Using Variables
Using Variables
Constants
Introduction
A constant is a value that doesn't change. There are two types of constants you will use in your programs: those supplied to you and those you define yourself.
To create a constant to use in your program type the Const keyword followed by a name for the variable, followed by the assignment operator "=", and followed by the value that the constant will hold. Here is an example:
Module Exercise
Sub Main()
Dim DateOfBirth = #12/5/1974#
Console.WriteLine(DateOfBirth)
End Sub
End Module
When defining a constant like this, the compiler would know the type of data to apply to the variable. In this case the DateOfBirth constant holds a Date value. Still, to be more explicit, you can indicate the type of value of the constant by following its name with the As keyword and the desired data type. Based on this, the above program would be:
Module Exercise
Sub Main()
Dim DateOfBirth As Date = #12/5/1974#
Console.WriteLine("Date of Birth: " & DateOfBirth)
End Sub
End Module
When creating a constant, if its value supports a type character, instead of using the As keyword, you can use that type character. Here is an example:
Module Exercise
Sub Main()
Dim Temperature% = 52
End Sub
End Module
As mentioned earlier, the second category of constants are those that are built in the Visual Basic language. Because there are many of them and used in different circumstances, when we need one, we will introduce and then use it.
Practical Learning: Using Constants
Module CleaningOrder
Sub Main()
Const UnitPriceShirts As Double = 1.25
Const UnitPricePants As Double = 1.95
Const UnitPriceOtherItems As Double = 3.25
Const TaxRate As Double = 5.75
Console.WriteLine("====================================================")
Console.WriteLine(vbTab & "-/- Georgetown Dry Cleaner -/-")
Console.WriteLine("====================================================")
' Request order information from the user
Console.Write("Enter Customer Name: ")
Dim CustomerName As String = Console.ReadLine()
Console.Write("Enter Customer Phone: ")
Dim CustomerPhone As String = Console.ReadLine()
Console.Write("Enter the order date: ")
Dim OrderDate As Date = Console.ReadLine()
Console.Write("Enter the order time: ")
Dim OrderTime As Date = Console.ReadLine()
Console.Write("Enter Number of Shirts: ")
Dim NumberOfShirts As UInteger = Console.ReadLine()
Console.Write("Enter Number of Pants: ")
Dim NumberOfPants As UInteger = Console.ReadLine()
Console.Write("Enter Number of Other Items: ")
Dim NumberOfOtherItems As UInteger = Console.ReadLine()
' Perform the necessary calculations
Dim SubTotalShirts As Double = NumberOfShirts * UnitPriceShirts
Dim SubTotalPants As Double = NumberOfPants * UnitPricePants
Dim SubTotalOtherItems As Double = NumberOfOtherItems * UnitPriceOtherItems
' Calculate the "temporary" total of the order
Dim TotalOrder As Double = SubTotalShirts + SubTotalPants + SubTotalOtherItems
' Calculate the tax amount using a constant rate
Dim TaxAmount As Double = TotalOrder * TaxRate / 100
' Add the tax amount to the total order
Dim NetTotal As Double = TotalOrder + TaxAmount
' Communicate the total to the user...
