Aggregate functions (Count, Sum, Average) in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count the number of elements in the list.

C Sharp (C#)

int count = numbers.[1]();

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

ACount

BSum

CAverage

DMax

Attempts:

3 left

💡 Hint

Common Mistakes

Using Sum() instead of Count()

Using Average() which calculates the mean

Using Max() which finds the largest value

The Count() method returns the number of elements in the list.

2fill in blank

medium

Complete the code to calculate the sum of all elements in the list.

C Sharp (C#)

int total = numbers.[1]();

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

AMin

BSum

CAverage

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count() which returns the number of elements

Using Average() which calculates the mean

Using Min() which finds the smallest value

The Sum() method adds all the numbers in the list together.

3fill in blank

hard

Fix the error in the code to calculate the average of the list elements.

C Sharp (C#)

double avg = numbers.[1]();

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

AAverage

BCount

CSum

DMax

Attempts:

3 left

💡 Hint

Common Mistakes

Using Sum() which adds all elements

Using Count() which counts elements

Using Max() which finds the largest element

The Average() method calculates the mean value of the list elements.

4fill in blank

hard

Fill both blanks to create a dictionary with words as keys and their lengths as values.

C Sharp (C#)

var lengths = new Dictionary<string, int> { { [1], [2] } };

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

Aword

Bword.Length

Cword.Length > 3

D"length"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a boolean expression as a value

Using a string 'length' instead of the actual length

Using the length as key instead of value

Use the word itself as the key and its length as the value.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths, including only words longer than 4 characters.

C Sharp (C#)

var result = words.Where(word => word.Length [1] 4).ToDictionary([2] => [3], word => word.Length);

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

A>

Bw

Cw.ToUpper()

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the filter

Using the original word as key instead of uppercase

Using the wrong lambda parameter name

Filter words longer than 4, then map each word's uppercase form as key and length as value.

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