Set operations (Union, Intersect, Except) in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Union operation on two sets

What is the output of the following C# code that uses Union on two integer sets?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
    static void Main() {
        var setA = new HashSet<int> {1, 2, 3};
        var setB = new HashSet<int> {3, 4, 5};
        var unionSet = setA.Union(setB);
        foreach(var item in unionSet) {
            Console.Write(item + " ");
        }
    }
}

A 1 2 3 4 5

B 3 4 5

C 1 2 3

D 1 2 4 5

💡 Hint

Union combines all unique elements from both sets.

The Union method returns all distinct elements from both sets. Since 3 is common, it appears once. So the output is 1 2 3 4 5.

❓ Predict Output

intermediate

Output of Intersect operation on two sets

What does the following C# code print when using Intersect on two sets?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
    static void Main() {
        var setA = new HashSet<int> {10, 20, 30, 40};
        var setB = new HashSet<int> {30, 40, 50, 60};
        var intersectSet = setA.Intersect(setB);
        foreach(var item in intersectSet) {
            Console.Write(item + " ");
        }
    }
}

A 50 60

B 10 20 30 40 50 60

C 30 40

D 10 20

💡 Hint

Intersect returns only elements present in both sets.

Only 30 and 40 are in both sets, so the output is 30 40.

❓ Predict Output

advanced

Output of Except operation on two sets

What is the output of this C# code using Except to find elements in setA not in setB?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
    static void Main() {
        var setA = new HashSet<string> {"apple", "banana", "cherry"};
        var setB = new HashSet<string> {"banana", "date"};
        var exceptSet = setA.Except(setB);
        foreach(var item in exceptSet) {
            Console.Write(item + " ");
        }
    }
}

A date

B apple cherry

C apple banana cherry date

D banana date

💡 Hint

Except returns elements in the first set that are not in the second.

Only "apple" and "cherry" are in setA but not in setB, so output is apple cherry.

🔧 Debug

advanced

Identify the error in set operation code

This C# code tries to get the union of two sets but causes a compile error. What is the error?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        var setA = new HashSet<int> {1, 2, 3};
        var setB = new HashSet<int> {4, 5, 6};
        var unionSet = setA.Union(setB);
        foreach(int item in unionSet) {
            Console.WriteLine(item);
        }
    }
}

A Missing using System.Linq directive causes Union() to be undefined.

B HashSet<int> cannot use Union method; use AddRange instead.

C Union method requires a List<int> argument, not HashSet<int>.

D Cannot iterate unionSet because it is not enumerable.

💡 Hint

Check if all required namespaces are included for LINQ methods.

Union is an extension method in System.Linq namespace. Without 'using System.Linq;', Union() is not recognized, causing a compile error.

🧠 Conceptual

expert

Number of elements after combined set operations

Given these sets:

Set A = {1, 2, 3, 4, 5}
Set B = {4, 5, 6, 7}
Set C = {5, 7, 8, 9}

What is the number of elements in the set resulting from:
var result = A.Union(B).Intersect(C);

A 4

B 3

C 5

D 2

💡 Hint

First find A union B, then find intersection with C.

A union B = {1,2,3,4,5,6,7}. Intersect with C = {5,7,8,9} gives {5,7}. So the count is 2.

Practice

1.

Which set operation in C# returns all unique elements from two collections combined?

easy

A. Union

B. Intersect

C. Except

D. Concat

Solution

1. Step 1: Understand the meaning of Union

   Union combines all unique elements from both collections without duplicates.

2. Step 2: Compare with other operations

   Intersect returns only common elements, Except returns elements only in the first collection, and Concat just joins collections without removing duplicates.

3. Final Answer:

   Union -> Option A

4. Quick Check:

   Union = all unique elements combined [OK]

Hint: Union = combine all unique items from both sets [OK]

Common Mistakes:

• Confusing Union with Intersect
• Thinking Concat removes duplicates
• Mixing Except with Union

2.

