HashSet for unique elements in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HashSet code?

Consider the following C# code using HashSet<int>. What will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> numbers = new HashSet<int>() {1, 2, 3, 2, 4, 1};
        Console.WriteLine(numbers.Count);
    }
}

A 4

B 5

C 6

D 3

💡 Hint

HashSet stores only unique elements. Duplicate values are ignored.

The HashSet contains unique elements: 1, 2, 3, 4. Duplicates like the second 1 and 2 are ignored. So the count is 4.

❓ Predict Output

intermediate

What does this code print after adding elements?

What will be the output of this C# program?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<string> fruits = new HashSet<string>();
        fruits.Add("apple");
        fruits.Add("banana");
        fruits.Add("apple");
        fruits.Add("orange");
        Console.WriteLine(fruits.Count);
    }
}

A 4

B 3

C 2

D 1

💡 Hint

HashSet ignores duplicates when adding.

"apple" is added twice but only stored once. The unique fruits are "apple", "banana", and "orange". Count is 3.

🔧 Debug

advanced

Why does this HashSet code throw an exception?

Examine the code below. It throws a runtime exception. What is the cause?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> set = new HashSet<int>() {1, 2, 3};
        foreach (var item in set) {
            if (item == 2) {
                set.Remove(item);
            }
        }
    }
}

A The HashSet is empty, so Remove fails.

B HashSet does not support Remove method.

C The foreach loop variable 'item' is read-only and cannot be used in Remove.

D Modifying a collection while iterating causes an InvalidOperationException.

💡 Hint

Think about what happens if you change a collection while looping over it.

Removing items from a HashSet during foreach iteration causes an InvalidOperationException because the collection is modified during enumeration.

❓ Predict Output

advanced

What is the output of this HashSet intersection code?

What will this program print?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> setA = new HashSet<int>() {1, 2, 3, 4};
        HashSet<int> setB = new HashSet<int>() {3, 4, 5, 6};
        setA.IntersectWith(setB);
        Console.WriteLine(string.Join(",", setA));
    }
}

A 5,6

B 1,2

C 3,4

D 1,2,3,4,5,6

💡 Hint

IntersectWith keeps only elements present in both sets.

After intersection, setA contains only elements common to setA and setB: 3 and 4.

🧠 Conceptual

expert

What is the time complexity of HashSet operations?

Which statement best describes the average time complexity of Add, Remove, and Contains operations in a C# HashSet<T>?

A Add, Remove, and Contains operations run on average in O(1) time due to hashing.

B Add and Remove are O(1), but Contains is O(n) because it searches linearly.

C All operations run in O(n) time because HashSet uses a list internally.

D All operations run in O(log n) time because HashSet uses a balanced tree.

💡 Hint

Think about how hashing helps find elements quickly.

HashSet uses a hash table internally, so Add, Remove, and Contains usually run in constant time O(1) on average.

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