GroupBy operation in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to group numbers by their remainder when divided by 3.

C Sharp (C#)

var groups = numbers.[1](n => n % 3);

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

ASelect

BWhere

CGroupBy

DOrderBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Select instead of GroupBy will transform elements but not group them.

Where filters elements but does not group.

OrderBy sorts elements but does not group.

The GroupBy method groups elements based on a key selector function.

2fill in blank

medium

Complete the code to print each group key and its elements.

C Sharp (C#)

foreach (var group in groups) {
    Console.WriteLine($"Key: {group.[1]");
    foreach (var num in group) {
        Console.WriteLine(num);
    }
}

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

ACount

BKey

CToList

DElementAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count instead of Key will print the number of elements, not the key.

ToList is a method, not a property, and does not represent the key.

ElementAt requires an index and is not the key.

The Key property gives the key value of each group.

3fill in blank

hard

Fix the error in the code to group words by their first letter.

C Sharp (C#)

var groupedWords = words.[1](w => w[0]);

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

AGroupBy

BWhere

CSelect

DOrderBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Select or Where instead of GroupBy will not create groups.

OrderBy sorts but does not group.

GroupBy is needed to group elements by a key, here the first letter.

4fill in blank

hard

Fill both blanks to create a dictionary from groups with key and count of elements.

C Sharp (C#)

var dict = groupedWords.ToDictionary(g => g.[1], g => g.[2]());

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

AKey

BCount

CLength

DSum

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length is invalid for groups; Count() is the correct method.

Sum is for adding values, not counting elements.

Use Key for the dictionary key and Count() to get the number of elements in each group.

5fill in blank

hard

Fill all three blanks to group numbers by even/odd and select the key and sum of each group.

C Sharp (C#)

var result = numbers.[1](n => n % 2 == 0).Select(g => new {
    [2] = g.Key,
    [3] = g.Sum()
});

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

AGroupBy

BKey

CSum

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count instead of Sum will count elements, not sum them.

Using Select instead of GroupBy will not group elements.

GroupBy groups numbers by even/odd, Key accesses the group key, and Sum calculates the total of each group.

Practice

1. What does the GroupBy method do in C#?

easy

A. It sorts elements in ascending order.

B. It groups elements of a collection based on a key selector.

C. It filters elements based on a condition.

D. It removes duplicate elements from a collection.

Solution

1. Step 1: Understand the purpose of GroupBy

   The GroupBy method organizes elements by a key, creating groups of items sharing the same key.

2. Step 2: Compare with other operations

   Sorting arranges order, filtering selects items, and removing duplicates eliminates repeats, which are different from grouping.

3. Final Answer:

   It groups elements of a collection based on a key selector. -> Option B

4. Quick Check:

   GroupBy = grouping by key [OK]

Hint: GroupBy always creates groups by a key, not sorting or filtering [OK]

Common Mistakes:

• Confusing GroupBy with sorting methods
• Thinking GroupBy filters items
• Assuming GroupBy removes duplicates

2. Which of the following is the correct syntax to group a list of strings by their first character using LINQ in C#?

easy

A. var groups = list.GroupBy(s => s[0]);

B. var groups = list.GroupBy(s => s.Length);

C. var groups = list.GroupBy(s => s.ToUpper());

D. var groups = list.GroupBy(s => s.Substring(1));

Solution

1. Step 1: Identify grouping key for first character

   Grouping by the first character means using s => s[0] as the key selector.

2. Step 2: Check other options

   Grouping by length, uppercase string, or substring starting at index 1 does not group by first character.

3. Final Answer:

   var groups = list.GroupBy(s => s[0]); -> Option A

4. Quick Check:

   First char key = s[0] [OK]

Hint: Use s => s[0] to group by first character [OK]

Common Mistakes:

• Using substring starting at 1 instead of 0
• Grouping by string length instead of character
• Using ToUpper() changes case but not grouping key

3. What is the output of the following C# code?

var numbers = new[] {1, 2, 3, 4, 5, 6};
var groups = numbers.GroupBy(n => n % 2 == 0 ? "Even" : "Odd");
foreach (var group in groups) {
    Console.WriteLine($"{group.Key}: {string.Join(",", group)}");
}

medium

A. 1: 1
2: 2
3: 3
4: 4
5: 5
6: 6

B. Even: 2,4,6
Odd: 1,3,5

C. Odd: 1,3,5
Even: 2,4,6

D. Even: 1,3,5
Odd: 2,4,6

Solution

1. Step 1: Understand grouping key logic

   Numbers are grouped by whether they are even or odd using the key "Even" or "Odd".

2. Step 2: Determine group contents and order

   Group "Odd" contains 1,3,5; group "Even" contains 2,4,6. The foreach prints groups in order they appear, which is "Odd" then "Even".

3. Final Answer:

   Odd: 1,3,5 Even: 2,4,6 -> Option C

4. Quick Check:

   Group keys "Odd" then "Even" with correct items [OK]

Hint: GroupBy preserves order of first occurrence of keys [OK]

Common Mistakes:

• Assuming groups print in alphabetical order
• Mixing up even and odd groups
• Expecting separate groups for each number

4. Identify the error in this C# code that tries to group words by their length:

var words = new List<string> {"apple", "bat", "car", "dog"};
var groups = words.GroupBy(word => word.Length);
foreach (var group in groups)
    Console.WriteLine(group.Key + ": " + group.ToString());

medium

A. Using group.ToString() instead of joining group elements.

B. GroupBy cannot be used on List<string>.

C. Missing semicolon after GroupBy statement.

D. word.Length is not a valid key selector.

Solution

1. Step 1: Check GroupBy usage

   GroupBy on List<string> with word.Length is valid syntax and logic.

2. Step 2: Analyze output statement

   Using group.ToString() prints the type name, not the grouped items. We should join group elements to display them.

3. Final Answer:

   Using group.ToString() instead of joining group elements. -> Option A

4. Quick Check:

   Print grouped items by joining, not ToString() [OK]

Hint: Join group elements to print, not group.ToString() [OK]

Common Mistakes:

• Thinking GroupBy can't be used on lists
• Forgetting to join group elements for display
• Misunderstanding word.Length as key selector

5. Given a list of employees with properties Name and Department, how would you use GroupBy to create a dictionary where keys are departments and values are lists of employee names?

hard

A. var dict = employees.GroupBy(e => e.Name).ToDictionary(g => g.Key, g => g.Select(e => e.Department).ToList());

B. var dict = employees.ToDictionary(e => e.Department, e => e.Name);

C. var dict = employees.GroupBy(e => e.Department).ToList();

D. var dict = employees.GroupBy(e => e.Department).ToDictionary(g => g.Key, g => g.Select(e => e.Name).ToList());

Solution

1. Step 1: Group employees by Department

   Use GroupBy with key selector e => e.Department to group employees by their department.

2. Step 2: Convert groups to dictionary with employee names list

   Use ToDictionary with key as group.Key (department) and value as list of employee names using g.Select(e => e.Name).ToList().

3. Final Answer:

   var dict = employees.GroupBy(e => e.Department).ToDictionary(g => g.Key, g => g.Select(e => e.Name).ToList()); -> Option D

4. Quick Check:

   GroupBy + ToDictionary with Select names = correct [OK]

Hint: GroupBy then ToDictionary with Select for values list [OK]

Common Mistakes:

• Using ToDictionary directly without grouping
• Grouping by Name instead of Department
• Not converting grouped items to list of names