How do I break out of a loop in Scala?

Question

How do I break out a loop?

var largest=0
for(i<-999 to 1 by -1) {
    for (j<-i to 1 by -1) {
        val product=i*j
        if (largest>product)
            // I want to break out here
        else
           if(product.toString.equals(product.toString.reverse))
              largest=largest max product
    }
}

How do I turn nested for loops into tail recursion?

From Scala Talk at FOSDEM 2009 http://www.slideshare.net/Odersky/fosdem-2009-1013261 on the 22nd page:

Break and continue Scala does not have them. Why? They are a bit imperative; better use many smaller functions Issue how to interact with closures. They are not needed!

What is the explanation?

Answer 1

import scala.util.control._

object demo_brk_963 
{
   def main(args: Array[String]) 
   {
      var a = 0;
      var b = 0;
      val numList1 = List(1,2,3,4,5,6,7,8,9,10);
      val numList2 = List(11,12,13);

      val outer = new Breaks; //object for break
      val inner = new Breaks; //object for break

      outer.breakable // Outer Block
      {
         for( a <- numList1)
         {
            println( "Value of a: " + a);

            inner.breakable // Inner Block
            {
               for( b <- numList2)
               {
                  println( "Value of b: " + b);

                  if( b == 12 )
                  {
                      println( "break-INNER;");
                       inner.break;
                  }
               }
            } // inner breakable
            if( a == 6 )
            {
                println( "break-OUTER;");
                outer.break;
            }
         }
      } // outer breakable.
   }
}

Basic method to break the loop, using Breaks class. By declaring the loop as breakable.

Answer 2

It is never a good idea to break out of a for-loop. If you are using a for-loop it means that you know how many times you want to iterate. Use a while-loop with 2 conditions.

for example

var done = false
while (i <= length && !done) {
  if (sum > 1000) {
     done = true
  }
}

Answer 3

Simply We can do in scala is

scala> import util.control.Breaks._

scala> object TestBreak{
       def main(args : Array[String]){
       breakable {
       for (i <- 1 to 10){
       println(i)
       if (i == 5){
       break;
       } } } } }

output :

scala> TestBreak.main(Array())
1
2
3
4
5

Answer 4

Since there is no break in Scala yet, you could try to solve this problem with using a return-statement. Therefore you need to put your inner loop into a function, otherwise the return would skip the whole loop.

Scala 2.8 however includes a way to break

http://www.scala-lang.org/api/rc/scala/util/control/Breaks.html

Answer 5

// import following package
import scala.util.control._

// create a Breaks object as follows
val loop = new Breaks;

// Keep the loop inside breakable as follows
loop.breakable{
// Loop will go here
for(...){
   ....
   // Break will go here
   loop.break;
   }
}

use Break module http://www.tutorialspoint.com/scala/scala_break_statement.htm

Answer 6

The third-party breakable package is one possible alternative

https://github.com/erikerlandson/breakable

Example code:

scala> import com.manyangled.breakable._
import com.manyangled.breakable._

scala> val bkb2 = for {
     |   (x, xLab) <- Stream.from(0).breakable   // create breakable sequence with a method
     |   (y, yLab) <- breakable(Stream.from(0))  // create with a function
     |   if (x % 2 == 1) continue(xLab)          // continue to next in outer "x" loop
     |   if (y % 2 == 0) continue(yLab)          // continue to next in inner "y" loop
     |   if (x > 10) break(xLab)                 // break the outer "x" loop
     |   if (y > x) break(yLab)                  // break the inner "y" loop
     | } yield (x, y)
bkb2: com.manyangled.breakable.Breakable[(Int, Int)] = com.manyangled.breakable.Breakable@34dc53d2

scala> bkb2.toVector
res0: Vector[(Int, Int)] = Vector((2,1), (4,1), (4,3), (6,1), (6,3), (6,5), (8,1), (8,3), (8,5), (8,7), (10,1), (10,3), (10,5), (10,7), (10,9))