Stack.Peek Method

Definition

Namespace:

System.Collections

Assembly:

System.Collections.NonGeneric.dll

Source:

Stack.cs

Returns the object at the top of the Stack without removing it.

public:
 virtual System::Object ^ Peek();

public virtual object? Peek ();

abstract member Peek : unit -> obj
override this.Peek : unit -> obj

Public Overridable Function Peek () As Object

Returns

Object

The Object at the top of the Stack.

Exceptions

InvalidOperationException

The Stack is empty.

Examples

The following example shows how to add elements to the Stack, remove elements from the Stack, or view the element at the top of the Stack.

using namespace System;
using namespace System::Collections;
void PrintValues( IEnumerable^ myCollection, char mySeparator );
int main()
{
   
   // Creates and initializes a new Stack.
   Stack^ myStack = gcnew Stack;
   myStack->Push( "The" );
   myStack->Push( "quick" );
   myStack->Push( "brown" );
   myStack->Push( "fox" );
   
   // Displays the Stack.
   Console::Write( "Stack values:" );
   PrintValues( myStack, '\t' );
   
   // Removes an element from the Stack.
   Console::WriteLine( "(Pop)\t\t{0}", myStack->Pop() );
   
   // Displays the Stack.
   Console::Write( "Stack values:" );
   PrintValues( myStack, '\t' );
   
   // Removes another element from the Stack.
   Console::WriteLine( "(Pop)\t\t{0}", myStack->Pop() );
   
   // Displays the Stack.
   Console::Write( "Stack values:" );
   PrintValues( myStack, '\t' );
   
   // Views the first element in the Stack but does not remove it.
   Console::WriteLine( "(Peek)\t\t{0}", myStack->Peek() );
   
   // Displays the Stack.
   Console::Write( "Stack values:" );
   PrintValues( myStack, '\t' );
}

void PrintValues( IEnumerable^ myCollection, char mySeparator )
{
   IEnumerator^ myEnum = myCollection->GetEnumerator();
   while ( myEnum->MoveNext() )
   {
      Object^ obj = safe_cast<Object^>(myEnum->Current);
      Console::Write( "{0}{1}", mySeparator, obj );
   }

   Console::WriteLine();
}

/* 
 This code produces the following output.
 
 Stack values:    fox    brown    quick    The
 (Pop)        fox
 Stack values:    brown    quick    The
 (Pop)        brown
 Stack values:    quick    The
 (Peek)        quick
 Stack values:    quick    The
 */

using System;
using System.Collections;
public class SamplesStack  {

   public static void Main()  {

      // Creates and initializes a new Stack.
      Stack myStack = new Stack();
      myStack.Push( "The" );
      myStack.Push( "quick" );
      myStack.Push( "brown" );
      myStack.Push( "fox" );

      // Displays the Stack.
      Console.Write( "Stack values:" );
      PrintValues( myStack, '\t' );

      // Removes an element from the Stack.
      Console.WriteLine( "(Pop)\t\t{0}", myStack.Pop() );

      // Displays the Stack.
      Console.Write( "Stack values:" );
      PrintValues( myStack, '\t' );

      // Removes another element from the Stack.
      Console.WriteLine( "(Pop)\t\t{0}", myStack.Pop() );

      // Displays the Stack.
      Console.Write( "Stack values:" );
      PrintValues( myStack, '\t' );

      // Views the first element in the Stack but does not remove it.
      Console.WriteLine( "(Peek)\t\t{0}", myStack.Peek() );

      // Displays the Stack.
      Console.Write( "Stack values:" );
      PrintValues( myStack, '\t' );
   }

   public static void PrintValues( IEnumerable myCollection, char mySeparator )  {
      foreach ( Object obj in myCollection )
         Console.Write( "{0}{1}", mySeparator, obj );
      Console.WriteLine();
   }
}


/*
This code produces the following output.

Stack values:    fox    brown    quick    The
(Pop)        fox
Stack values:    brown    quick    The
(Pop)        brown
Stack values:    quick    The
(Peek)        quick
Stack values:    quick    The
*/

Imports System.Collections

Public Class SamplesStack    

    Public Shared Sub Main()

        ' Creates and initializes a new Stack.
        Dim myStack As New Stack()
        myStack.Push("The")
        myStack.Push("quick")
        myStack.Push("brown")
        myStack.Push("fox")

        ' Displays the Stack.
        Console.Write("Stack values:")
        PrintValues(myStack, ControlChars.Tab)

        ' Removes an element from the Stack.
        Console.WriteLine("(Pop)" & ControlChars.Tab & ControlChars.Tab & _
           "{0}", myStack.Pop())

        ' Displays the Stack.
        Console.Write("Stack values:")
        PrintValues(myStack, ControlChars.Tab)

        ' Removes another element from the Stack.
        Console.WriteLine("(Pop)" & ControlChars.Tab & ControlChars.Tab & _
           "{0}", myStack.Pop())

        ' Displays the Stack.
        Console.Write("Stack values:")
        PrintValues(myStack, ControlChars.Tab)

        ' Views the first element in the Stack but does not remove it.
        Console.WriteLine("(Peek)" & ControlChars.Tab & ControlChars.Tab & _
           "{0}", myStack.Peek())

        ' Displays the Stack.
        Console.Write("Stack values:")
        PrintValues(myStack, ControlChars.Tab)

    End Sub

    Public Shared Sub PrintValues(myCollection As IEnumerable, mySeparator As Char)
        Dim obj As [Object]
        For Each obj In  myCollection
            Console.Write("{0}{1}", mySeparator, obj)
        Next obj
        Console.WriteLine()
    End Sub

End Class


' This code produces the following output.
' 
' Stack values:    fox    brown    quick    The
' (Pop)        fox
' Stack values:    brown    quick    The
' (Pop)        brown
' Stack values:    quick    The
' (Peek)        quick
' Stack values:    quick    The

Remarks

This method is similar to the Pop method, but Peek does not modify the Stack.

null can be pushed onto the Stack as a placeholder, if needed. To distinguish between a null value and the end of the stack, check the Count property or catch the InvalidOperationException, which is thrown when the Stack is empty.

This method is an O(1) operation.

See also

• Pop()
• Push(Object)