HashSet for unique elements in C Sharp (C#) - Step-by-Step Execution

Concept Flow - HashSet for unique elements

Create empty HashSet

↓

Add element?

No→Skip add

Yes↓

Element added if unique

↓

More elements?

No→End

↩Back to Add element?

Start with an empty HashSet, try to add elements one by one. Only unique elements get added. Repeat until no more elements.

Execution Sample

C Sharp (C#)

var set = new HashSet<int>();
set.Add(1);
set.Add(2);
set.Add(1);
Console.WriteLine(set.Count);

Adds numbers to a HashSet and prints how many unique numbers are inside.

Execution Table

Step	Action	Element Added	Set Contents	Output
1	Create empty HashSet	N/A	{}
2	Add 1	1	{1}
3	Add 2	2	{1, 2}
4	Add 1 again	No (duplicate)	{1, 2}
5	Print Count	N/A	{1, 2}	2

💡 All elements processed; duplicates ignored; final count is 2

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
set	{}	{1}	{1, 2}	{1, 2}	{1, 2}

Key Moments - 2 Insights

Why doesn't adding '1' the second time change the set?

Because HashSet only keeps unique elements. Step 4 in the execution_table shows 'Add 1 again' is ignored since '1' is already inside.

What does set.Count represent?

It shows how many unique elements are in the set. Step 5 prints '2' because only two unique numbers (1 and 2) were added.

Visual Quiz - 3 Questions

Look at the execution_table, what is the set content after step 3?

A {}

B {1, 2}

C {1}

D {2}

💡 Hint

Check the 'Set Contents' column at step 3 in the execution_table.

At which step is a duplicate element detected and not added?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for 'No (duplicate)' in the 'Element Added' column in the execution_table.

If we add '3' after step 4, what will be the new count?

A 4

B 2

C 3

D 1

💡 Hint

Adding a new unique element increases the count by 1; see variable_tracker for how set changes.

Concept Snapshot

HashSet stores unique elements only.
Use Add() to insert elements; duplicates are ignored.
Count property shows number of unique items.
Ideal for removing duplicates quickly.

Full Transcript

This example shows how a HashSet in C# keeps only unique elements. We start with an empty set. Adding '1' puts it inside. Adding '2' adds another unique element. Trying to add '1' again does nothing because it's a duplicate. Finally, printing Count shows 2, the number of unique elements. HashSet is useful when you want to avoid duplicates automatically.

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