List<T>.Find(Predicate<T>) Método
Definición
Importante
Parte de la información hace referencia a la versión preliminar del producto, que puede haberse modificado sustancialmente antes de lanzar la versión definitiva. Microsoft no otorga ninguna garantía, explícita o implícita, con respecto a la información proporcionada aquí.
Busca un elemento que coincida con las condiciones definidas por el predicado especificado y devuelve la primera aparición en toda la matriz List<T>.
public:
T Find(Predicate<T> ^ match);
public T Find (Predicate<T> match);
public T? Find (Predicate<T> match);
member this.Find : Predicate<'T> -> 'T
Public Function Find (match As Predicate(Of T)) As T
Parámetros
- match
- Predicate<T>
Delegado Predicate<T> que define las condiciones del elemento que se va a buscar.
Devoluciones
Primer elemento que coincide con las condiciones definidas por el predicado especificado, si se encuentra; de lo contrario, valor predeterminado para el tipo T
.
Excepciones
match
es null
.
Ejemplos
En el ejemplo siguiente se muestra el Find método en un List<T> objeto que contiene un objeto complejo simple.
using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify a part
// but the part name can change.
public class Part : IEquatable<Part>
{
public string PartName { get; set; }
public int PartId { get; set; }
public override string ToString()
{
return "ID: " + PartId + " Name: " + PartName;
}
public override bool Equals(object obj)
{
if (obj == null) return false;
Part objAsPart = obj as Part;
if (objAsPart == null) return false;
else return Equals(objAsPart);
}
public override int GetHashCode()
{
return PartId;
}
public bool Equals(Part other)
{
if (other == null) return false;
return (this.PartId.Equals(other.PartId));
}
// Should also override == and != operators.
}
public class Example
{
public static void Main()
{
// Create a list of parts.
List<Part> parts = new List<Part>();
// Add parts to the list.
parts.Add(new Part() { PartName = "crank arm", PartId = 1234 });
parts.Add(new Part() { PartName = "chain ring", PartId = 1334 });
parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ;
// Write out the parts in the list. This will call the overridden ToString method
// in the Part class.
Console.WriteLine();
foreach (Part aPart in parts)
{
Console.WriteLine(aPart);
}
// Check the list for part #1734. This calls the IEquatable.Equals method
// of the Part class, which checks the PartId for equality.
Console.WriteLine("\nContains: Part with Id=1734: {0}",
parts.Contains(new Part { PartId = 1734, PartName = "" }));
// Find items where name contains "seat".
Console.WriteLine("\nFind: Part where name contains \"seat\": {0}",
parts.Find(x => x.PartName.Contains("seat")));
// Check if an item with Id 1444 exists.
Console.WriteLine("\nExists: Part with Id=1444: {0}",
parts.Exists(x => x.PartId == 1444));
/*This code example produces the following output:
ID: 1234 Name: crank arm
ID: 1334 Name: chain ring
ID: 1434 Name: regular seat
ID: 1444 Name: banana seat
ID: 1534 Name: cassette
ID: 1634 Name: shift lever
Contains: Part with Id=1734: False
Find: Part where name contains "seat": ID: 1434 Name: regular seat
Exists: Part with Id=1444: True
*/
}
}
Imports System.Collections.Generic
' Simple business object. A PartId is used to identify a part
' but the part name can change.
Public Class Part
Implements IEquatable(Of Part)
Public Property PartName() As String
Get
Return m_PartName
End Get
Set(value As String)
m_PartName = Value
End Set
End Property
Private m_PartName As String
Public Property PartId() As Integer
Get
Return m_PartId
End Get
Set(value As Integer)
m_PartId = Value
End Set
End Property
Private m_PartId As Integer
Public Overrides Function ToString() As String
Return Convert.ToString("ID: " & PartId & " Name: ") & PartName
End Function
Public Overrides Function Equals(obj As Object) As Boolean
If obj Is Nothing Then
Return False
End If
Dim objAsPart As Part = TryCast(obj, Part)
If objAsPart Is Nothing Then
Return False
Else
Return Equals(objAsPart)
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return PartId
End Function
Public Overloads Function Equals(other As Part) As Boolean _
Implements IEquatable(Of Part).Equals
If other Is Nothing Then
Return False
End If
Return (Me.PartId.Equals(other.PartId))
End Function
' Should also override == and != operators.
End Class
Public Class Example
Public Shared Sub Main()
' Create a list of parts.
