C# Academy · Lesson

Indexing and Length

Access elements safely.

Lesson 2 of 413 steps

Indexing and Length is a free C# Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Reading by Index

Each element in an array has a position called an index. You read a value by putting the index in square brackets after the array name.

Indexes start at 0, so the very first element is at index 0.

string[] days = { "Mon", "Tue", "Wed" };
Console.WriteLine(days[0]); // Mon

Zero-Based Counting

Because indexes start at 0, the second element is index 1 and the third is index 2.

This zero-based counting trips up beginners, so remember: index = position minus one.

string[] days = { "Mon", "Tue", "Wed" };
Console.WriteLine(days[1]); // Tue
Console.WriteLine(days[2]); // Wed

Changing an Element

You can overwrite any element by assigning a new value to its index.

Here we replace the value at index 1. The array length does not change, only the stored value.

int[] nums = { 5, 6, 7 };
nums[1] = 99;
Console.WriteLine(nums[1]); // 99

The Length Property

Every array has a Length property that tells you how many elements it holds.

This is handy because you do not have to remember the size yourself.

int[] nums = { 5, 6, 7, 8 };
Console.WriteLine(nums.Length); // 4

Last Element

The last valid index is always Length - 1, because counting starts at 0.

Using Length - 1 lets you find the final element without hard-coding a number.

int[] nums = { 5, 6, 7, 8 };
Console.WriteLine(nums[nums.Length - 1]); // 8

The Index From End Operator

C# offers a shortcut for counting from the end using the ^ operator. ^1 means the last element, ^2 the second to last.

It is a clean way to grab the end without writing Length - 1.

int[] nums = { 5, 6, 7, 8 };
Console.WriteLine(nums[^1]); // 8

A Full Program

This complete program prints the first element, the last element, and the total length.

Run it to watch all three values appear.

int[] nums = { 10, 20, 30 };
Console.WriteLine("First: " + nums[0]);
Console.WriteLine("Last: " + nums[nums.Length - 1]);
Console.WriteLine("Count: " + nums.Length);

Out of Range Errors

If you use an index that is too big or negative, C# throws an IndexOutOfRangeException and the program crashes.

For an array of length 3, valid indexes are only 0, 1, and 2. Index 3 is one step too far.

int[] nums = { 1, 2, 3 };
// nums[3] would throw IndexOutOfRangeException
Console.WriteLine(nums[2]); // safe: 3

Checking Before Access

To avoid crashes, check that your index is within range before using it.

A safe index must be at least 0 and less than Length.

int[] nums = { 1, 2, 3 };
int i = 5;
if (i >= 0 && i < nums.Length)
    Console.WriteLine(nums[i]);
else
    Console.WriteLine("Out of range");

Using Length in Math

Because Length is just an integer, you can use it in calculations, such as finding a middle index.

Here we compute the center position of the array.

int[] nums = { 1, 2, 3, 4, 5 };
int mid = nums.Length / 2;
Console.WriteLine(nums[mid]); // 3

Length Is Read-Only

You cannot change Length to resize an array. Arrays have a fixed size once created.

If you need a different size, you create a new array. Collections like List<T> are better when the size must grow.

int[] nums = { 1, 2, 3 };
// nums.Length = 5; // error: read-only
Console.WriteLine(nums.Length); // 3

Quick Check

Test your understanding of indexes and length.

Answer it in the interactive lesson

Recap

You learned to read and change elements by index, that indexes are zero-based, and that the last index is Length - 1.

The Length property counts elements, the ^ operator counts from the end, and going out of range throws an exception. Next you will loop over every element.

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Frequently Asked Questions

Is the “Indexing and Length” lesson free?

Yes — the full text of “Indexing and Length” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Indexing and Length”?

Access elements safely. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Indexing and Length” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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