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.