Dim parts As New List(Of Part)()
' Add parts to the list.
parts.Add(New Part() With { _
.PartName = "crank arm", _
.PartId = 1234 _
})
parts.Add(New Part() With { _
.PartName = "chain ring", _
.PartId = 1334 _
})
parts.Add(New Part() With { _
.PartName = "regular seat", _
.PartId = 1434 _
})
parts.Add(New Part() With { _
.PartName = "banana seat", _
.PartId = 1444 _
})
parts.Add(New Part() With { _
.PartName = "cassette", _
.PartId = 1534 _
})
parts.Add(New Part() With { _
.PartName = "shift lever", _
.PartId = 1634 _
})
' Write out the parts in the list. This will call the overridden ToString method
' in the Part class.
Console.WriteLine()
For Each aPart As Part In parts
Console.WriteLine(aPart)
Next
' Check the list for part #1734. This calls the IEquatable.Equals method
' of the Part class, which checks the PartId for equality.
Console.WriteLine(vbLf & "Contains: Part with Id=1734: {0}",
parts.Contains(New Part() With { _
.PartId = 1734, _
.PartName = "" _
}))
' Find items where name contains "seat".
Console.WriteLine(vbLf & "Find: Part where name contains ""seat"": {0}",
parts.Find(Function(x) x.PartName.Contains("seat")))
' Check if an item with Id 1444 exists.
Console.WriteLine(vbLf & "Exists: Part with Id=1444: {0}",
parts.Exists(Function(x) x.PartId = 1444))
'This code example produces the following output:
'
' ID: 1234 Name: crank arm
' ID: 1334 Name: chain ring
' ID: 1434 Name: regular seat
' ID: 1444 Name: banana seat
' ID: 1534 Name: cassette
' ID: 1634 Name: shift lever
'
' Contains: Part with Id=1734: False
'
' Find: Part where name contains "seat": ID: 1434 Name: regular seat
'
' Exists: Part with Id=1444: True
'
End Sub
End Class
En el ejemplo siguiente se muestran los métodos find de la List<T> clase . El ejemplo de la List<T> clase contiene book
objetos, de clase Book
, utilizando los datos del archivo XML de ejemplo: Libros (LINQ to XML). El FillList
método del ejemplo usa LINQ to XML para analizar los valores del XML a los valores de propiedad de los book
objetos .
En la tabla siguiente se describen los ejemplos proporcionados para los métodos de búsqueda.
Método | Ejemplo |
---|---|
Find(Predicate<T>) | Busca un libro por un identificador mediante el delegado de IDToFind predicado.En el ejemplo de C# se usa un delegado anónimo. |
FindAll(Predicate<T>) | Busque todos los libros cuya Genre propiedad es "Computer" mediante el delegado de FindComputer predicado. |
FindLast(Predicate<T>) | Busca el último libro de la colección que tiene una fecha de publicación anterior a 2001, mediante el delegado de PubBefore2001 predicado.En el ejemplo de C# se usa un delegado anónimo. |
FindIndex(Predicate<T>) | Busca el índice del primer libro de equipos mediante el delegado de FindComputer predicado. |
FindLastIndex(Predicate<T>) | Busca el índice del último libro de equipos mediante el delegado de FindComputer predicado. |
FindIndex(Int32, Int32, Predicate<T>) | Busca el índice del primer libro de equipos en la segunda mitad de la colección, utilizando el delegado de FindComputer predicado. |
FindLastIndex(Int32, Int32, Predicate<T>) | Busca el índice del último libro de equipos en la segunda mitad de la colección, utilizando el delegado de FindComputer predicado. |
using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;
namespace Find
{
class Program
{
private static string IDtoFind = "bk109";
private static List<Book> Books = new List<Book>();
public static void Main(string[] args)
{
FillList();
// Find a book by its ID.
Book result = Books.Find(
delegate(Book bk)
{
return bk.ID == IDtoFind;
}
);
if (result != null)
{
DisplayResult(result, "Find by ID: " + IDtoFind);
}
else
{
Console.WriteLine("\nNot found: {0}", IDtoFind);
}
// Find last book in collection published before 2001.
result = Books.FindLast(
delegate(Book bk)
{
DateTime year2001 = new DateTime(2001,01,01);
return bk.Publish_date < year2001;
});
if (result != null)
{
DisplayResult(result, "Last book in collection published before 2001:");
}
else
{
Console.WriteLine("\nNot found: {0}", IDtoFind);
}
// Find all computer books.
