📝 Lesson 4.1: Collections — Arrays and Lists

Why Collections?

Imagine storing five test scores. With what you know so far, you might write:

int score1 = 85;
int score2 = 92;
int score3 = 78;
int score4 = 90;
int score5 = 88;

That's already awkward — and hopeless for 500 scores. A collection holds many values under a single name, so you can store them together and process them with a loop.

Arrays

An array stores a fixed number of values of the same type. You declare the type followed by []. Here are two ways to create one:

// 1) Create with known values
int[] scores = { 85, 92, 78, 90, 88 };

// 2) Create empty with a fixed size, then fill it
int[] temps = new int[3];   // three slots, all start at 0
temps[0] = 20;
temps[1] = 25;
temps[2] = 22;

Picture an array as a row of numbered boxes, all holding the same type of thing:

Indexing and Zero-Based Counting

You access an item by its index — its position number — using square brackets. Here's the twist that surprises every beginner:

int[] scores = { 85, 92, 78, 90, 88 };

Console.WriteLine(scores[0]);   // 85  (first item)
Console.WriteLine(scores[4]);   // 88  (last item)

scores[1] = 100;                // change the second item
Console.WriteLine(scores[1]);   // 100

Console.WriteLine(scores.Length);   // 5  (how many items)

Looping Through Arrays

This is where collections and loops (Lesson 3.2) come together beautifully. To visit every item, use a loop instead of writing a line per element.

With foreach (simplest)

int[] scores = { 85, 92, 78, 90, 88 };

foreach (int score in scores)
{
    Console.WriteLine(score);
}

With for (when you need the index)

int[] scores = { 85, 92, 78, 90, 88 };

for (int i = 0; i < scores.Length; i++)
{
    Console.WriteLine($"Score #{i + 1} is {scores[i]}");
}

Score #1 is 85
Score #2 is 92
Score #3 is 78
Score #4 is 90
Score #5 is 88

Here's a practical combination — summing an array and computing an average:

int[] scores = { 85, 92, 78, 90, 88 };
int total = 0;

foreach (int score in scores)
{
    total += score;
}

double average = (double)total / scores.Length;
Console.WriteLine($"Total: {total}, Average: {average:F1}");
// Total: 433, Average: 86.6

(The (double) converts total to a double so we get a decimal average instead of falling into the integer-division trap from Lesson 2.2.)

Lists: Collections That Grow

Arrays are great when you know the size ahead of time. But often you don't — think of a shopping list you add to as you go. For that, use a List<T>, which can grow and shrink as your program runs.

The <T> part is the type of item the list holds — List<int> is a list of ints, List<string> a list of strings. (You may need using System.Collections.Generic; at the top in older project styles; modern templates include it automatically.)

// A list of strings, starting empty
List<string> tasks = new List<string>();

tasks.Add("Buy milk");
tasks.Add("Walk the dog");
tasks.Add("Write C# code");

Console.WriteLine(tasks.Count);   // 3

// You can also start with items:
List<int> scores = new List<int> { 85, 92, 78 };

Indexing and looping work just like arrays:

Console.WriteLine(tasks[0]);   // Buy milk

foreach (string task in tasks)
{
    Console.WriteLine($"- {task}");
}

Common List Operations

Lists come with handy built-in methods. Here are the ones you'll use most:

List<string> tasks = new List<string> { "Buy milk", "Walk the dog" };

tasks.Add("Write C# code");         // now 3 items
tasks.Remove("Buy milk");           // now 2 items
Console.WriteLine(tasks.Count);     // 2
Console.WriteLine(tasks.Contains("Walk the dog"));   // True

foreach (string task in tasks)
{
    Console.WriteLine(task);
}
// Walk the dog
// Write C# code

Exercise & Quiz

List<string> tasks = new List<string>();

Console.Write("Enter a task (or 'done' to finish): ");
string input = Console.ReadLine();

while (input != "done")
{
    // TODO: add input to the list, then prompt + read again
}

// TODO: print the count, then each task numbered

List<string> tasks = new List<string>();

Console.Write("Enter a task (or 'done' to finish): ");
string input = Console.ReadLine();

while (input != "done")
{
    tasks.Add(input);
    Console.Write("Enter a task (or 'done' to finish): ");
    input = Console.ReadLine();
}

Console.WriteLine($"\nYou have {tasks.Count} task(s):");
for (int i = 0; i < tasks.Count; i++)
{
    Console.WriteLine($"{i + 1}. {tasks[i]}");
}

Enter a task (or 'done' to finish): Buy milk
Enter a task (or 'done' to finish): Walk the dog
Enter a task (or 'done' to finish): done

You have 2 task(s):
1. Buy milk
2. Walk the dog

Summary

📚 Additional Resources

• Arrays — C# Programming Guide
• List<T> — API reference
• Arrays and collections tutorial

🚀 What's Next?

You can now group data together. Next, we learn to bundle data and behavior into custom types of our own. In Lesson 4.2: Introduction to Object-Oriented Programming, you'll create your own classes — the foundation of modern C#.