27. C# (Arrays)

Full Runnable Code

using System;

class Program
{
    static void Main()
    {
        // 1) Declare + allocate (check default values)
        int[] numbers = new int[3];

        Console.WriteLine("Default values:");
        Console.WriteLine(numbers[0]);
        Console.WriteLine(numbers[1]);
        Console.WriteLine(numbers[2]);
        Console.WriteLine();

        // 2) Update values
        numbers[0] = 10;
        numbers[1] = 20;
        numbers[2] = 30;

        Console.WriteLine("After updates:");
        for (int i = 0; i < numbers.Length; i++)
        {
            Console.WriteLine(numbers[i]);
        }
        Console.WriteLine();

        // 3) Index from End (^)
        int[] numbers2 = { 10, 20, 30, 40, 50 };

        Console.WriteLine("Access from the end:");
        Console.WriteLine(numbers2[^1]); // 50
        Console.WriteLine(numbers2[^2]); // 40
        Console.WriteLine();

        // 4) Sum all values
        int sum = 0;

        for (int i = 0; i < numbers2.Length; i++)
        {
            sum += numbers2[i];
        }

        Console.WriteLine("Sum:");
        Console.WriteLine(sum);

        Console.ReadKey();
    }
}

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0. The Real Goal of This Lesson

An array is not “multiple variables.”
It is one structure that stores many values of the same type under a single reference.

If you miss this:

• Index logic feels random
• Off-by-one bugs keep happening
• Runtime errors feel “mysterious”

This lesson is about understanding the execution model.

---

1. What Is an Array?

A variable is one box:

int number = 10;

An array is multiple boxes as one structure:

int[] numbers = new int[3];

Meaning:

• Allocate space for 3 int values
• Each element is filled with the default value (0)

---

2. Index Starts at 0

If the array size is 3:

index: 0 1 2

The last valid index is always:

Length - 1

This is non-negotiable.

---

3. Compile-Time Error vs Runtime Error

Compile-Time Error (Type Mismatch)

numbers[0] = "hello"; // invalid type

The compiler blocks execution.

The program cannot run.

---

Runtime Error (Index Out of Range)

numbers[3] = 100; // index out of range

Compilation succeeds.

The program starts.

Then it crashes during execution.

Typical error:

Index was outside the bounds of the array

This is the exact difference:

• Type mistakes fail early (compile-time)
• Index mistakes fail late (runtime)

---

4. The Most Common Beginner Mistake

numbers[numbers.Length]

If Length = 3, valid indices are:

0, 1, 2

So numbers[3] is invalid.

Rule:

Last index = Length - 1

---

5. The ^ Operator (Index from End)

numbers2[^1]

Meaning:

• Last element

Equivalent to:

numbers2[numbers2.Length - 1]

So:

• ^1 → last
• ^2 → second last

It is a convenience feature.

The indexing rules are still the same.

---

6. The One Rule You Must Not Break in for Loops

Correct:

i < numbers.Length

Incorrect:

i <= numbers.Length

Why?

Because the final iteration will produce:

i == Length

And numbers[Length] is out of range.

This is the classic off-by-one error.

---

7. The Biggest Limitation of Arrays

Arrays have fixed size.

new int[3]

Those 3 slots stay 3 slots.

You cannot grow or shrink the array.

So:

• Fixed count → array
• Variable count → later use List<T>

---

Final Summary

An array is an index-based structure that stores a fixed number of same-type values.

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Learning Check (One Question)

Explain exactly when the runtime error occurs, step by step, tracking i:

int[] numbers = { 10, 20, 30 };

for (int i = 0; i <= numbers.Length; i++)
{
    Console.WriteLine(numbers[i]);
}

You must answer:

• How does i change starting from 0?
• What is numbers.Length here?
• What is the final value of i?
• At which exact moment does the crash happen, and why?