List<Book> results = Books.FindAll(FindComputer);
if (results.Count != 0)
{
DisplayResults(results, "All computer:");
}
else
{
Console.WriteLine("\nNo books found.");
}
// Find all books under $10.00.
results = Books.FindAll(
delegate(Book bk)
{
return bk.Price < 10.00;
}
);
if (results.Count != 0)
{
DisplayResults(results, "Books under $10:");
}
else
{
Console.WriteLine("\nNo books found.");
}
// Find index values.
Console.WriteLine();
int ndx = Books.FindIndex(FindComputer);
Console.WriteLine("Index of first computer book: {0}", ndx);
ndx = Books.FindLastIndex(FindComputer);
Console.WriteLine("Index of last computer book: {0}", ndx);
int mid = Books.Count / 2;
ndx = Books.FindIndex(mid, mid, FindComputer);
Console.WriteLine("Index of first computer book in the second half of the collection: {0}", ndx);
ndx = Books.FindLastIndex(Books.Count - 1, mid, FindComputer);
Console.WriteLine("Index of last computer book in the second half of the collection: {0}", ndx);
}
// Populates the list with sample data.
private static void FillList()
{
// Create XML elements from a source file.
XElement xTree = XElement.Load(@"c:\temp\books.xml");
// Create an enumerable collection of the elements.
IEnumerable<XElement> elements = xTree.Elements();
// Evaluate each element and set set values in the book object.
foreach (XElement el in elements)
{
Book book = new Book();
book.ID = el.Attribute("id").Value;
IEnumerable<XElement> props = el.Elements();
foreach (XElement p in props)
{
if (p.Name.ToString().ToLower() == "author")
{
book.Author = p.Value;
}
else if (p.Name.ToString().ToLower() == "title")
{
book.Title = p.Value;
}
else if (p.Name.ToString().ToLower() == "genre")
{
book.Genre = p.Value;
}
else if (p.Name.ToString().ToLower() == "price")
{
book.Price = Convert.ToDouble(p.Value);
}
else if (p.Name.ToString().ToLower() == "publish_date")
{
book.Publish_date = Convert.ToDateTime(p.Value);
}
else if (p.Name.ToString().ToLower() == "description")
{
book.Description = p.Value;
}
}
Books.Add(book);
}
DisplayResults(Books, "All books:");
}
// Explicit predicate delegate.
private static bool FindComputer(Book bk)
{
if (bk.Genre == "Computer")
{
return true;
}
else
{
return false;
}
}
private static void DisplayResult(Book result, string title)
{
Console.WriteLine();
Console.WriteLine(title);
Console.WriteLine("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", result.ID,
result.Author, result.Title, result.Genre, result.Price,
result.Publish_date.ToShortDateString());
Console.WriteLine();
}
private static void DisplayResults(List<Book> results, string title)
{
Console.WriteLine();
Console.WriteLine(title);
foreach (Book b in results)
{
Console.Write("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", b.ID,
b.Author, b.Title, b.Genre, b.Price,
b.Publish_date.ToShortDateString());
}
Console.WriteLine();
}
}
public class Book
{
public string ID { get; set; }
public string Author { get; set; }
public string Title { get; set; }
public string Genre { get; set; }
public double Price { get; set; }
public DateTime Publish_date { get; set; }
public string Description { get; set; }
}
}
Imports System.Collections.Generic
Imports System.Linq
Imports System.Xml.Linq
Module Module1
Private IDToFind As String = "bk109"
Public Books As New List(Of Book)
Sub Main()
FillList()
' Find a book by its ID.
Dim result As Book = Books.Find(AddressOf FindID)
If result IsNot Nothing Then
DisplayResult(result, "Find by ID: " & IDToFind)
Else
Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
End If
Console.WriteLine()
' Find last book in collection that has a publish date before 2001.
result = Books.FindLast(AddressOf PubBefore2001)
If result IsNot Nothing Then
DisplayResult(result, "Last book in collection published before 2001:")
Else
Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
End If
Console.WriteLine()
' Find all computer books.
Dim results As List(Of Book) = Books.FindAll(AddressOf FindComputer)
If results.Count <> 0 Then
DisplayResults(results, "All computer books:")
Else
Console.WriteLine(vbCrLf & "No books found.")
End If
Console.WriteLine()
' Find all books under $10.00.
results = Books.FindAll(AddressOf FindUnderTen)
If results.Count <> 0 Then
DisplayResults(results, "Books under $10:")
Else
Console.WriteLine(vbCrLf & "No books found.")
End If
Console.WriteLine()
' Find index values.
