Basic Scala for loop Issue

Question

I am trying to learn scala, here I am using basic for loop, but I am getting errors while compiling.

object App {    
  def main(args: Array[String]) {

    for (i <- 1 to 10; i % 2 == 0)
      Console.println("Counting " + i)

  }
}

Errors while compiling :

fortest.scala:5: error: '<-' expected but ')' found.

    for (i <- 1 to 10; i % 2 == 0)
                                 ^
fortest.scala:7: error: illegal start of simple expression

  }
  ^

two errors found

I am using scala version 2.9.1

Any idea what is the problem..............?

dhg · Accepted Answer · 2012-11-01 06:51:52Z

8

for (i <- 1 to 10 if i % 2 == 0)
  println("Counting " + i)

answered Nov 1, 2012 at 6:51

dhg

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Comments

gilad hoch · Accepted Answer · 2012-11-01 13:59:22Z

7

Scala is not Java, thus you cannot use a regular Java syntax. Instead you have to do:

for{
    i <- 1 to 10
    if(i % 2 == 0)
}{println("Counting " + i)}

or with ; delimeters, inside the (,) parentheses:

for(i <- 1 to 10;if(i % 2 == 0)){
    println("Counting " + i)
}

Also, note that Scala's for expressions, have some pretty nifty capabilities. you can use a for expression with multiple "loop iterators" and conditions.

For instance, instead of writing:

for(i <- 1 to n; if(someCondition(i)){
    for(j <- 1 to m; if(otherCondition(j)){
         //Do something
    }
}

You can simply write:

for{
    i <- 1 to n
    if(someCondition(i))
    j <- 1 to m
    if(otherCondition(j))
}{
    //Do something
}

SIDE NOTE:
When you extend App (there's a trait of that name in Predef), you don't need to define a main method. You can simply write your code between the curly braces of object:

object MyClazz extends App {
    for(i <- 1 to 10;if(i % 2 == 0)){
        println("Counting " + i)
    }
}

edited Nov 1, 2012 at 13:59

answered Nov 1, 2012 at 6:45

gilad hoch

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3 Comments

Cloud Over a year ago

Thanks, what the basic concept behind scala, which prevents multiple conditions in a for loop?

gilad hoch Over a year ago

you can invoke as many conditions as you want. just add more if lines... (i'll edit my answer to include more info on that)

Sergey Passichenko Over a year ago

you can skip semicolon between generator and if-guard: for (i <- 1 to 10 if i % 2 == 0) println(i)

RKumsher · Accepted Answer · 2012-11-01 19:58:34Z

6

Take a look at the "by" method of the Range class to count by 2

object App {    
  def main(args: Array[String]) {
    for (i <- 2 to 10 by 2)
      Console.println("Counting " + i)
  }
}

Or, like others have already stated you can fix your loop by doing

object App {    
  def main(args: Array[String]) {
    for {
         i <- 1 to 10
         if i % 2 == 0
        }
      Console.println("Counting " + i)
  }
}

Or another way:

object App {    
  def main(args: Array[String]) {
    val evenNumbers = for {
         i <- 1 to 10
         if i % 2 == 0
        } yield i
    Console.println(evenNumbers.mkString("\n"))
  }
}

edited Nov 1, 2012 at 19:58

answered Nov 1, 2012 at 19:48

RKumsher

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Comments

Brian · Accepted Answer · 2012-11-01 06:45:46Z

1

The modulo 2 condition can be moved to an if clause.

object App {    
  def main(args: Array[String]) {

    for (i <- 1 to 10)
      if(i % 2 == 0)
      Console.println("Counting " + i)

  }

}

answered Nov 1, 2012 at 6:45

Brian

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Comments

Haimei · Accepted Answer · 2014-11-13 02:36:09Z

0

Here is the simply example;

 for (i <- List(1, 2, 3) if i < 2) println(i)

The best way to exam your code is to use scala shell.

answered Nov 13, 2014 at 2:36

Haimei

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Comments

Community · Accepted Answer · 2018-06-02 19:59:14Z

0

Basically, you are trying to use for-loop + iterator gaurd. Please find below syntax

for ( i <- 1 to 10 if (i%2==0) ) yield i

edited Jun 2, 2018 at 19:59

CommunityBot

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answered Oct 24, 2017 at 13:59

Shiva Garg

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1 Comment

Mathieu VIALES Over a year ago

I don't think that your answer, being the 7th on the topic, adds much to this thread.

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