Stack in Scala

A stack is a data structure that follows the last-in, first-out(LIFO) principle. We can add or remove element only from one end called top. Scala has both mutable and immutable versions of a stack.
Syntax :

import scala.collection.mutable.Stack
var s = Stack[type]()

// OR
var s = Stack(val1, val2, val3, ...)

Operations on Stack

Once stack has been created we can either push elements to the stack or pop them out of the stack.

• Push: We can push element of any type to the stack using push() function. All elements must have same data type.
  Example :

  // Scala program to
  // push element
  // to the stack
  
  import scala.collection.mutable.Stack
  
  // Creating object
  object GfG
  {
  
      // Main method
      def main(args:Array[String])
      {
  
          var s = Stack[Int]()
  
          // pushing values
          // one at a time
          s.push(5)
          s.push(1)
          s.push(2)
          println("s:" + s)
  
          var s2 = Stack[Int]()
  
          // pushing multiple values
          s2.push(5,1,2)
          println("s2:" + s2)
  
      }
  }

  Output:

  s:Stack(2, 1, 5)
  s2:Stack(2, 1, 5)

• Pop: We can pop element from top of the stack using pop function. The function returns the same type as that of elements of the stack.
  Example :

  // Scala program to
  // pop element from
  // top of the stack
  
  import scala.collection.mutable.Stack
  
  // Creating object
  object GfG
  {
  
      // Main method
      def main(args:Array[String])
      {
  
          var s = Stack[Int]()
  
          s.push(5)
          s.push(1)
          s.push(2)
          println(s)
  
          // pop element from
          // top of the stack
  
          println("Popped:" + s.pop)
          println("Popped:" + s.pop)
          println("Popped:" + s.pop)
      }
  }

  Output:

  Stack(2, 1, 5)
  Popped:2
  Popped:1
  Popped:5

Other Functions

Other Functions :
Let’s discuss some more functions with examples.

• isEmpty: To check whether the stack is empty. Returns true if it is empty.
  Example :

  // Scala program to
  // check if the stack
  // is empty
  
  import scala.collection.mutable.Stack
  
  // Creating object
  object GfG
  {
  
      // Main method
      def main(args:Array[String])
      {
  
          var s = Stack[Int]()
  
          s.push(5)
          s.push(1)
          s.push(2)
          println(s)
  
          // pop element from
          // top of the stack
  
          println("Popped:" + s.pop)
          println("Popped:" + s.pop)
          println("Empty:" + s.isEmpty)
          println("Popped:" + s.pop)
  
          // all three elements popped
          println("Empty:" + s.isEmpty)
      }
  }

  Output:

  Stack(2, 1, 5)
  Popped:2
  Popped:1
  Empty:false
  Popped:5
  Empty:true

• top: Returns the element that is currently at the top of the stack.
  Example :

  // Scala program to
  // print top of stack
  
  import scala.collection.mutable.Stack
  
  // Creating object
  object GfG
  {
  
      // Main method
      def main(args:Array[String])
      {
  
          var s = Stack[Int]()
  
          s.push(5)
          s.push(1)
          s.push(2)
          println(s)
          println("Top: " + s.top)
          println("Popped:" + s.pop)
          println("Top: " + s.top)
      }
  }

  Output:

  Stack(2, 1, 5)
  Top: 2
  Popped:2
  Top: 1

• size:Returns the number of elements present in the stack.
  Example :

  // Scala program to
  // print size of the stack
  
  import scala.collection.mutable.Stack
  
  // Creating object
  object GfG
  {
  
      // Main method
      def main(args:Array[String])
      {
  
          var s = Stack[Int]()
  
          s.push(5)
          s.push(1)
          s.push(2)
          println(s)
          println("Size: " + s.size)
          println("Popped:" + s.pop)
          println("Size: " + s.size)
      }
  }

  Output:

  Stack(2, 1, 5)
  Size: 3
  Popped:2
  Size: 2