Queue<T>.Peek Method
Definition
Important
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Returns the object at the beginning of the Queue<T> without removing it.
public:
T Peek();public T Peek ();member this.Peek : unit -> 'TPublic Function Peek () As TReturns
The object at the beginning of the Queue<T>.
Exceptions
The Queue<T> is empty.
Examples
The following code example demonstrates several methods of the Queue<T> generic class, including the Peek method.
The code example creates a queue of strings with default capacity and uses the Enqueue method to queue five strings. The elements of the queue are enumerated, which does not change the state of the queue. The Dequeue method is used to dequeue the first string. The Peek method is used to look at the next item in the queue, and then the Dequeue method is used to dequeue it.
The ToArray method is used to create an array and copy the queue elements to it, then the array is passed to the Queue<T> constructor that takes IEnumerable<T>, creating a copy of the queue. The elements of the copy are displayed.
An array twice the size of the queue is created, and the CopyTo method is used to copy the array elements beginning at the middle of the array. The Queue<T> constructor is used again to create a second copy of the queue containing three null elements at the beginning.
The Contains method is used to show that the string "four" is in the first copy of the queue, after which the Clear method clears the copy and the Count property shows that the queue is empty.
using System;
using System.Collections.Generic;
class Example
{
public static void Main()
{
Queue<string> numbers = new Queue<string>();
numbers.Enqueue("one");
numbers.Enqueue("two");
numbers.Enqueue("three");
numbers.Enqueue("four");
numbers.Enqueue("five");
// A queue can be enumerated without disturbing its contents.
foreach( string number in numbers )
{
Console.WriteLine(number);
}
Console.WriteLine("\nDequeuing '{0}'", numbers.Dequeue());
Console.WriteLine("Peek at next item to dequeue: {0}",
numbers.Peek());
Console.WriteLine("Dequeuing '{0}'", numbers.Dequeue());
// Create a copy of the queue, using the ToArray method and the
// constructor that accepts an IEnumerable<T>.
Queue<string> queueCopy = new Queue<string>(numbers.ToArray());
Console.WriteLine("\nContents of the first copy:");
foreach( string number in queueCopy )
{
Console.WriteLine(number);
}
// Create an array twice the size of the queue and copy the
// elements of the queue, starting at the middle of the
// array.
string[] array2 = new string[numbers.Count * 2];
numbers.CopyTo(array2, numbers.Count);
// Create a second queue, using the constructor that accepts an
// IEnumerable(Of T).
Queue<string> queueCopy2 = new Queue<string>(array2);
Console.WriteLine("\nContents of the second copy, with duplicates and nulls:");
foreach( string number in queueCopy2 )
{
Console.WriteLine(number);
}
Console.WriteLine("\nqueueCopy.Contains(\"four\") = {0}",
queueCopy.Contains("four"));
Console.WriteLine("\nqueueCopy.Clear()");
queueCopy.Clear();
Console.WriteLine("\nqueueCopy.Count = {0}", queueCopy.Count);
}
}
/* This code example produces the following output:
one
two
three
four
five
Dequeuing 'one'
Peek at next item to dequeue: two
Dequeuing 'two'
Contents of the first copy:
three
four
five
Contents of the second copy, with duplicates and nulls:
three
four
five
queueCopy.Contains("four") = True
queueCopy.Clear()
queueCopy.Count = 0
*/
open System
open System.Collections.Generic
let numbers = Queue()
numbers.Enqueue "one"
numbers.Enqueue "two"
numbers.Enqueue "three"
numbers.Enqueue "four"
numbers.Enqueue "five"
// A queue can be enumerated without disturbing its contents.
for number in numbers do
printfn $"{number}"
printfn $"\nDequeuing '{numbers.Dequeue()}'"
printfn $"Peek at next item to dequeue: {numbers.Peek()}"
printfn $"Dequeuing '{numbers.Dequeue()}'"
// Create a copy of the queue, using the ToArray method and the
// constructor that accepts an IEnumerable<T>.
