List FindLastIndex() Method in C# | Set -2
Last Updated :
05 Dec, 2019
This method is used to search for an element which 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. There are 3 methods in the overload list of this method:
Here, we will discuss only the last two methods.
FindLastIndex(Int32, Predicate<T>) Method
This method 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.
Syntax: public int FindLastIndex (int start, Predicate<T> match);
Parameters:
start: It is the starting index from the searching will starts.
match: It is the Predicate delegate that defines the conditions of the searched element.
Return Value: If the element is found then it returns the zero-based index of type
Int32 of the last occurrence of an element that matches a specified condition by the parameter “
match”. And if not found then it returns “
-1”.
Exceptions:
- ArgumentNullException: If the match is null.
- ArgumentOutOfRangeException: If the startis outside the range of valid indexes for the List<T>.
Below programs illustrate the use of above-discussed method:
Example 1:
csharp
// C# program to illustrate the
// List<T>.FindLastIndex(Int32,
// Predicate <T>) Method
using System;
using System.Collections.Generic;
class GFG {
// Main Method
public static void Main()
{
// List creation
// List name is "PC"
List<string> PC = new List<string>();
// elements in the List
PC.Add("Computer");
PC.Add("keyboard");
PC.Add("laptop");
PC.Add("mouse");
// the search will starts from index 2
int indx = PC.FindLastIndex(2, FindIndex);
Console.WriteLine(indx);
}
// Conditional method
private static bool FindIndex(string g)
{
if (g == "Computer")
{
return true;
}
else
{
return false;
}
}
}
0
Example 2:
csharp
// C# program to illustrate the
// List<T>.FindLastIndex(Int32,
// Predicate <T>) Method
using System;
using System.Collections.Generic;
class GFG {
// Main Method
public static void Main()
{
// List creation
// List name is "PC"
List<int> PC = new List<int>();
// elements in the List
PC.Add(3);
PC.Add(4);
PC.Add(5);
PC.Add(6);
// condition is "FindIndex"
int indx = PC.FindLastIndex(2, FindIndex);
Console.WriteLine(indx);
}
// Conditional method
private static bool FindIndex(int g)
{
// search for "5"
if (g == 5)
{
return true;
}
else
{
return false;
}
}
}
2
Example 3: In this example we use an XML file and search an item from a starting index and prints the index of that item, if the item is not found then prints “-1” and if found then prints the index. The item is “
GeeksForGeeks”. But here we don’t have the XML file, and here the compiler gives an exception.
csharp
// C# program to illustrate the
// List<T>.FindLastIndex(Int32,
// Predicate <T>) Method
using System;
using System.Collections.Generic;
using System.Linq;
class GFG {
// here List<T> contains the object "gfg" using
// data from a sample XML file
// List initialize
private static List<gfg> geeks = new List<gfg>();
public static void Main()
{
// if the item is found then
// it prints the index
// if not found prints "-1"
int x = geeks.FindLastIndex(3, FindGFG);
Console.WriteLine(x);
}
// conditional method
private static bool FindGFG(gfg g)
{
// item is "GeeksForGeeks"
if (g.G == "GeeksForGeeks")
{
return true;
}
else
{
return false;
}
}
}
public class gfg {
public string G
{
get;
set;
}
}
Unhandled Exception:
System.ArgumentOutOfRangeException: ArgumentOutOfRange_Index
Parameter name: startIndex
FindLastIndex(Int32, Int32, Predicate<T>) Method
This method 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 and the list contains the specified number of elements and ends at the specified index.
Syntax: public int FindLastIndex (int startIndex, int count, Predicate<T> match);
Parameters:
startIndex: It is the zero-based starting index of the backward search.
count: It is the number of elements in the section to search.
match: It is the Predicate<T> delegate that defines the conditions of the element to search for.
Return Value: If the element is found then it returns the zero-based index of type Int32 of the last element that matches a specified condition by the parameter “match”. And if not found then it returns “-1”.
Exceptions:
- ArgumentNullException: if the “match” is null.
- ArgumentOutOfRangeException: if “startIndex” is outside the range or “count” is less than 0(Zero) or “startIndex” and “count” do not specify a valid section in the List
Example:
CSharp
// C# Program to illustrate the
// FindLastIndex(Int32, Int32,
// Predicate<T>) Method
using System;
using System.Collections.Generic;
class GFG
{
public static void Main()
{
// List name is "mylist"
List<string> mylist = new List<string>();
// Elements in the List
mylist.Add("C");
mylist.Add("C++");
mylist.Add("Java");
mylist.Add("Python");
mylist.Add("C#");
mylist.Add("HTML");
mylist.Add("Java");
mylist.Add("PHP");
// the search will starts from index 2
// the number of element is 3
int indx = mylist.FindLastIndex(2, 3, FindIndex);
Console.WriteLine("The index of Java is: "+indx);
}
// Conditional method
private static bool FindIndex(string g)
{
if (g == "Java")
{
return true;
}
else
{
return false;
}
}
}
The index of Java is: 2
Note:
- 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 start.