Exercise - Evaluate an expression

  • 11 minutes

Decision logic is used to establish alternative pathways through your code, where the decision about which path to follow is based on the evaluation of an expression. For example, you might write some code that executes one of two paths based on a user's input. If the user enters the letter "a", your code will execute one code block. If they enter the letter "b", your code will execute a different code block. In this example, you're controlling the execution path based on the value assigned to a string. Your code selects an execution path based on an expression, how that expression is evaluated, and the underlying logic used to define the paths.

Examining how to construct and evaluate an expression is a good place to start.

What is an expression?

An expression is any combination of values (literal or variable), operators, and methods that return a single value. A statement is a complete instruction in C#, and statements are comprised of one or more expressions. For example, the following if statement contains a single expression that returns a single value:

if (myName == "Luiz")

You might have been thinking that the value returned by an expression would be a number or maybe a string. It's true that application developers use different types of expressions for different purposes. In this case, when you're developing an if selection statement, you'll be using an expression that returns either true or false. Developers refer to this type of expression as a Boolean expression. When your code includes a Boolean expression, return value is always a single true or false value.

Boolean expressions are important because your code can use these expressions to decide which block of code to execute.

There are many different types of operators that you can use within a Boolean expression. For example, the if statement above uses the equality operator == to check whether a string variable is assigned a particular value. The operator that you choose will depend on the available code paths, the conditions associated with the paths, and the underlying application logic.

Evaluating equality and inequality

One of the most common code evaluations is a check to see whether two values are equal. When checking for equality, you'll locate the equality operator == between the two values being checked. If the values on either side of the equality operator are equivalent, then the expression will return true. Otherwise, it will return false.

Conversely, you might also need to check whether two values aren't equal. To check for inequality, you'll use the inequality operator != between the two values.

You might wonder why you need both equality and inequality operators. The reason will become clearer as you learn how to create branching statements and begin to write real world code. Two operators that perform opposite tasks allow you to be more expressive and compact.

Now it's time to prepare your coding environment and begin writing code that evaluates Boolean expressions.

Prepare your coding environment

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

  1. Open Visual Studio Code.

    You can use the Windows Start menu (or equivalent resource for another OS) to open Visual Studio Code.

  2. On the Visual Studio Code File menu, select Open Folder.

  3. 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.

  4. In the Open Folder dialog, select Select Folder.

    If you see a security dialog asking if you trust the authors, select Yes.

  5. 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.

  6. At the Terminal command prompt, to create a new console application in a specified folder, type dotnet new console -o ./CsharpProjects/TestProject and then press Enter.

    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.

  7. In the EXPLORER panel, 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.

  8. In the EXPLORER panel, to view your code file in the Editor panel, select Program.cs.

  9. Delete the existing code lines.

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

  10. Close the Terminal panel.

Use the equality operator

  1. Ensure that you have Visual Studio Code open and Program.cs displayed in the Editor panel.

    Note

    Program.cs should be empty. If it isn't, select and delete all code lines.

  2. Type the following code into the Visual Studio Code Editor.

    Console.WriteLine("a" == "a");
Console.WriteLine("a" == "A");
Console.WriteLine(1 == 2);

string myValue = "a";
Console.WriteLine(myValue == "a");

  3. On the Visual Studio Code File menu, select Save.

    The Program.cs file must be saved before building or running the code.

  4. In the EXPLORER panel, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal.

    A Terminal panel will open. The Terminal should include a command prompt showing that the Terminal is open to your TestProject folder location.

  5. At the Terminal command prompt, to run your code, type dotnet run and then press Enter.

    Note

    If you see a message saying "Couldn't find a project to run", ensure that the Terminal command prompt displays the expected TestProject folder location. For example: C:\Users\someuser\Desktop\csharpprojects\TestProject>

    You should see the following output.

    True
False
False
True

Improve the check for string equality using the string's built-in helper methods

You might be surprised that the line Console.WriteLine("a" == "A"); outputs false. When comparing strings, case matters.

Also, consider this line of code:

Console.WriteLine("a" == "a ");

Here you've added a space character at the end of the string. This expression will also output false.

In some cases, having a space character before or after the text might be perfectly acceptable. However, if you need to accept a match that isn't exact, you can "massage" the data first. "Massaging" the data means that you perform some cleanup before you perform a comparison for equality.

For example, consider the case when you're collecting user input inside a loop. After each value is entered, you could provide the user with a prompt to determine if they want to continue, such as Do you want to continue (Y/N)?. If the user wants to continue, they will probably enter either y or Y. You'll want your code to interpret both values equally, even though y isn't equivalent to Y.

Before you check two string values for equality, especially when one or both values were entered by a user, you should:

  • Make sure both strings are all upper-case or all lower-case using the ToUpper() or ToLower() helper method on any string value.
  • Remove any leading or trailing blank spaces using the Trim() helper method on any string value.

