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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:

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// 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;
        }
    }
}

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Output:

0

Example 2:

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// 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;
        }
    }
}

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Output:



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.

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// 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;
    }
}

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Runtime Error:

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:

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// 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;
        }
    }
}

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Output:

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.
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