Aggregate functions (Count, Sum, Average) in C Sharp (C#) - Mini Project: Build & Apply

Aggregate functions (Count, Sum, Average)

📖 Scenario: You work in a small store and want to analyze sales data to understand how many items were sold, the total sales amount, and the average price per item.

🎯 Goal: Build a simple C# program that uses aggregate functions to count items, sum prices, and calculate the average price from a list of sales.

📋 What You'll Learn

Create a list of sales with exact prices

Create a variable to hold the number of sales

Calculate the total sum of sales prices

Calculate the average price of sales

Print the count, sum, and average values

💡 Why This Matters

🌍 Real World

Stores and businesses often analyze sales data to understand performance and make decisions.

💼 Career

Knowing how to use aggregate functions like count, sum, and average is essential for data analysis and reporting in many programming jobs.

1

Create the sales data list

Create a List<double> called sales with these exact values: 10.5, 20.0, 15.75, 30.0, 25.25.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Create the sales list with exact values
        // Your code here
    }
}

Hint

Use List<double> and initialize it with the exact numbers inside curly braces.

2

Create a variable to count sales

Create an int variable called count and set it to the number of items in the sales list using sales.Count.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List sales = new List { 10.5, 20.0, 15.75, 30.0, 25.25 };
        // Create an int variable count and set it to sales.Count
        // Your code here
    }
}

Hint

Use int count = sales.Count; to get the number of items.

3

Calculate sum and average of sales

Create a double variable called sum and set it to the sum of all values in sales using sales.Sum(). Then create a double variable called average and set it to sum / count.

C Sharp (C#)

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

class Program
{
    static void Main()
    {
        List sales = new List { 10.5, 20.0, 15.75, 30.0, 25.25 };
        int count = sales.Count;
        // Calculate sum using sales.Sum()
        // Calculate average as sum / count
        // Your code here
    }
}

Hint

Use sales.Sum() to get the total sum. Then divide by count to get the average.

4

Print the count, sum, and average

Write three Console.WriteLine statements to print the values of count, sum, and average exactly as shown:
Count: 5
Sum: 101.5
Average: 20.3.
Use average.ToString("F1") to format the average to one decimal place.

C Sharp (C#)

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

class Program
{
    static void Main()
    {
        List sales = new List { 10.5, 20.0, 15.75, 30.0, 25.25 };
        int count = sales.Count;
        double sum = sales.Sum();
        double average = sum / count;
        // Print count, sum, and average with formatting
        // Your code here
    }
}

Hint

Use Console.WriteLine with string interpolation and format average with ToString("F1").

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