Stack<T>.Peek メソッド

定義

Stack<T> の先頭にあるオブジェクトを削除せずに返します。Returns the object at the top of the Stack<T> without removing it.

public:
 T Peek();
public T Peek ();
member this.Peek : unit -> 'T
Public Function Peek () As T

戻り値

T

Stack<T> の一番上にあるオブジェクト。The object at the top of the Stack<T>.

例外

Stack<T> が空です。The Stack<T> is empty.

次のコード例は、メソッドを含む、ジェネリッククラスのいくつかのメソッドを示して Stack<T> Peek います。The following code example demonstrates several methods of the Stack<T> generic class, including the Peek method.

このコード例では、既定の容量を持つ文字列のスタックを作成し、メソッドを使用して Push 5 つの文字列をスタックにプッシュします。The code example creates a stack of strings with default capacity and uses the Push method to push five strings onto the stack. スタックの要素が列挙され、スタックの状態が変更されることはありません。The elements of the stack are enumerated, which does not change the state of the stack. メソッドは、 Pop スタックから最初の文字列をポップするために使用されます。The Pop method is used to pop the first string off the stack. メソッドは、 Peek スタック上の次の項目を確認するために使用され、その後、メソッドを Pop 使用してポップします。The Peek method is used to look at the next item on the stack, and then the Pop method is used to pop it off.

ToArrayメソッドは、配列を作成してスタック要素をコピーするために使用されます。次に、によって取得されるコンストラクターに配列が渡され Stack<T> ます。その後 IEnumerable<T> 、要素の順序を逆にしてスタックのコピーが作成されます。The ToArray method is used to create an array and copy the stack elements to it, then the array is passed to the Stack<T> constructor that takes IEnumerable<T>, creating a copy of the stack with the order of the elements reversed. コピーの要素が表示されます。The elements of the copy are displayed.

スタックのサイズの2倍の配列が作成され、メソッドを使用して配列 CopyTo の途中から配列の要素がコピーされます。An array twice the size of the stack is created, and the CopyTo method is used to copy the array elements beginning at the middle of the array. Stack<T>コンストラクターは、要素の順序を逆にしてスタックのコピーを作成するために再び使用されます。したがって、3つの null 要素が末尾にあります。The Stack<T> constructor is used again to create a copy of the stack with the order of elements reversed; thus, the three null elements are at the end.

メソッドは、 Contains 文字列 "4" がスタックの最初のコピーに含まれていることを示すために使用されます。その後、 Clear メソッドはコピーをクリアし、 Count プロパティはスタックが空であることを示します。The Contains method is used to show that the string "four" is in the first copy of the stack, after which the Clear method clears the copy and the Count property shows that the stack is empty.

using System;
using System.Collections.Generic;

class Example
{
    public static void Main()
    {
        Stack<string> numbers = new Stack<string>();
        numbers.Push("one");
        numbers.Push("two");
        numbers.Push("three");
        numbers.Push("four");
        numbers.Push("five");

        // A stack can be enumerated without disturbing its contents.
        foreach( string number in numbers )
        {
            Console.WriteLine(number);
        }

        Console.WriteLine("\nPopping '{0}'", numbers.Pop());
        Console.WriteLine("Peek at next item to destack: {0}",
            numbers.Peek());
        Console.WriteLine("Popping '{0}'", numbers.Pop());

        // Create a copy of the stack, using the ToArray method and the
        // constructor that accepts an IEnumerable<T>.
        Stack<string> stack2 = new Stack<string>(numbers.ToArray());

        Console.WriteLine("\nContents of the first copy:");
        foreach( string number in stack2 )
        {
            Console.WriteLine(number);
        }

        // Create an array twice the size of the stack and copy the
        // elements of the stack, starting at the middle of the
        // array.
        string[] array2 = new string[numbers.Count * 2];
        numbers.CopyTo(array2, numbers.Count);

        // Create a second stack, using the constructor that accepts an
        // IEnumerable(Of T).
        Stack<string> stack3 = new Stack<string>(array2);

        Console.WriteLine("\nContents of the second copy, with duplicates and nulls:");
        foreach( string number in stack3 )
        {
            Console.WriteLine(number);
        }

        Console.WriteLine("\nstack2.Contains(\"four\") = {0}",
            stack2.Contains("four"));

        Console.WriteLine("\nstack2.Clear()");
        stack2.Clear();
        Console.WriteLine("\nstack2.Count = {0}", stack2.Count);
    }
}

/* This code example produces the following output:

five
four
three
two
one

Popping 'five'
Peek at next item to destack: four
Popping 'four'

Contents of the first copy:
one
two
three

Contents of the second copy, with duplicates and nulls:
one
two
three




stack2.Contains("four") = False

stack2.Clear()

stack2.Count = 0
 */
Imports System.Collections.Generic

Module Example

    Sub Main

        Dim numbers As New Stack(Of String)
        numbers.Push("one")
        numbers.Push("two")
        numbers.Push("three")
        numbers.Push("four")
        numbers.Push("five")

        ' A stack can be enumerated without disturbing its contents.
        For Each number As String In numbers
            Console.WriteLine(number)
        Next

        Console.WriteLine(vbLf & "Popping '{0}'", numbers.Pop())
        Console.WriteLine("Peek at next item to pop: {0}", _
            numbers.Peek())    
        Console.WriteLine("Popping '{0}'", numbers.Pop())

        ' Create another stack, using the ToArray method and the
        ' constructor that accepts an IEnumerable(Of T). Note that
        ' the order of items on the new stack is reversed.
        Dim stack2 As New Stack(Of String)(numbers.ToArray())

        Console.WriteLine(vbLf & "Contents of the first copy:")
        For Each number As String In stack2
            Console.WriteLine(number)
        Next
        
        ' Create an array twice the size of the stack, compensating
        ' for the fact that Visual Basic allocates an extra array 
        ' element. Copy the elements of the stack, starting at the
        ' middle of the array. 
        Dim array2((numbers.Count * 2) - 1) As String
        numbers.CopyTo(array2, numbers.Count)
        
        ' Create a second stack, using the constructor that accepts an
        ' IEnumerable(Of T). The elements are reversed, with the null
        ' elements appearing at the end of the stack when enumerated.
        Dim stack3 As New Stack(Of String)(array2)

        Console.WriteLine(vbLf & _
            "Contents of the second copy, with duplicates and nulls:")
        For Each number As String In stack3
            Console.WriteLine(number)
        Next

        Console.WriteLine(vbLf & "stack2.Contains(""four"") = {0}", _
            stack2.Contains("four"))

        Console.WriteLine(vbLf & "stack2.Clear()")
        stack2.Clear()
        Console.WriteLine(vbLf & "stack2.Count = {0}", _
            stack2.Count)
    End Sub
End Module

' This code example produces the following output:
'
'five
'four
'three
'two
'one
'
'Popping 'five'
'Peek at next item to pop: four
'Popping 'four'
'
'Contents of the first copy:
'one
'two
'three
'
'Contents of the second copy, with duplicates and nulls:
'one
'two
'three
'
'
'
'
'stack2.Contains("four") = False
'
'stack2.Clear()
'
'stack2.Count = 0

注釈

このメソッドは Pop メソッドに類似していますが、PeekStack<T> を変更しません。This method is similar to the Pop method, but Peek does not modify the Stack<T>.

T が参照型の場合、必要に応じて null をプレースホルダーとして Stack<T> にプッシュすることができます。If type T is a reference type, null can be pushed onto the Stack<T> as a placeholder, if needed.

このメソッドは、O(1) 操作です。This method is an O(1) operation.

適用対象

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