Set operations (Union, Intersect, Except) in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a union of two sets.

C Sharp (C#)

var set1 = new HashSet<int> {1, 2, 3};
var set2 = new HashSet<int> {3, 4, 5};
var unionSet = set1.[1](set2);
Console.WriteLine(string.Join(", ", unionSet));

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

AExcept

BUnionWith

CIntersect

DUnion

Attempts:

3 left

💡 Hint

Common Mistakes

Using UnionWith which modifies the original set instead of returning a new one.

Using Intersect which returns only common elements.

The Union method returns a new set that contains all elements from both sets.

2fill in blank

medium

Complete the code to find the intersection of two sets.

C Sharp (C#)

var setA = new HashSet<string> {"apple", "banana", "cherry"};
var setB = new HashSet<string> {"banana", "date", "fig"};
var common = setA.[1](setB);
Console.WriteLine(string.Join(", ", common));

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

AExcept

BUnion

CIntersect

DUnionWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using Except which returns elements only in the first set.

Using Union which returns all elements from both sets.

The Intersect method returns elements that appear in both sets.

3fill in blank

hard

Fix the error in the code to get the difference of two sets.

C Sharp (C#)

var firstSet = new HashSet<int> {10, 20, 30, 40};
var secondSet = new HashSet<int> {30, 40, 50};
var difference = firstSet.[1](secondSet);
Console.WriteLine(string.Join(", ", difference));

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

AIntersect

BExcept

CUnionWith

DUnion

Attempts:

3 left

💡 Hint

Common Mistakes

Using Intersect which returns common elements.

Using Union which returns all elements combined.

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

4fill in blank

hard

Fill both blanks to create a set of fruits that are in set1 but not in set2, then print the count.

C Sharp (C#)

var set1 = new HashSet<string> {"apple", "banana", "cherry", "date"};
var set2 = new HashSet<string> {"banana", "date", "fig"};
var result = new HashSet<string>(set1.[1](set2));
Console.WriteLine(result.[2]);

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

AExcept

BCount

CLength

DIntersect

Attempts:

3 left

💡 Hint

Common Mistakes

Using Intersect instead of Except.

Using Length which is not a property of HashSet.

Except returns elements in set1 not in set2. Count gives the number of elements in the result.

5fill in blank

hard

Fill the blanks to create a union of two sets, remove elements in the third set, and print the final elements.

C Sharp (C#)

var setA = new HashSet<int> {1, 2, 3};
var setB = new HashSet<int> {3, 4, 5};
var setC = new HashSet<int> {2, 5};
var union = setA.[1](setB);
var finalSet = union.[2](setC);
Console.WriteLine(string.Join(", ", finalSet));

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

AUnion

BExcept

CIntersect

DUnionWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using Intersect instead of Union.

Using UnionWith where a method returning a new set is needed.

Union combines setA and setB. Then Except removes elements in setC from the union.

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