You can improve the previous equality check by chaining these two helper methods on both values, as shown in the following code listing:

  1. Replace the code in the Visual Studio Code Editor with the following code:

    string value1 = " a";
string value2 = "A ";
Console.WriteLine(value1.Trim().ToLower() == value2.Trim().ToLower());

  2. Save your code file, and then use Visual Studio Code to run your code.

  3. Notice that when you run the code this time, it outputs True.

Use the inequality operator

  1. Use the line comment operator // to comment out all of the code from the previous step.

  2. Type the following code into the Visual Studio Code Editor.

    Console.WriteLine("a" != "a");
Console.WriteLine("a" != "A");
Console.WriteLine(1 != 2);

string myValue = "a";
Console.WriteLine(myValue != "a");

  3. Save your code file, and then use Visual Studio Code to run your code.

    You should see the following output.

    False
True
True
False

As you would expect, the result when using the inequality operator is the opposite of what you saw when using the equality operator. That means that your code will branch in the opposite manner as well, which can be exactly what you want.

Evaluating comparisons

When working with numeric data types, you might want to determine if a value is larger or smaller than another value. Use the following operators to perform these types of comparisons:

  • Greater than >
  • Less than <
  • Greater than or equal to >=
  • Less than or equal to <=

Naturally, the == and != operators that you used to compare string values above will also work when comparing numeric data types.

Use the Comparison operators

  1. Use the line comment operator // to comment out all of the code from the previous task.

  2. Type the following code into the Visual Studio Code Editor.

    Console.WriteLine(1 > 2);
Console.WriteLine(1 < 2);
Console.WriteLine(1 >= 1);
Console.WriteLine(1 <= 1);

  3. Save your code file, and then use Visual Studio Code to build and run your code.

    You should see the following output:

    False
True
True
True

Methods that return a Boolean value

Some methods return a Boolean value (true or false). In the following exercise, you'll use a built-in method of the String class to determine whether or not a larger string contains a specific word or phrase that's significant to your application.

Note

Some data types have methods that perform helpful utility tasks. The String data type has many of these. Several return a Boolean value including Contains(), StartsWith(), and EndsWith(). You can learn more about them in the Microsoft Learn module "Manipulate alphanumeric data using String class methods in C#".

Use a method that returns a Boolean

  1. Use the line comment operator // to comment out all of the code from the previous step.

  2. Type the following code into the Visual Studio Code Editor.

    string pangram = "The quick brown fox jumps over the lazy dog.";
Console.WriteLine(pangram.Contains("fox"));
Console.WriteLine(pangram.Contains("cow"));

  3. Save your code file, and then use Visual Studio Code to build and run your code.

    You should see the following output.

    True
False

What is logical negation?

The term "Logical Negation" refers to the unary negation operator !. Some people call this operator the "not operator". When you place the ! operator before a conditional expression (or any code that's evaluated to either true or false), it forces your code to reverse its evaluation of the operand. When logical negation is applied, the evaluation produces true , if the operand evaluates to false , and false , if the operand evaluates to true.

Here is an example that might help you to see the connection between these ideas. The following two lines of code produce the same result. The second line is more compact.

// These two lines of code will create the same output

Console.WriteLine(pangram.Contains("fox") == false);
Console.WriteLine(!pangram.Contains("fox"));

Use the Logical Negation operator

  1. Use the line comment operator // to comment out all of the code from the previous step.

  2. Type the following code into the Visual Studio Code Editor.

    string pangram = "The quick brown fox jumps over the lazy dog.";
Console.WriteLine(!pangram.Contains("fox"));
Console.WriteLine(!pangram.Contains("cow"));

  3. Save your code file, and then use Visual Studio Code to build and run your code.

    You should see the following output.

    False
True

Inequality operator versus logical negation

The inequality operator != includes a ! character, but should not be confused with logical negation. The inequality operator returns true if its operands aren't equal, and returns false if the operands are equal. For the operands of the built-in types, the expression x != y produces the same result as the expression !(x == y) (an example of logical negation).

The following code sample demonstrates the use of the != operator:

int a = 7;
int b = 6;
Console.WriteLine(a != b); // output: True
string s1 = "Hello";
string s2 = "Hello";
Console.WriteLine(s1 != s2); // output: False

Recap

Here's the main takeaways you learned about evaluating Boolean expressions so far:

  • There are many different kinds of expressions that evaluate to either true or false.
  • Evaluate equality using the == operator.
  • Evaluating equality of strings requires you to consider the possibility that the strings have different case and leading or trailing spaces. Depending on your situation, use the ToLower() or ToUpper() helper methods, and the Trim() helper method to improve the likelihood that two strings are equal.
  • Evaluate inequality using the != operator.
  • Evaluate greater than, less than and similar operations using comparison operators like >, <, >=, and <=.
  • If a method returns a bool, it can be used as a Boolean expression.
  • Use the logical negation operator ! to evaluate the opposite of a given expression.

Check your knowledge

1.

Which of the following is a list of valid comparison operators?

2.

Which of the following code lines uses logical negation?

3.

An application contains two string variables named myValue1 and myValue2. The user enters the following two values for these variables: "Y " and " y". Which of the following expressions return false?