let queueCopy = numbers.ToArray() |> Queue
printfn $"\nContents of the first copy:"
for number in queueCopy do
printfn $"{number}"
// Create an array twice the size of the queue and copy the
// elements of the queue, starting at the middle of the
// array.
let array2 = numbers.Count * 2 |> Array.zeroCreate
numbers.CopyTo(array2, numbers.Count)
// Create a second queue, using the constructor that accepts an
// IEnumerable(Of T).
let queueCopy2 = Queue array2
printfn $"\nContents of the second copy, with duplicates and nulls:"
for number in queueCopy2 do
printfn $"{number}"
printfn $"""\nqueueCopy.Contains "four" = {queueCopy.Contains "four"}"""
printfn $"\nqueueCopy.Clear()"
queueCopy.Clear()
printfn $"queueCopy.Count = {queueCopy.Count}"
// This code example produces the following output:
// one
// two
// three
// four
// five
//
// Dequeuing 'one'
// Peek at next item to dequeue: two
// Dequeuing 'two'
//
// Contents of the first copy:
// three
// four
// five
//
// Contents of the second copy, with duplicates and nulls:
//
//
//
// three
// four
// five
//
// queueCopy.Contains "four" = True
//
// queueCopy.Clear()
//
// queueCopy.Count = 0
Imports System.Collections.Generic
Module Example
Sub Main
Dim numbers As New Queue(Of String)
numbers.Enqueue("one")
numbers.Enqueue("two")
numbers.Enqueue("three")
numbers.Enqueue("four")
numbers.Enqueue("five")
' A queue can be enumerated without disturbing its contents.
For Each number As String In numbers
Console.WriteLine(number)
Next
Console.WriteLine(vbLf & "Dequeuing '{0}'", numbers.Dequeue())
Console.WriteLine("Peek at next item to dequeue: {0}", _
numbers.Peek())
Console.WriteLine("Dequeuing '{0}'", numbers.Dequeue())
' Create a copy of the queue, using the ToArray method and the
' constructor that accepts an IEnumerable(Of T).
Dim queueCopy As New Queue(Of String)(numbers.ToArray())
Console.WriteLine(vbLf & "Contents of the first copy:")
For Each number As String In queueCopy
Console.WriteLine(number)
Next
' Create an array twice the size of the queue, compensating
' for the fact that Visual Basic allocates an extra array
' element. Copy the elements of the queue, starting at the
' middle of the array.
Dim array2((numbers.Count * 2) - 1) As String
numbers.CopyTo(array2, numbers.Count)
' Create a second queue, using the constructor that accepts an
' IEnumerable(Of T).
Dim queueCopy2 As New Queue(Of String)(array2)
Console.WriteLine(vbLf & _
"Contents of the second copy, with duplicates and nulls:")
For Each number As String In queueCopy2
Console.WriteLine(number)
Next
Console.WriteLine(vbLf & "queueCopy.Contains(""four"") = {0}", _
queueCopy.Contains("four"))
Console.WriteLine(vbLf & "queueCopy.Clear()")
queueCopy.Clear()
Console.WriteLine(vbLf & "queueCopy.Count = {0}", _
queueCopy.Count)
End Sub
End Module
' This code example produces the following output:
'
'one
'two
'three
'four
'five
'
'Dequeuing 'one'
'Peek at next item to dequeue: two
'
'Dequeuing 'two'
'
'Contents of the copy:
'three
'four
'five
'
'Contents of the second copy, with duplicates and nulls:
'
'
'
'three
'four
'five
'
'queueCopy.Contains("four") = True
'
'queueCopy.Clear()
'
'queueCopy.Count = 0
Remarks
This method is similar to the Dequeue method, but Peek does not modify the Queue<T>.
If type T is a reference type, null can be added to the Queue<T> as a value.
This method is an O(1) operation.
