HashSet for unique elements in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a HashSet of integers.

C Sharp (C#)

HashSet<int> numbers = new [1]<int>();

Drag options to blanks, or click blank then click option'

AHashSet

BList

CDictionary

DQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using List instead of HashSet, which allows duplicates.

Using Dictionary which requires key-value pairs.

The HashSet class is used to store unique elements. Here, we create a new HashSet of integers.

2fill in blank

medium

Complete the code to add an element to the HashSet.

C Sharp (C#)

numbers.[1](5);

Drag options to blanks, or click blank then click option'

APush

BAppend

CAdd

DInsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using Insert which is not a method of HashSet.

Using Push or Append which are not valid methods here.

The Add method adds an element to the HashSet if it is not already present.

3fill in blank

hard

Fix the error in the code to check if the HashSet contains the number 10.

C Sharp (C#)

bool exists = numbers.[1](10);

Drag options to blanks, or click blank then click option'

AContains

BExists

CIncludes

DHas

Attempts:

3 left

💡 Hint

Common Mistakes

Using Has or Includes which are not valid in C# HashSet.

Using Exists which is a LINQ method, not a HashSet method.

The Contains method checks if an element exists in the HashSet.

4fill in blank

hard

Fill both blanks to create a HashSet from an array and check if it contains 3.

C Sharp (C#)

int[] arr = {1, 2, 3, 4};
HashSet<int> set = new [1](arr);
bool hasThree = set.[2](3);

Drag options to blanks, or click blank then click option'

AHashSet

BContains

CList

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using List instead of HashSet for unique elements.

Using Add instead of Contains to check membership.

We create a HashSet from the array arr. Then we use Contains to check if 3 is in the set.

5fill in blank

hard

Fill all three blanks to add elements and check the count of unique items.

C Sharp (C#)

HashSet<string> fruits = new [1]();
fruits.[2]("apple");
fruits.[3]("banana");
int count = fruits.Count;

Drag options to blanks, or click blank then click option'

AHashSet

BAdd

DInsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using Insert which is not a method of HashSet.

Confusing Add with other collection methods.

We create a HashSet of strings, add "apple" and "banana" using Add, then get the count of unique items.

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