Reading Array Elements

In this lesson, we will look at how array elements are indexed, how to get them by index, and what happens when you access a non-existent element.

Indexing array elements

An index is the position number of an element in an array. Indexing starts at zero. Let's continue using the movie theater row analogy. Suppose a row has 4 seats total, and the first 3 are occupied; then the array might look like this:

Seat 1 (index 0) - Ivan
Seat 2 (index 1) - Kate
Seat 3 (index 2) - Pete
Seat 4 (index 3) - empty

Or in C#:


1
string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
Line 1, Char 1

Reading elements

Knowing the index, you can easily get an array element.


1
2
3
4
5
6
string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
string ivan = people[0];
Console.WriteLine(ivan);
// or directly, without using a variable
Console.WriteLine(people[1]);
Console.WriteLine(people[2]);
Line 1, Char 1

Going out of bounds

If you try to access an element that does not exist, the program will throw an IndexOutOfRangeException


1
2
string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
Console.WriteLine(people[4]);
Line 1, Char 1


1
2
string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
Console.WriteLine(people[-1]);
Line 1, Char 1

How to check an index before reading

An array has a Length property that indicates the number of elements in the array. To avoid an exception when reading, it is common to check that the index is within the bounds of the array


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
Console.WriteLine($"Array length: {people.Length}");
PrintName(2);
PrintName(4);
PrintName(-1);
void PrintName(int index)
{
    if (index >= 0 && index < people.Length)
    {
        Console.WriteLine(people[index]);
    }
    else
    {
        Console.WriteLine("Invalid index!");
    }
Line 1, Char 1

Summary

In this lesson you learned:

how to read array elements by index
that indexing in C# starts at zero
what IndexOutOfRangeException is
how to avoid errors and check the index

Exercises

Print the first and last elements of the array.


1
2
string[] fruits = new string[] { "Apple", "Banana", "Cherry", "Pear" };
// Your code
Line 1, Char 1

Try to print elements with index 4 and index -1. What will happen? Why?


1
2
string[] fruits = new string[] { "Apple", "Banana", "Cherry", "Pear" };
// Your code
Line 1, Char 1

Write a method PrintFruit that takes an index and prints the array element if the index is valid; otherwise prints the message "Invalid index!"


1
2
3
4
5
6
7
8
string[] fruits = new string[] { "Apple", "Banana", "Cherry", "Pear" };
PrintFruit(1);
PrintFruit(4);
PrintFruit(-1);
void PrintFruit(int index)
{
    // Your code
Line 1, Char 1