main extends App {
  def sum1(xs: List[Int]): Int =
    xs.filter(_ > 0).sum

  def sum2(xs: List[Int]): Unit = {
    val mx = xs.filter(_ > 0).sum
    println(mx) // 10
  }

  println(sum1(List(1, 2))) // 3
  sum2(List(1, 2, 3, 4))
}