Exercise - Get started with array basics

19 minutes

Arrays can be used to store multiple values of the same type in a single variable. The values stored in an array are generally related. For example, a list of student names could be stored in a string array named students.

Your work in the security department is focused on finding a pattern for fraudulent orders. You want your code to review past customer orders and identify markers associated with fraudulent orders. Your company hopes the markers can be used to identify potential fraudulent purchase orders in the future before they're processed. Since you don't always know in advance how many orders you need to review, you can't create individual variables to hold each Order ID. How can you create a data structure to hold multiple related values?

In this exercise, you use arrays to store and analyze a collection of Order IDs.

What is an array?

An array is a collection of individual data elements accessible through a single variable name. You use a zero-based numeric index to access each element of an array. Arrays allow you to create a collection of data values that shares a common purpose or characteristics under a single variable name for easier processing.

Declaring arrays and accessing array elements

An array is a special type of variable that can hold multiple values of the same data type. The declaration syntax is slightly different for an array because you have to specify both the data type and the size of the array.

Prepare your coding environment

This module includes activities that guide you through the process of building and running sample code. You're encouraged to complete these activities using Visual Studio Code as your development environment. Using Visual Studio Code for these activities helps you to become more comfortable writing and running code in a developer environment that's used by professionals worldwide.

Open Visual Studio Code.

You can use the Windows Start menu (or equivalent resource for another OS) to open Visual Studio Code.
On the Visual Studio Code File menu, select Open Folder.
In the Open Folder dialog, navigate to the Windows Desktop folder.

If you have a different folder location where you keep code projects, you can use that folder location instead. For this training, the important thing is to have a location that’s easy to locate and remember.
In the Open Folder dialog, select Select Folder.

If you see a security dialog asking if you trust the authors, select Yes.
On the Visual Studio Code Terminal menu, select New Terminal.

Notice that a command prompt in the Terminal panel displays the folder path for the current folder. For example:
```
C:\Users\someuser\Desktop>
```
Note

If you are working on your own PC rather than in a sandbox or hosted environment and you have completed other Microsoft Learn modules in this C# series, you may have already created a project folder for code samples. If that's the case, you can skip over the next step, which is used to create a console app in the TestProject folder.
At the Terminal command prompt, to create a new console application in a specified folder, enter the following prompt:
```
dotnet new console -o ./CsharpProjects/TestProject
```
This .NET CLI command uses a .NET program template to create a new C# console application project in the specified folder location. The command creates the CsharpProjects and TestProject folders for you, and uses TestProject as the name of your .csproj file.

If a message is displayed telling you that the files already exist, continue with the next steps. You'll reuse the existing project files.
In the EXPLORER view, expand the CsharpProjects folder.

You should see the TestProject folder and two files, a C# program file named Program.cs and a C# project file named TestProject.csproj.
On the Visual Studio Code File menu, select Open Folder.
In the Open Folder dialog, select the CsharpProjects folder, and then select Select Folder.
In the EXPLORER view, expand the TestProject folder, and then select Program.cs.
Delete the existing code lines.

You'll be using this C# console project to create, build, and run code samples during this module.
Close the Terminal panel.

Declare a new array

To declare a new array of strings that can hold three elements, enter the following code:
```
string[] fraudulentOrderIDs = new string[3];
```
Take a minute to examine your code.

The new operator creates a new instance of an array in the computer's memory that can hold three string values. For more information about the new keyword, see the module "Call methods from the .NET Class Library using C#".

Notice that the first set of square brackets [] merely tells the compiler that the variable named fraudulentOrderIDs is an array, but the second set of square brackets [3] indicates the number of elements that the array can hold.

Note

This example demonstrates how to declare an array of strings, however, you can create an array of every data type including primitives like int and bool as well as more complex data types like classes. This example uses the simplicity of strings to minimize the number of new ideas you need to grasp as you're getting started.

Assign values to elements of an array

At this point, you've declared an array of strings, but each element of the array is empty. To access an element of an array, you use a numeric zero-based index inside of square brackets. You can assign a value to an array element using the = as if it were a regular variable.

To assign Order ID values to your fraudulentOrderIDs array, update your code as follows:

string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";

Take a minute to examine your code.

Notice that you're using the name of the array to access array elements. Each element is accessed individually by specifying zero-based index number inside the square brackets.

Since your array is declared as a string, the values that you assign must also be strings. In this scenario, you're assigning Order IDs to the elements of the array.

Attempt to use an index that is out of bounds of the array

It might not seem intuitive at first, but it's important to remember that you're declaring the count of elements in the array. However, you access each element of the array starting with zero. So, to access the second item in the array, you use index 1.

It's common for beginners to forget that arrays are zero-based and attempt to access an element of the array that doesn't exist. If you make this mistake, a runtime exception occurs informing you that you attempted to access an element that is outside the boundary of the array.

To intentionally "break" your application, attempt to access a fourth element of your array using index value of 3.

At the bottom of your code file, enter the following code line:
```
fraudulentOrderIDs[3] = "D000";
```

Ensure that your code matches this example:

string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";
fraudulentOrderIDs[3] = "D000";

On the Visual Studio Code File menu, select Save.
In the EXPLORER view, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.

A Terminal panel should open, and should include a command prompt showing that the Terminal is open to your TestProject folder location.
At the Terminal command prompt, to compile your code, type dotnet build and then press Enter.

You should see the following message:
```
Build succeeded.        
    0 Warning(s)        
    0 Error(s)
```
At the Terminal command prompt, to run your code, type dotnet run and then press Enter.

