Section 15.5. Stacks

15.5 Stacks

A stack is a last-in first-out (LIFO) collection, like a stack of dishes at a buffet table or a stack of coins on your desk. You add a dish on top, and it is the first dish you take off the stack.

The classic example of a stack is the stack, the portion of memory on which parameters and local variables are stored. See Chapter 8 for more about the stack.

The principal methods for adding to and removing from an instance of the Stack class are Push( ) and Pop( ); Stack also offers a Peek( ) method, very much like Queue. The most important methods and properties for Stack are shown in Table 15-5.

Table 15-5. Stack members

Method or property	Purpose
Count	Public property that gets the number of elements in the Stack
Clear( )	Method that removes all objects from the Stack
Contains( )	Method that determines if an element is in the Stack
CopyTo( )	Method that copies the Stack elements to an existing one-dimensional array
GetEnumerator( )	Method that returns an enumerator for the Stack
Peek( )	Method that returns the object at the top of the Stack without removing it
Pop( )	Method that removes and returns the object at the top of the Stack
Push( )	Method that inserts an object at the top of the Stack
ToArray( )	Method that copies the elements to a new array

In Example 15-5, you rewrite Example 15-4 to use a Stack rather than a Queue. The logic is almost identical. The key difference is that a Stack is Last In First Out, while a Queue is First In First Out.

Example 15-5. Using a Stack

 Option Strict On Imports System Namespace StackDemo     Class Tester         Public Sub Run( )             Dim intStack As New Stack( )             ' populate the stack             Dim i As Integer             For i = 0 To 7                 intStack.Push((i * 5))             Next i             ' Display the Stack.             Console.WriteLine("intStack values:")             DisplayValues(intStack)             ' Remove an element from the stack.             Console.WriteLine("(Pop){0}", intStack.Pop( ))             ' Display the Stack.             Console.WriteLine("intStack values:")             DisplayValues(intStack)             ' Remove another element from the stack.             Console.WriteLine("(Pop){0}", intStack.Pop( ))             ' Display the Stack.             Console.WriteLine("intStack values:")             DisplayValues(intStack)             ' View the first element in the              ' Stack but do not remove.             Console.WriteLine("(Peek)   {0}", intStack.Peek( ))             ' Display the Stack.             Console.WriteLine("intStack values:")             DisplayValues(intStack)         End Sub 'Run         Public Shared Sub DisplayValues(ByVal myCollection As IEnumerable)             Dim o As Object             For Each o In myCollection                 Console.WriteLine(o)             Next o         End Sub 'DisplayValues         Shared Sub Main( )             Dim t As New Tester( )             t.Run( )         End Sub 'Main     End Class 'Tester End Namespace 'StackDemo 

  Output:  intStack values: 35 30 25 20 15 10 5 0 (Pop)35 intStack values: 30 25 20 15 10 5 0 (Pop)30 intStack values: 25 20 15 10 5 0 (Peek)   25 intStack values: 25 20 15 10 5 

You start Example 15-5 by creating a Stack object called intStack:

 Dim intStack As New Stack( ) 

You populate the stack with integers by calling the Push( ) method, which pushes each integer object onto the stack (i.e., adds it to the top of the Stack):

 For i = 0 To 7     intStack.Push((i * 5)) Next i 

You remove an object from the stack by popping it off the stack with the Pop( ) method:

 Console.WriteLine("(Pop){0}", intStack.Pop( )) 

Just as you could peek at the object at the beginning of the Queue without dequeing it, you can Peek( ) at the object on top of the stack without popping it:

 Console.WriteLine("(Peek)   {0}", intStack.Peek( ))