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How to remove items in HashSet present in a specified collection

Surabhi Sharma

The HashSet<T> generic class in the System.Collections.Generic namespaceIn C-sharp, we use namespaces to organize too many classes and handle applications easily. provides the ExceptWith() method.

We use the ExceptWith() method to remove all elements from the HashSet<T> present in the specified collectionspecialized classes for data storage and retrieval..

Syntax

public void ExceptWith (System.Collections.Generic.IEnumerable<T> other);

This method takes an IEnumerable<T> collection as input, and removes all elements from the HashSet<T> present in the collection of elements.

Mathematical set subtraction using ExceptWith()

Things to note

  • The ExceptWith() method represents the mathematical set subtraction operation.

  • It is an O(n) operation where n is the number of elements of the passed collection of elements.

  • The method throws an exception if the input collection is null.

  • This method changes the state of the HashSet<T>.

Code

In the example below, we created a HashSet of integers and added a few numbers. We also created a new List of integers and added some elements to it.

using System;
using System.Collections.Generic;

class HashSetRemover
{
    static void Main()
    {
        HashSet<int> numSet = new HashSet<int>();

        numSet.Add(1);
        numSet.Add(5);
        numSet.Add(9);
        numSet.Add(11);
        numSet.Add(15);
        numSet.Add(6);

        Console.Write("HashSet Elements : ");
        PrintSet(numSet);

        List<int> otherList = new List<int>{9,11};
        
        Console.WriteLine("Input Collection : {0}", string.Join(" ", otherList.ToArray()));
        numSet.ExceptWith(otherList);

        Console.Write("HashSet Elements After ExceptWith() : ");
        PrintSet(numSet);
    }

    private static void PrintSet(HashSet<int> set)
    {
      Console.Write("{");
      foreach (int i in set) {
        Console.Write(" {0}", i);
      }
      Console.WriteLine(" }");
    }
}
Use ExceptWith() to remove element from list

Explanation

We call ExceptWith() with the list of integers as input. This removes all the elements from the HashSet present in the input list of integers.

The list contains two elements, 9 and 11, which were removed from the HashSet.

We print the elements of the HashSet before and after the ExceptWith() operation. The program prints the following output and exits.

HashSet Elements : { 1 5 9 11 15 6 }
Input Collection : 9 11
HashSet Elements After ExceptWith() : { 1 5 15 6 }

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