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, ...)

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 :
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