meet it

Every item in an array has an , starting at 0.

Each item has an index: its position number. C# starts counting at 0, so the first item is planets[0] and the second is planets[1]. Square brackets after the name pick one out.

So the last item is always at Length - 1. To change an item, put a new value in its slot: planets[1] = "Mars";. An array's size is fixed, so you cannot add slots.

Ask for a slot that is not there and C# stops with an , an error that happens while the program runs. This one is called IndexOutOfRangeException.

using System;

string[] planets = { "Mercury", "Venus", "Earth" };
Console.WriteLine(planets[0]);
Console.WriteLine(planets[planets.Length - 1]);
planets[1] = "Mars";
Console.WriteLine(string.Join(", ", planets));

Think of the index as how many steps from the start, so the first item is 0 steps away.

guess first

Which planet is at index 1?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;

string[] planets = { "Mercury", "Venus", "Earth" };
Console.WriteLine(planets[1]);

your turn

Your turn. Join the team.

Swap one player for you. Type an index, 0, 1 or 2, in the square brackets, and your name between the quotes. Then press run.

Your code

output

press run to see what happens

fix it

We want the last cake, but C# crashes. Fix it!

Broken code to fix

output

press run to see what happens

C# stopped with an exception. Count the cakes, then count their indexes.
Three cakes have indexes 0, 1 and 2. There is no slot 3.
Change 3 to 2 on line 4, or use cakes.Length - 1, then press run.

nailed it

you can pick any item you like. :D

The index of the first item is 0.
The last item is at Length - 1.
A missing slot stops the program with an exception.

new words in your collection

index
exception

This lesson isn’t saved. A free account remembers every lesson you finish.

Every item in an array has an index, starting at 0.

Which planet is at index 1?

Your turn. Join the team.

We want the last cake, but C# crashes. Fix it!

you can pick any item you like. :D

Every item in an array has an , starting at 0.