Aggregate functions (Count, Sum, Average) in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Count example?

Consider this C# code that counts elements in a list:

var numbers = new List<int> {1, 2, 3, 4, 5};
int count = numbers.Count(n => n > 3);
Console.WriteLine(count);

What will be printed?

C Sharp (C#)

var numbers = new List<int> {1, 2, 3, 4, 5};
int count = numbers.Count(n => n > 3);
Console.WriteLine(count);

A 2

B 3

C 5

D 4

💡 Hint

Count counts how many items satisfy the condition.

Only numbers greater than 3 are 4 and 5, so count is 2.

❓ Predict Output

intermediate

What is the sum of these numbers?

Look at this code that sums numbers:

var values = new List<int> {10, 20, 30};
int total = values.Sum();
Console.WriteLine(total);

What is printed?

C Sharp (C#)

var values = new List<int> {10, 20, 30};
int total = values.Sum();
Console.WriteLine(total);

A 0

B 60

C 100

D 30

💡 Hint

Sum adds all numbers in the list.

10 + 20 + 30 equals 60.

❓ Predict Output

advanced

What is the average value computed here?

Analyze this code that calculates average:

var scores = new List<double> {4.0, 5.0, 6.0};
double avg = scores.Average();
Console.WriteLine(avg);

What will be printed?

C Sharp (C#)

var scores = new List<double> {4.0, 5.0, 6.0};
double avg = scores.Average();
Console.WriteLine(avg);

A 5

B 15

C 4

D 6

💡 Hint

Average is total sum divided by count.

(4 + 5 + 6) / 3 = 15 / 3 = 5.

❓ Predict Output

advanced

What error occurs when calling Average on an empty list?

Consider this code:

var emptyList = new List<int>();
double avg = emptyList.Average();
Console.WriteLine(avg);

What happens when this runs?

C Sharp (C#)

var emptyList = new List<int>();
double avg = emptyList.Average();
Console.WriteLine(avg);

A 0

B null

C InvalidOperationException

D DivideByZeroException

💡 Hint

Average on empty collections throws an error.

Calling Average on an empty list throws InvalidOperationException because there is no element to average.

🧠 Conceptual

expert

How many items are counted here?

Given this code:

var data = new List<int> {1, 2, 3, 4, 5, 6};
int count = data.Count(n => n % 2 == 0 && n > 3);
Console.WriteLine(count);

How many items satisfy the condition?

C Sharp (C#)

var data = new List<int> {1, 2, 3, 4, 5, 6};
int count = data.Count(n => n % 2 == 0 && n > 3);
Console.WriteLine(count);

A 4

B 3

C 1

D 2

💡 Hint

Count numbers that are even and greater than 3.

Numbers 4 and 6 are even and greater than 3, so count is 2.

Practice

1.

Which aggregate function in C# is used to find how many items are in a list?

easy

A. Count

B. Sum

C. Average

D. Max

Solution

1. Step 1: Understand the purpose of Count

   The Count function returns the number of elements in a collection.

2. Step 2: Compare with other aggregate functions

   Sum adds values, Average calculates mean, Max finds the largest value, so they do not count items.

3. Final Answer:

   Count -> Option A

4. Quick Check:

   Count = number of items [OK]

Hint: Count counts items, Sum adds, Average finds mean [OK]

Common Mistakes:

• Confusing Sum with Count
• Using Average to count items
• Thinking Max counts items

2.

Which of the following is the correct syntax to calculate the sum of integers in a list named numbers?

var total = ???;

easy

A. numbers.Sum()

B. numbers.Count()

C. Sum(numbers)

D. numbers.Average()

Solution

1. Step 1: Identify the method to sum list elements

   The Sum() method is called on the list to add all elements.

2. Step 2: Check syntax correctness

   numbers.Sum() is the correct syntax; Count() counts items, Average() finds mean, Sum(numbers) is invalid.

3. Final Answer:

   numbers.Sum() -> Option A

4. Quick Check:

   Sum() adds list values [OK]

Hint: Use list.Sum() to add all numbers [OK]

Common Mistakes:

• Using Count() instead of Sum()
• Calling Sum as a standalone function
• Using Average() to sum values

3.

What is the output of this C# code?

var numbers = new List<int> { 2, 4, 6, 8 };
var result = numbers.Average();
Console.WriteLine(result);

medium

A. 20

B. 6

C. 4

D. 5

Solution

1. Step 1: Calculate the sum of the list elements

   Sum = 2 + 4 + 6 + 8 = 20

2. Step 2: Calculate the average

   Average = Sum / Count = 20 / 4 = 5.0

3. Step 3: Check the output type

   Average returns a double, so output is 5 (printed as 5)

4. Final Answer:

   5 -> Option D

5. Quick Check:

   Average = 20 / 4 = 5 [OK]

Hint: Average = sum of values divided by count [OK]

Common Mistakes:

• Adding values but not dividing by count
• Confusing sum with average
• Miscounting number of elements

4.

Identify the error in this code snippet and choose the correct fix:

var numbers = new int[] { 1, 2, 3 };
var total = numbers.Count + 5;
Console.WriteLine(total);

medium

A. No error, code is correct

B. Replace Count with Sum()

C. Replace Count with Count()

D. Replace Count with Average()

Solution

1. Step 1: Identify the error in Count usage

   Count is a method, so it requires parentheses: Count()

2. Step 2: Fix the syntax

   Change numbers.Count to numbers.Count() to get the number of elements.

3. Final Answer:

   Replace Count with Count() -> Option C

4. Quick Check:

   Count() is a method, not a property [OK]

Hint: Count is a method, always use parentheses [OK]

Common Mistakes:

• Using Count without parentheses
• Replacing Count with Sum or Average incorrectly
• Assuming Count is a property

5.

You have a list of exam scores: var scores = new List<int> { 70, 85, 90, 100, 65 };. You want to find the average score but only for scores above 80. Which code snippet correctly calculates this?

hard

A. var avg = scores.Average(s => s > 80);

B. var avg = scores.Where(s => s > 80).Average();

C. var avg = scores.Sum(s => s > 80) / scores.Count();

D. var avg = scores.Count(s => s > 80) / scores.Sum();

Solution

1. Step 1: Filter scores greater than 80

   Use Where(s => s > 80) to select only scores above 80.

2. Step 2: Calculate average of filtered scores

   Call Average() on the filtered list to get the mean of those scores.

3. Final Answer:

   var avg = scores.Where(s => s > 80).Average(); -> Option B

4. Quick Check:

   Filter then average = correct approach [OK]

Hint: Filter with Where(), then call Average() [OK]

Common Mistakes:

• Passing condition directly to Average()
• Dividing sum by total count instead of filtered count
• Using Count divided by Sum incorrectly