Console.WriteLine()
Dim ndx As Integer = Books.FindIndex(AddressOf FindComputer)
Console.WriteLine("Index of first computer book: " & ndx)
ndx = Books.FindLastIndex(AddressOf FindComputer)
Console.WriteLine("Index of last computer book: " & ndx)
Dim mid As Integer = Books.Count / 2
ndx = Books.FindIndex(mid, mid, AddressOf FindComputer)
Console.WriteLine("Index of first computer book in the second half of the collection: " & ndx)
ndx = Books.FindLastIndex(Books.Count - 1, mid, AddressOf FindComputer)
Console.WriteLine("Index of last computer book in the second half of the collection: " & ndx)
End Sub
Private Sub FillList()
' Create XML elements from a source file.
Dim xTree As XElement = XElement.Load("c:\temp\books.xml")
' Create an enumerable collection of the elements.
Dim elements As IEnumerable(Of XElement) = xTree.Elements
' Evaluate each element and set values in the book object.
For Each el As XElement In elements
Dim Book As New Book()
Book.ID = el.Attribute("id").Value
Dim props As IEnumerable(Of XElement) = el.Elements
For Each p As XElement In props
If p.Name.ToString.ToLower = "author" Then
Book.Author = p.Value
End If
If p.Name.ToString.ToLower = "title" Then
Book.Title = p.Value
End If
If p.Name.ToString.ToLower = "genre" Then
Book.Genre = p.Value
End If
If p.Name.ToString.ToLower = "price" Then
Book.Price = Convert.ToDouble(p.Value)
End If
If p.Name.ToString.ToLower = "publish_date" Then
Book.Publish_date = Convert.ToDateTime(p.Value)
End If
If p.Name.ToString.ToLower = "description" Then
Book.Description = p.Value
End If
Next
Books.Add(Book)
Next
DisplayResults(Books, "All books:")
Console.WriteLine()
End Sub
' Predicate delegates for
' Find and FindAll methods.
Private Function FindID(ByVal bk As Book) As Boolean
If bk.ID = IDToFind Then
Return True
Else
Return False
End If
End Function
Private Function FindComputer(ByVal bk As Book) As Boolean
If bk.Genre = "Computer" Then
Return True
Else
Return False
End If
End Function
Private Function FindUnderTen(ByVal bk As Book) As Boolean
Dim tendollars As Double = 10.0
If bk.Price < tendollars Then
Return True
Else
Return False
End If
End Function
Private Function PubBefore2001(ByVal bk As Book) As Boolean
Dim year2001 As DateTime = New DateTime(2001, 1, 1)
Return bk.Publish_date < year2001
End Function
Private Sub DisplayResult(ByVal result As Book, ByVal title As String)
Console.WriteLine()
Console.WriteLine(title)
Console.WriteLine(vbLf & result.ID & vbTab & result.Author & _
vbTab & result.Title & vbTab & result.Genre & _
vbTab & result.Publish_date & vbTab & result.Price)
Console.WriteLine()
End Sub
Private Sub DisplayResults(ByVal results As List(Of Book), ByVal title As String)
Console.WriteLine()
Console.WriteLine(title)
For Each b As Book In results
Console.Write(vbLf & b.ID & vbTab & b.Author & _
vbTab & b.Title & vbTab & b.Genre & _
vbTab & b.Publish_date & vbTab & b.Price)
Next
Console.WriteLine()
End Sub
Public Class Book
Public ID As String
Public Author As String
Public Title As String
Public Genre As String
Public Price As Double
Public Publish_date As DateTime
Public Description As String
End Class
End Module
Comentarios
Predicate<T> es un delegado de un método que devuelve true
si el objeto pasado a él coincide con las condiciones definidas en el delegado. Los elementos del actual List<T> se pasan individualmente al Predicate<T> delegado, avanzando en List<T>, empezando por el primer elemento y finalizando con el último elemento. El procesamiento se detiene cuando se encuentra una coincidencia.
Importante
Al buscar una lista que contenga tipos de valor, asegúrese de que el valor predeterminado del tipo no satisface el predicado de búsqueda. De lo contrario, no hay ninguna manera de distinguir entre un valor predeterminado que indica que no se encontró ninguna coincidencia y un elemento de lista que tiene el valor predeterminado para el tipo. Si el valor predeterminado satisface el predicado de búsqueda, use el FindIndex método en su lugar.
Este método realiza una búsqueda lineal; por lo tanto, este método es una operación O(n), donde n es Count.