When you run the app, you get the following runtime error message:
```
Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.     
   at Program.<Main>$(String[] args) in C:\Users\someuser\Desktop\CsharpProjects\TestProject\Program.cs:line 6
```
Notice the following parts of the error:
- Error message: System.IndexOutOfRangeException: Index was outside the bounds of the array.
- Error location: Program.cs:line 6
Comment out the line that generated the runtime error.
```
// fraudulentOrderIDs[3] = "D000";
```

You've seen how to assign a value to an array element. Now look at how to access a value that's being stored in an array element.

Retrieve values from elements of an array

Accessing the value of an array element works the same way as assigning a value to an array element. You just specify the index of the element whose value you want to retrieve.

To write the value of each fraudulent Order ID, update your code as follows:

string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";
// fraudulentOrderIDs[3] = "D000";

Console.WriteLine($"First: {fraudulentOrderIDs[0]}");
Console.WriteLine($"Second: {fraudulentOrderIDs[1]}");
Console.WriteLine($"Third: {fraudulentOrderIDs[2]}");

On the Visual Studio Code File menu, select Save.
In the EXPLORER view, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.
At the Terminal command prompt, type dotnet run and then press Enter.

You should see the following message:
```
First: A123
Second: B456
Third: C789
```

Reassign the value of an array

The elements of an array are just like any other variable value. You can assign, retrieve, and reassign a value to each element of the array.

At the end of your code file, to reassign and then print the value of the first array element, enter the following code:
```
fraudulentOrderIDs[0] = "F000";

Console.WriteLine($"Reassign First: {fraudulentOrderIDs[0]}");
```

Ensure that your code matches the following example:

string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";
// fraudulentOrderIDs[3] = "D000";

Console.WriteLine($"First: {fraudulentOrderIDs[0]}");
Console.WriteLine($"Second: {fraudulentOrderIDs[1]}");
Console.WriteLine($"Third: {fraudulentOrderIDs[2]}");

fraudulentOrderIDs[0] = "F000";

Console.WriteLine($"Reassign First: {fraudulentOrderIDs[0]}");

On the Visual Studio Code File menu, select Save.
In the EXPLORER view, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.
At the Terminal command prompt, type dotnet run and then press Enter.

You should see the following message:
```
First: A123
Second: B456
Third: C789
Reassign First: F000
```

Initialize an array

You can initialize an array during declaration just like you would a regular variable.

Comment out the lines where you declare the fraudulentOrderIDs variable.

You can use a multi-line comment (/* ... */) to comment out the declaration of fraudulentOrderIDs and the lines used to assign values to the array elements.
To declare the array and initialize values in a single statement, enter the following code:
```
string[] fraudulentOrderIDs = [ "A123", "B456", "C789" ];
```
This example uses the Collection expression syntax, which was introduced in C# 12.

You may also see an older syntax used to initialize an array.
```
string[] fraudulentOrderIDs = { "A123", "B456", "C789" };
```
Notice that this older syntax uses curly braces {} to enclose the values of the array. Both syntaxes are valid.

Note

You may see a combination of the older syntax and the Collection expression syntax used in this training.

Ensure that your code matches the following example:

/*
string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";
// fraudulentOrderIDs[3] = "D000";
*/

string[] fraudulentOrderIDs = [ "A123", "B456", "C789" ];

Console.WriteLine($"First: {fraudulentOrderIDs[0]}");
Console.WriteLine($"Second: {fraudulentOrderIDs[1]}");
Console.WriteLine($"Third: {fraudulentOrderIDs[2]}");

fraudulentOrderIDs[0] = "F000";

Console.WriteLine($"Reassign First: {fraudulentOrderIDs[0]}");

Take a minute to examine the declaration statement.

Notice that this syntax is both compact and easy to read. When you run the application, there should be no change to the output.
On the Visual Studio Code File menu, select Save.
In the EXPLORER view, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.
At the Terminal command prompt, type dotnet run and then press Enter.

You should see the same message as before:
```
First: A123
Second: B456
Third: C789
Reassign First: F000
```

Use the Length property of an array

Depending on how the array is created, you may not know in advance how many elements an array contains. To determine the size of an array, you can use the Length property.

Note

The Length property of an array is not zero-based.

At the end of your code file, to report the number of fraudulent orders, enter the following code:
```
Console.WriteLine($"There are {fraudulentOrderIDs.Length} fraudulent orders to process.");
```
This code uses the array's Length property, an integer, to return the number of elements in your fraudulentOrderIDs array.

Ensure that your code matches this example:

/*
string[] fraudulentOrderIDs = new string[3];

fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";
// fraudulentOrderIDs[3] = "D000";
*/

string[] fraudulentOrderIDs = [ "A123", "B456", "C789" ];

Console.WriteLine($"First: {fraudulentOrderIDs[0]}");
Console.WriteLine($"Second: {fraudulentOrderIDs[1]}");
Console.WriteLine($"Third: {fraudulentOrderIDs[2]}");

fraudulentOrderIDs[0] = "F000";

Console.WriteLine($"Reassign First: {fraudulentOrderIDs[0]}");

Console.WriteLine($"There are {fraudulentOrderIDs.Length} fraudulent orders to process.");

Save the changes to your Program.cs file, and then run the application.

You should see the following output:
```
First: A123
Second: B456
Third: C789
Reassign First: F000
There are 3 fraudulent orders to process.
```

Recap

Here's the most important things to remember when working with arrays:

An array is a special variable that holds a collection of related data elements.
You should memorize the basic format of an array variable declaration.
Access each element of an array to set or get its values using a zero-based index inside of square brackets.
If you attempt to access an index outside of the boundary of the array, you get a run time exception.
The Length property gives you a programmatic way to determine the number of elements in an array.

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Exercise - Get started with array basics

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