`Stack`

A Stack adds elements to its top. Elements are pushed to the top, and then popped from the top. The last element added to the Stack is the first element removed from the Stack.

It is possible to loop over a VB.NET Stack, push and pop elements, and also peek and count. The Stack can be used for parsing text.

Push example

We Push elements to the top of a Stack. Then, we use the For-Each loop to display the Stack. The most recently added element (1000) is at the top of the Stack.

Module Module1
    Sub Main()
        Dim stack As Stack(Of Integer) = New Stack(Of Integer)
        stack.Push(10)
        stack.Push(100)
        stack.Push(1000)

        ' Display stack elements.
        For Each value As Integer In stack
            Console.WriteLine(value)
        Next
    End Sub
End Module1000
100
10

Pop, Peek

We use the Pop and Peek functions. With Pop, you take the top element off of the Stack and it is stored in the return value. It is no longer is part the Stack.

Info With Peek, you take the top element and copy it to the return value, but it is not removed.

Tip To avoid exceptions with an empty stack, use the Count property and test it against zero.

Module Module1
    Sub Main()
        Dim stack As Stack(Of Integer) = New Stack(Of Integer)
        stack.Push(10)
        stack.Push(100)
        stack.Push(1000)

        ' Test Pop.
        Dim value As Integer = stack.Pop
        Console.WriteLine("Element popped: {0}", value)

        ' Test Peek.
        value = stack.Peek
        Console.WriteLine("Element peeked: {0}", value)
    End Sub
End ModuleElement popped: 1000
Element peeked: 100

Array

We show how to copy an array into a new Stack. Please note that the types of the two collections must match. The array of Strings must be copied to a Stack of Strings.

Next We use the Contains function, which works in much the same way as the List Contains function. It returns True or False.

Module Module1
    Sub Main()
        Dim array(2) As String
        array(0) = "dot"
        array(1) = "net"
        array(2) = "perls"

        ' Copy array to a new stack.
        Dim stack As Stack(Of String) = New Stack(Of String)(array)

        ' Use Contains.
        Console.WriteLine(stack.Contains("net"))
        Console.WriteLine(stack.Contains("not"))
    End Sub
End ModuleTrue
False

The Stack collection can be used to implement certain parsers that track nesting levels, such as HTML parsers. As each nested element is encountered, a new element is added to the Stack.

Stacks are useful in any system where the most recently added element is the most urgent. The VB.NET Stack type is used to implement a variety of important algorithms.