Lesson 9: Arrays

At the end of this lesson, you should be able to answer the following:

• What is an array?

• What is an element? What is an index?

• How do I declare an array in C#?

• How do I access the values in an array?

So far, we have been storing single values in our variables. But in some situations, we need variables that store more than one value.

An array is a data structure that holds multiple values of the same type. It's denoted by the square brackets at the end of the type name.

int[] scores;

Above we have declared an array of integers. Because we have not initialised it yet, its value is null.

To be able to use the array, we need to initialise it first by allocating how many values we need. This number determines the size of the array. Note that the size of the array is fixed and cannot be changed once it's allocated.

int[] scores = new int[10];
var words = new string[5];

To initialise the array, we use the new keyword, followed by the type of the array, and the number of values we want in the square brackets. Line 2 shows another way of declaring and initialising an array, using the var keyword that we learned in Lesson 5.

We can access the values in the array by their position using the square brackets. Each value is called an element and its position is its index.

Let's try printing some values. Type the following in your code box and run the program.

int[] scores = new int[10];

Console.WriteLine(scores[1]);

The array is initialised, but the elements in the array are still the default values of each type. That's why scores[1] prints 0, the default value for int.

Try accessing the last element of the array. Do you know which index to use?

Since we have 10 elements, wouldn't the last element be the 10th one? If you tried to run the code below, you might get a surprise!

int[] scores = new int[10];

Console.WriteLine(scores[10]);

An error happens because 10 is not the index of the last element! Arrays in C# are zero-based, meaning the index starts at 0. So an array of 10 elements would have indexes from 0 to 9.

To demonstrate this more clearly, let's first put some actual values into our array. We can do this during our array declaration. The following are all valid ways to declare and initialise an array:

int[] arr1 = new int[3] { 10, 20, 30 };
int[] arr2 = new int[] { 10, 20, 30 }; 
int[] arr3 = { 10, 20, 30 }

We can declare the array elements inside curly brackets next to the type.

For Lines 1 and 2, we can also replace the first int[] in Lines 1 and 2 with a var keyword and the syntax will be valid. We can't do the same with Line 3, though.

For Lines 2 and 3, the size of the array is inferred by C# from the number of values.

Use one of the ways above to declare a scores array with some values. Then print the first and last element by accessing the elements with the index 0 and 9 respectively.

// Declare an array with 10 elements
int[] scores = { 100, 85, 80, 93, 98, 100, 74, 88, 90, 99 };

// Print the first element
Console.WriteLine(scores[0]);

// Print the last element
Console.WriteLine(scores[9]);

Run the program. The first element is 100, and the last element is 99.

We can get the total number of elements in an array using the Length property. Add the following to the code above and run the program. The output should be 10.

Console.WriteLine(scores.Length);

Can you recall where we used a Length property before? In Lesson 6 we learned that strings have a Length property that returns the number of characters in the string. Coincidence?

No, because strings are actually arrays of char values! You can do the following with a string:

var move = "consecutive normal punches";
Console.WriteLine($"The 4th character of '{move}' is '{move[3]}'.");

Questions

True or False:

• Once allocated, the size of an array is fixed and cannot change.

• You can have both int and string values in one array.

• The first element of an arrayarr is in arr[0].

• The last element of an array arr2 is in arr2[arr2.Length-1].

• This is a valid array declaration: int[] arr3 = new int[];

Challenge

Create a string array with your desired number of elements. Populate it using one of the initialisation expressions shown in the lesson. Then print the first element, last element, and the total number of elements in the array. Use string interpolation to make the output like this:

First element:
Last element:
Total number of elements:

Challenge

Using the string array in the previous challenge, print out all the elements in the array. (Hint: You can use a for loop!)