Which of the following is the correct syntax to get the intersection of two sets setA and setB in C#?

var result = ???;

easy

A. setA.Join(setB)

B. setA.Intersect(setB)

C. setA.Except(setB)

D. setA.Union(setB)

Solution

1. Step 1: Identify method for intersection

   The Intersect method returns elements common to both collections.

2. Step 2: Verify syntax correctness

   setA.Intersect(setB) is the correct syntax; Union and Except do different operations, and Join is unrelated.

3. Final Answer:

   setA.Intersect(setB) -> Option B

4. Quick Check:

   Intersect = common elements [OK]

Hint: Use Intersect() to find common elements between sets [OK]

Common Mistakes:

• Using Union instead of Intersect
• Trying to use Join for sets
• Incorrect method call syntax

3.

What is the output of the following C# code?

var setA = new HashSet<int>() {1, 2, 3, 4};
var setB = new HashSet<int>() {3, 4, 5, 6};
var result = setA.Except(setB);
Console.WriteLine(string.Join(",", result));

medium

A. 1,2,5,6

B. 3,4

C. 5,6

D. 1,2

Solution

1. Step 1: Understand Except operation

   Except returns elements in the first set that are not in the second set.

2. Step 2: Apply Except to given sets

   setA has 1,2,3,4; setB has 3,4,5,6; elements in setA but not in setB are 1 and 2.

3. Final Answer:

   1,2 -> Option D

4. Quick Check:

   Except = elements only in first set [OK]

Hint: Except returns items only in first set, not in second [OK]

Common Mistakes:

• Confusing Except with Intersect
• Including elements from second set
• Expecting all unique elements combined

4.

Find the error in this C# code snippet that tries to get the union of two sets set1 and set2:

var set1 = new HashSet<string>() {"apple", "banana"};
var set2 = new HashSet<string>() {"banana", "cherry"};
var result = set1.Union(set2).ToList();
Console.WriteLine(result);

medium

A. Union method does not exist for HashSet

B. ToList() should be called before Union

C. Console.WriteLine cannot print a List directly

D. HashSet cannot contain strings

Solution

1. Step 1: Check method usage

   Union exists and works on HashSet; ToList() is valid after Union.

2. Step 2: Identify Console.WriteLine issue

   Console.WriteLine cannot print a List directly; it prints the type name instead.

3. Final Answer:

   Console.WriteLine cannot print a List directly -> Option C

4. Quick Check:

   Printing List needs string.Join or loop [OK]

Hint: Use string.Join to print collections, not Console.WriteLine directly [OK]

Common Mistakes:

• Assuming Union is invalid for HashSet
• Calling ToList before Union
• Thinking HashSet can't hold strings

5.

You have two lists of integers representing IDs: list1 = {1, 2, 3, 4, 5} and list2 = {4, 5, 6, 7}. You want to find IDs that are either in list1 or list2 but not in both (exclusive or). Which C# code snippet correctly achieves this using set operations?

hard

A. var result = list1.Except(list2).Union(list2.Except(list1));

B. var result = list1.Intersect(list2).Except(list1.Union(list2));

C. var result = list1.Union(list2).Except(list1.Except(list2));

D. var result = list1.Except(list2).Intersect(list2.Except(list1));

Solution

1. Step 1: Understand exclusive or (XOR) logic

   XOR means elements in one set or the other but not both.

2. Step 2: Analyze each option

   var result = list1.Union(list2).Except(list1.Except(list2)); subtracts elements unique to list1 from the union, leaving common and list2-unique elements - incorrect. var result = list1.Except(list2).Union(list2.Except(list1)); combines elements unique to each list using Except and Union, which directly matches XOR.

3. Final Answer:

   var result = list1.Except(list2).Union(list2.Except(list1)); -> Option A

4. Quick Check:

   XOR = unique elements from each set combined [OK]

Hint: XOR = (A except B) union (B except A) [OK]

Common Mistakes:

• Using Intersect instead of Except for uniqueness
• Confusing union and intersection order
• Trying to intersect except results