# List<T>.FindLastIndex Method

## Definition

Searches for an element that matches the conditions defined by a specified predicate, and returns the zero-based index of the last occurrence within the List<T> or a portion of it.

 FindLastIndex(Predicate) Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the entire List. FindLastIndex(Int32, Predicate) Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the range of elements in the List that extends from the first element to the specified index. FindLastIndex(Int32, Int32, Predicate) Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the range of elements in the List that contains the specified number of elements and ends at the specified index.

## FindLastIndex(Predicate<T>)

Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the entire List<T>.

``````public:
int FindLastIndex(Predicate<T> ^ match);``````
``public int FindLastIndex (Predicate<T> match);``
``member this.FindLastIndex : Predicate<'T> -> int``
``Public Function FindLastIndex (match As Predicate(Of T)) As Integer``

#### Parameters

match
Predicate<T>

The Predicate<T> delegate that defines the conditions of the element to search for.

#### Returns

The zero-based index of the last occurrence of an element that matches the conditions defined by `match`, if found; otherwise, -1.

#### Exceptions

`match` is `null`.

### Examples

The following example demonstrates the find methods for the List<T> class. The example for the List<T> class contains `book` objects, of class `Book`, using the data from the Sample XML File: Books (LINQ to XML). The `FillList` method in the example uses LINQ to XML to parse the values from the XML to property values of the `book` objects.

The following table describes the examples provided for the find methods.

Method Example
Find(Predicate<T>) Finds a book by an ID using the `IDToFind` predicate delegate.

C# example uses an anonymous delegate.
FindAll(Predicate<T>) Find all books that whose `Genre` property is "Computer" using the `FindComputer` predicate delegate.
FindLast(Predicate<T>) Finds the last book in the collection that has a publish date before 2001, using the `PubBefore2001` predicate delegate.

C# example uses an anonymous delegate.
FindIndex(Predicate<T>) Finds the index of first computer book using the `FindComputer` predicate delegate.
FindLastIndex(Predicate<T>) Finds the index of the last computer book using the `FindComputer` predicate delegate.
FindIndex(Int32, Int32, Predicate<T>) Finds the index of first computer book in the second half of the collection, using the `FindComputer` predicate delegate.
FindLastIndex(Int32, Int32, Predicate<T>) Finds the index of last computer book in the second half of the collection, using the `FindComputer` predicate delegate.
``````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
{
}

// 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
{
}

// 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.

// 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;
}
}

}

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

End If
Console.WriteLine()

' Find last book in collection that has a publish date before 2001.
If result IsNot Nothing Then
DisplayResult(result, "Last book in collection published before 2001:")
Else
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.
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)
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
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
``````

### Remarks

The List<T> is searched backward starting at the last element and ending at the first element.

The Predicate<T> is a delegate to a method that returns `true` if the object passed to it matches the conditions defined in the delegate. The elements of the current List<T> are individually passed to the Predicate<T> delegate.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

## FindLastIndex(Int32, Predicate<T>)

Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the range of elements in the List<T> that extends from the first element to the specified index.

``````public:
int FindLastIndex(int startIndex, Predicate<T> ^ match);``````
``public int FindLastIndex (int startIndex, Predicate<T> match);``
``member this.FindLastIndex : int * Predicate<'T> -> int``
``Public Function FindLastIndex (startIndex As Integer, match As Predicate(Of T)) As Integer``

#### Parameters

startIndex
Int32

The zero-based starting index of the backward search.

match
Predicate<T>

The Predicate<T> delegate that defines the conditions of the element to search for.

#### Returns

The zero-based index of the last occurrence of an element that matches the conditions defined by `match`, if found; otherwise, -1.

#### Exceptions

`match` is `null`.

`startIndex` is outside the range of valid indexes for the List<T>.

### Remarks

The List<T> is searched backward starting at `startIndex` and ending at the first element.

The Predicate<T> is a delegate to a method that returns `true` if the object passed to it matches the conditions defined in the delegate. The elements of the current List<T> are individually passed to the Predicate<T> delegate.

This method performs a linear search; therefore, this method is an O(n) operation, where n is the number of elements from the beginning of the List<T> to `startIndex`.

## FindLastIndex(Int32, Int32, Predicate<T>)

Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the last occurrence within the range of elements in the List<T> that contains the specified number of elements and ends at the specified index.

``````public:
int FindLastIndex(int startIndex, int count, Predicate<T> ^ match);``````
``public int FindLastIndex (int startIndex, int count, Predicate<T> match);``
``member this.FindLastIndex : int * int * Predicate<'T> -> int``
``Public Function FindLastIndex (startIndex As Integer, count As Integer, match As Predicate(Of T)) As Integer``

#### Parameters

startIndex
Int32

The zero-based starting index of the backward search.

count
Int32

The number of elements in the section to search.

match
Predicate<T>

The Predicate<T> delegate that defines the conditions of the element to search for.

#### Returns

The zero-based index of the last occurrence of an element that matches the conditions defined by `match`, if found; otherwise, -1.

#### Exceptions

`match` is `null`.

`startIndex` is outside the range of valid indexes for the List<T>.

-or-

`count` is less than 0.

-or-

`startIndex` and `count` do not specify a valid section in the List<T>.

### Examples

The following example demonstrates the find methods for the List<T> class. The example for the List<T> class contains `book` objects, of class `Book`, using the data from the Sample XML File: Books (LINQ to XML). The `FillList` method in the example uses LINQ to XML to parse the values from the XML to property values of the `book` objects.

The following table describes the examples provided for the find methods.

Method Example
Find(Predicate<T>) Finds a book by an ID using the `IDToFind` predicate delegate.

C# example uses an anonymous delegate.
FindAll(Predicate<T>) Find all books that whose `Genre` property is "Computer" using the `FindComputer` predicate delegate.
FindLast(Predicate<T>) Finds the last book in the collection that has a publish date before 2001, using the `PubBefore2001` predicate delegate.

C# example uses an anonymous delegate.
FindIndex(Predicate<T>) Finds the index of first computer book using the `FindComputer` predicate delegate.
FindLastIndex(Predicate<T>) Finds the index of the last computer book using the `FindComputer` predicate delegate.
FindIndex(Int32, Int32, Predicate<T>) Finds the index of first computer book in the second half of the collection, using the `FindComputer` predicate delegate.
FindLastIndex(Int32, Int32, Predicate<T>) Finds the index of last computer book in the second half of the collection, using the `FindComputer` predicate delegate.
``````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
{
}

// 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
{
}

// 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.

// 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;
}
}

}

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

End If
Console.WriteLine()

' Find last book in collection that has a publish date before 2001.
If result IsNot Nothing Then
DisplayResult(result, "Last book in collection published before 2001:")
Else
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.
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)
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
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
``````

### Remarks

The List<T> is searched backward starting at `startIndex` and ending at `startIndex` minus `count` plus 1, if `count` is greater than 0.

The Predicate<T> is a delegate to a method that returns `true` if the object passed to it matches the conditions defined in the delegate. The elements of the current List<T> are individually passed to the Predicate<T> delegate.

This method performs a linear search; therefore, this method is an O(n) operation, where n is `count`.