HashSet for unique elements in C Sharp (C#) - Mini Project: Build & Apply

HashSet for unique elements

📖 Scenario: You are organizing a guest list for a party. Some guests accidentally appear more than once on the list. You want to make sure each guest is invited only once.

🎯 Goal: Build a program that uses a HashSet to store unique guest names and then display the list of unique guests.

📋 What You'll Learn

Create a HashSet<string> to hold guest names

Add guest names to the HashSet, including duplicates

Use a foreach loop to iterate over the HashSet

Print each unique guest name

💡 Why This Matters

🌍 Real World

Removing duplicates from lists is common when managing contacts, emails, or any collection where uniqueness matters.

💼 Career

Understanding HashSet helps in writing efficient code for data processing, filtering, and ensuring data integrity in software development.

1

Create the initial guest list

Create a List<string> called guestList with these exact names: "Alice", "Bob", "Alice", "Charlie", "Bob".

C Sharp (C#)

// Create the guestList with duplicates
// Your code here

Hint

Use new List<string> { ... } to create the list with the exact names.

2

Create a HashSet for unique guests

Create a HashSet<string> called uniqueGuests and initialize it with the guestList.

C Sharp (C#)

using System.Collections.Generic;

List guestList = new List { "Alice", "Bob", "Alice", "Charlie", "Bob" };

// Create the HashSet uniqueGuests from guestList
// Your code here

Hint

Use the HashSet constructor that takes a collection to remove duplicates.

3

Iterate over the unique guests

Use a foreach loop with the variable guest to iterate over uniqueGuests.

C Sharp (C#)

using System.Collections.Generic;

List guestList = new List { "Alice", "Bob", "Alice", "Charlie", "Bob" };
HashSet uniqueGuests = new HashSet(guestList);

// Use foreach to loop over uniqueGuests
// Your code here

Hint

Write foreach (string guest in uniqueGuests) to loop through the unique names.

4

Print the unique guest names

Inside the foreach loop, write Console.WriteLine(guest); to print each unique guest name.

C Sharp (C#)

using System;
using System.Collections.Generic;

List guestList = new List { "Alice", "Bob", "Alice", "Charlie", "Bob" };
HashSet uniqueGuests = new HashSet(guestList);

foreach (string guest in uniqueGuests)
{
    // Print each guest name
    // Your code here
}

Hint

Use Console.WriteLine(guest); inside the loop to print each name.

Practice

1. What is the main purpose of using a HashSet<T> in C#?

easy

A. To allow duplicate elements for faster access

B. To store elements in sorted order

C. To store unique elements without duplicates

D. To store key-value pairs like a dictionary

Solution

1. Step 1: Understand HashSet behavior

   A HashSet automatically ignores duplicate entries and stores only unique elements.

2. Step 2: Compare with other collections

   Unlike lists or dictionaries, HashSet does not allow duplicates and does not maintain order.

3. Final Answer:

   To store unique elements without duplicates -> Option C

4. Quick Check:

   HashSet = Unique elements [OK]

Hint: HashSet always keeps unique items only [OK]

Common Mistakes:

• Thinking HashSet keeps elements sorted
• Assuming HashSet allows duplicates
• Confusing HashSet with Dictionary

2. Which of the following is the correct way to declare and initialize a HashSet<int> with values 1, 2, and 3?

easy

A. var set = HashSet<int> = {1, 2, 3};

B. var set = new HashSet<int> {1, 2, 3};

C. var set = new HashSet<int>[1, 2, 3];

D. var set = new HashSet<int>(1, 2, 3);

Solution

1. Step 1: Check HashSet initialization syntax

   HashSet can be initialized with collection initializer syntax using curly braces after the constructor.

2. Step 2: Validate each option

   var set = new HashSet<int> {1, 2, 3}; uses correct syntax: new HashSet<int> {1, 2, 3}; Options A, B, and C have invalid syntax.

3. Final Answer:

   var set = new HashSet<int> {1, 2, 3}; -> Option B

4. Quick Check:

   Use curly braces after constructor for initialization [OK]

Hint: Use curly braces after new HashSet for values [OK]

Common Mistakes:

• Using parentheses with multiple values directly
• Trying to declare array instead of HashSet
• Incorrect assignment syntax

3. What will be the output of the following C# code?

var set = new HashSet<string>();
set.Add("apple");
set.Add("banana");
set.Add("apple");
Console.WriteLine(set.Count);

medium

A. 0

B. 3

C. 1

D. 2

Solution

1. Step 1: Add elements to HashSet

   "apple" is added first, then "banana", then "apple" again.

2. Step 2: Understand duplicate handling

   The second "apple" is ignored because HashSet stores unique elements only.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Duplicates ignored, count = 2 [OK]

Hint: Count equals unique items added, duplicates ignored [OK]

Common Mistakes:

• Counting duplicates as separate elements
• Assuming HashSet allows duplicates
• Confusing Count with number of Add calls

4. Identify the error in this code snippet using HashSet<int>:

HashSet<int> numbers = new HashSet<int>();
numbers.Add(1);
numbers.Add(2);
numbers.Add(1);
Console.WriteLine(numbers[0]);

medium

A. HashSet does not support indexing with []

B. Cannot add duplicate values to HashSet

C. HashSet must be initialized with values

D. Add method returns void, cannot be used like this

Solution

1. Step 1: Review HashSet usage

   HashSet stores unique elements but does not support accessing elements by index.

2. Step 2: Identify invalid operation

   Using numbers[0] causes a compile-time error because HashSet has no indexer.

3. Final Answer:

   HashSet does not support indexing with [] -> Option A

4. Quick Check:

   No index access on HashSet [OK]

Hint: HashSet has no indexer; use foreach or Contains [OK]

Common Mistakes:

• Trying to access elements by index
• Thinking Add returns a value
• Assuming duplicates cause errors

5. You have a list of integers with duplicates: List<int> nums = new List<int> {1, 2, 2, 3, 4, 4, 4, 5};
Which code snippet correctly creates a HashSet<int> containing only the unique elements from nums?

hard

A. var unique = new HashSet<int>(nums);

B. var unique = new HashSet<int>(); unique.Add(nums);

C. var unique = new HashSet<int>(); foreach(var n in nums) unique = n;

D. var unique = new HashSet<int>(); unique.AddRange(nums);

Solution

1. Step 1: Understand HashSet constructor

   HashSet has a constructor that accepts an IEnumerable<T> to initialize with unique elements.

2. Step 2: Analyze each option

   var unique = new HashSet<int>(nums); correctly passes the list to the constructor. var unique = new HashSet<int>(); unique.Add(nums); tries to add the whole list as one item (invalid). var unique = new HashSet<int>(); foreach(var n in nums) unique = n; assigns int to HashSet variable (invalid). var unique = new HashSet<int>(); unique.AddRange(nums); uses AddRange which HashSet does not have.

3. Final Answer:

   var unique = new HashSet<int>(nums); -> Option A

4. Quick Check:

   Use constructor with collection for unique set [OK]

Hint: Pass list to HashSet constructor for unique items [OK]

Common Mistakes:

• Using Add to add whole list at once
• Trying to assign int to HashSet variable
• Using AddRange which HashSet lacks