GroupBy operation in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of GroupBy with simple key selector

What is the output of this C# code snippet that groups numbers by their remainder when divided by 3?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        var numbers = new[] {1, 2, 3, 4, 5, 6};
        var groups = numbers.GroupBy(n => n % 3);
        foreach (var group in groups) {
            Console.WriteLine($"Key: {group.Key}, Values: {string.Join(",", group)}");
        }
    }
}

A

Key: 0, Values: 3,6
Key: 1, Values: 1,4
Key: 2, Values: 2,5

B

Key: 3, Values: 3,6
Key: 1, Values: 1,4
Key: 2, Values: 2,5

C

Key: 1, Values: 1,4
Key: 2, Values: 2,5
Key: 0, Values: 3,6

D

Key: 0, Values: 6,3
Key: 1, Values: 4,1
Key: 2, Values: 5,2

💡 Hint

Remember that GroupBy preserves the order of groups as they first appear in the source.

GroupBy preserves the order of groups based on the first occurrence of each key in the source sequence: key 1 (from 1), key 2 (from 2), key 0 (from 3). Within each group, elements appear in source order: 1 then 4, etc.

❓ Predict Output

intermediate

GroupBy with complex key and projection

What is the output of this C# code that groups words by their first letter and selects the count of words in each group?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        string[] words = {"apple", "apricot", "banana", "blueberry", "cherry"};
        var result = words.GroupBy(w => w[0])
                          .Select(g => new { Letter = g.Key, Count = g.Count() });
        foreach (var item in result) {
            Console.WriteLine($"{item.Letter}: {item.Count}");
        }
    }
}

A

a: 2
b: 2
c: 1

B

a: 2
b: 1
c: 2

C

a: 1
b: 2
c: 2

D

a: 2
b: 2
c: 2

💡 Hint

Count how many words start with each letter.

Words starting with 'a' are "apple" and "apricot" (2 words), with 'b' are "banana" and "blueberry" (2 words), and with 'c' is "cherry" (1 word). So counts are a:2, b:2, c:1.

🔧 Debug

advanced

Identify the error in GroupBy usage

What error does this C# code produce when run?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        var data = new[] {1, 2, 3, 4};
        var groups = data.GroupBy(n => n > 2 ? "High" : "Low");
        foreach (var group in groups) {
            Console.WriteLine(group.Key + ": " + group);
        }
    }
}

A System.InvalidOperationException at runtime

B

Output:
Low: System.Linq.Lookup`2+Grouping
High: System.Linq.Lookup`2+Grouping

C Compile-time error: Cannot convert from 'System.Linq.IGrouping<string,int>' to 'string'

D

Output:
Low: 1,2
High: 3,4

💡 Hint

Check what happens when you print a group object directly.

The code prints group.Key plus group.ToString(). The group.ToString() returns the type name, not the elements. So output shows keys and the type name of the group object, not the values.

❓ Predict Output

advanced

GroupBy with ordering inside groups

What is the output of this C# code that groups people by age and orders names inside each group alphabetically?

C Sharp (C#)

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

class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

class Program {
    static void Main() {
        var people = new List<Person> {
            new Person { Name = "John", Age = 30 },
            new Person { Name = "Jane", Age = 25 },
            new Person { Name = "Jake", Age = 30 },
            new Person { Name = "Jill", Age = 25 }
        };

        var groups = people.GroupBy(p => p.Age)
                           .Select(g => new {
                               Age = g.Key,
                               Names = g.OrderBy(p => p.Name).Select(p => p.Name)
                           });

        foreach (var group in groups) {
            Console.WriteLine($"Age: {group.Age}, Names: {string.Join(",", group.Names)}");
        }
    }
}

A

Age: 30, Names: Jake,John
Age: 25, Names: Jane,Jill

B

Age: 25, Names: Jill,Jane
Age: 30, Names: John,Jake

C

Age: 25, Names: Jane,Jill
Age: 30, Names: Jake,John

D

Age: 30, Names: John,Jake
Age: 25, Names: Jill,Jane

💡 Hint

Groups are ordered by first occurrence of keys, and names inside groups are alphabetically ordered.

People order: John(30), Jane(25), Jake(30), Jill(25). First key 30, then 25. Names alpha sorted: 30: Jake,John; 25: Jane,Jill.

❓ Predict Output

expert

GroupBy with nested grouping and flattening

What is the output of this C# code that groups numbers by even/odd, then groups each group by remainder mod 3, and finally flattens the results?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        var numbers = Enumerable.Range(1, 6);
        var nestedGroups = numbers.GroupBy(n => n % 2 == 0 ? "Even" : "Odd")
                                  .Select(g => new {
                                      Parity = g.Key,
                                      SubGroups = g.GroupBy(x => x % 3)
                                  });

        foreach (var group in nestedGroups) {
            Console.WriteLine(group.Parity + ":");
            foreach (var subGroup in group.SubGroups) {
                Console.WriteLine($"  Remainder {subGroup.Key}: {string.Join(",", subGroup)}");
            }
        }
    }
}

A

2 :2 redniameR  
4 :1 redniameR  
6 :0 redniameR  
:nevE
 :2 redniameR  
3 :0 redniameR  
5,1 :1 redniameR  
:ddO

B

Odd:
  Remainder 1: 1,4
  Remainder 0: 3
  Remainder 2: 5
Even:
  Remainder 2: 2,5
  Remainder 0: 6
  Remainder 1: 4

C

Odd:
  Remainder 1: 1,5
  Remainder 0: 3
  Remainder 2: 
Even:
  Remainder 0: 6
  Remainder 1: 4
  Remainder 2: 2

D

Odd:
  Remainder 1: 1
  Remainder 0: 3
  Remainder 2: 5
Even:
  Remainder 2: 2
  Remainder 1: 4
  Remainder 0: 6

💡 Hint

Group numbers by even/odd, then inside each group by remainder mod 3, watch carefully the numbers in each subgroup.

Odd numbers 1,3,5: rem 1,0,2 → subgroups order 1:1, 0:3, 2:5. Even 2,4,6: rem 2,1,0 → subgroups order 2:2, 1:4, 0:6. Matches B.

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