Scala for loop Example

In this article, we will see functional style of for expression. Let’s get started with the simplest form of for expression.

‘for’ Example

If you have come from java or c++, ‘<-‘ may look a bit odd, for now, think of it as ‘in’. Left side of ‘<-‘, we have declared a variable a, it is of type val and can’t be re-assigned within the block. In each loop, a character is assigned to it.

ForExample:

object ForExample {

  for (a <- "abc") print(a)

  def main(args: Array[String]): Unit = {
  }
}

Output:

abc

Let’s see one more example. Here we define an array, traverse through each element and print.

ForExample:

object ForExample {

  for (a <- "abc") print(a)

  println

  var array = Array("d", "e", "f")
  for (a <- array) print(a)

  def main(args: Array[String]): Unit = {
  }
}

Output:

abc
def

Use of ‘to’ in ‘for’ loop also called Generator

The to in this example is actually a method that takes one Int argument.
(0).to(2).1

object ForExample {

  for (a <- "abc") print(a)

  println

  var array = Array("d", "e", "f")
  for (a <- array) print(a)

  println

  array = Array("x", "y", "z")
  for (a <- 0 to 2) print(array(a))

  def main(args: Array[String]): Unit = {
  }
}

Output:

abc
def
xyz

Let’s further analyze the to function. In the below example, we use it as a method. Yo can eliminate the dot and parenthesis, if you explicitly specify the receiver of the method call. We call print(0 to 2) and it prints the object’s toString Range(0, 1, 2).

object ForExample {

  for (a <- "abc") print(a)

  println

  var array = Array("d", "e", "f")
  for (a <- array) print(a)

  println

  array = Array("x", "y", "z")
  for (a <- 0 to 2) print(array(a))
  
  println

  (0).to(2).foreach(print)

  println
  
  print(0 to 2)

  def main(args: Array[String]): Unit = {
  }
}

Output:

abc
def
xyz
012
Range(0, 1, 2)

If have seen how to feed a range of values using the to method. These ranges are also called as generators.

Multiple Generators

Let’s now see how to create multiple ranges.
Instead of inner for loops, you can provide the ranges in the same for loop. See the highlighted one.

MultipleRanges:

object MultipleRanges {

  val array = Array("a", "b", "c", "x", "y", "z")
  for (a <- 0 to 2) 
    for (b <- 3 to 5) 
      print(array(a) + array(b))
  
  println
  
  for (a <- 0 to 2; b <- 3 to 5) print(array(a) + array(b))
  

  def main(args: Array[String]): Unit = {
  }
}

Output:

axayazbxbybzcxcycz

Example of Guard

We can add filters to the for loop also called Guards. In the below example, we filter the odd indexed elements.

GuardExample:

object GuardExample {

  val array = Array("a0", "a1", "a2", "b0", "b1", "b2")

  for (a <- 0 to 5; if a % 2==0) print(array(a))
  
  def main(args: Array[String]): Unit = {
  }
}

Output:

a0a2b1

Multiple Guards

We can add multiple Guards. We now add one more filter, to filter elements which doesn’t start with ‘a’.

object GuardExample {

  val array = Array("a0", "a1", "a2", "b0", "b1", "b2")

  for (a <- 0 to 5; if a % 2==0) print(array(a))
  
  println
  
  for (a <- 0 to 5; if a % 2==0; if array(a).charAt(0) == 'a') print(array(a))
  

  def main(args: Array[String]): Unit = {
  }
}

Output:

a0a2b1
a0a2

If you think the ranges and filters in the for loop are getting bigger, you can use curly braces and have each statement in a separate line. It adds to readability.

object GuardExample {

  val array = Array("a0", "a1", "a2", "b0", "b1", "b2")

  for (a <- 0 to 5; if a % 2 == 0) print(array(a))

  println

  for (a <- 0 to 5; if a % 2 == 0; if array(a).charAt(0) == 'a') print(array(a))

  println

  for {
    a <- 0 to 5;
    if a % 2 == 0;
    if array(a).charAt(0) == 'a'
  } print(array(a))

  def main(args: Array[String]): Unit = {
  }
}

Download the source code

In this article, we have seen some examples of for loop in scala. You can download the source code here: forExample.zip