Console.WriteLine("The Total order is: " & NetTotal)
' and request money for the order
Console.Write("Amount Tended? ")
Dim AmountTended As Double = Console.ReadLine()
Console.WriteLine("====================================================")
Console.WriteLine(vbTab & "-/- Georgetown Dry Cleaner -/-" & vbCrLf &
"====================================================" & vbCrLf &
vbTab & "Customer Name: " & CustomerName & vbCrLf &
vbTab & "Customer Phone: " & CustomerPhone & vbCrLf &
vbTab & "Order Date: " & OrderDate & vbCrLf &
vbTab & "Order Time: " & OrderTime & vbCrLf &
"====================================================" &
vbCrLf & "Item Type" & vbTab & "Qty" & vbTab & "Unit/Price" & vbTab & "Sub-Total" & vbCrLf &
"----------------------------------------------------" & vbCrLf &
"Shirts " & vbTab & vbTab & NumberOfShirts & vbTab & UnitPriceShirts & vbTab & vbTab & SubTotalShirts & vbCrLf &
"Pants " & vbTab & vbTab & NumberOfPants & vbTab & UnitPricePants & vbTab & vbTab & SubTotalPants & vbCrLf &
"Other Items " & vbTab & NumberOfOtherItems & vbTab & UnitPriceOtherItems & vbTab & vbTab & SubTotalOtherItems & vbCrLf &
"---------------------------------------------------" & vbCrLf & vbTab & vbTab &
"Total Cleaning: " & vbTab & TotalOrder & vbCrLf & vbTab & vbTab &
"Tax Rate: " & vbTab & vbTab & TaxRate & "%" & vbCrLf & vbTab & vbTab &
"Tax Amount: " & vbTab & vbTab & TaxAmount & vbCrLf & vbTab & vbTab &
"Net Price: " & vbTab & vbTab & NetTotal & vbCrLf &
"===================================================")
End Sub
End Module| Customer Name: | Jeannette Sharma |
| Customer Phone: | (301) 218-9000 |
| Order Date: | 04/11/2008 |
| Order Time | 08:24 |
| Number of Shirts: | 6 |
| Number of Pants: | 4 |
| Number of Other Items: | 2 |
====================================================
-/- Georgetown Dry Cleaner -/-
====================================================
Enter Customer Name: Jeannette Sharma
Enter Customer Phone: (107) 493-8074
Enter the order date: 04/11/2026
Enter the order time: 08:24
Enter Number of Shirts: 6
Enter Number of Pants: 4
Enter Number of Other Items: 2
The Total order is: 23.0535
Amount Tended? 25
====================================================
-/- Georgetown Dry Cleaner -/-
====================================================
Customer Name: Jeannette Sharma
Customer Phone: (107) 493-8074
Order Date: 4/11/2026
Order Time: 8:24:00 AM
====================================================
Item Type Qty Unit/Price Sub-Total
----------------------------------------------------
Shirts 6 1.25 7.5
Pants 4 1.95 7.8
Other Items 2 3.25 6.5
---------------------------------------------------
Total Cleaning: 21.8
Tax Rate: 5.75%
Tax Amount: 1.2535
Net Price: 23.0535
===================================================
Press any key to close this window . . .Modules
Introduction
In the small programs we have created so far, we were using only one file. A typical application uses as many files as necessary. You can use one file to list some objects used in other files. As we move on, we will see different examples of creating different files in the same program.
In the Visual Basic language, a file that holds Visual Basic code is called a module.
Creating a Module
As mentioned above, a module is primarily a file that holds code. Therefore, there is no complication with creating one. It is simply a file that holds the .vb extension. If you create a console application using the Console Application option of the New Project dialog box, Microsoft Visual Studio would create a default file for and would insert the module template code.
To create a module in Microsoft Visual Studio or Microsoft Visual Basic 2008 Express Edition, on the main menu, you can click Project -> Add Module... This would display the Add New Item dialog box with the Module selected as default in the Templates list. The studio would also suggest a default name. You can accept that name or change it. The name of the module follows the rules of an object in the Visual Basic language. Once you are ready with the dialog box, you can click Add.
If you are manually creating your code from Notepad or any text editor, you can simply create any file in your folder and give it the .vb extension.
Probably the most important thing in a module is that the area that contains its code must start with a Module ModuleName and end with an End Module line:
Module ModuleName End Module
Anything between these two lines is part of the normal code and everything that is normal code of the module must be inserted between these two lines. No code should be written outside of these two lines.
After creating a module and adding its required two lines, you can add the necessary code. Of course, there are rules you must follow. At a minimum, you can declare one or more variables in a module, just as we have done so far. Here is an example:
Module Exercise
Dim FullName As String
End Module
Accessing or Opening a Module
Each module of a project is represented in the Solution Explorer by a name under the project node. To open a module using the Solution Explorer:
If there are many opened module, each is represented in the Code Editor by a label and by an entry in the Windows menu. Therefore, to access a module:
Renaming a Module
As you may have realised, when you start a console application, Microsoft Visual Basic creates a default module and names it Module1. Of course, you can add as many modules as necessary. At any time, you can change the name of a module.
To rename a module, in the Solution Explorer
Deleting a Module
If you have a module you don't need anymore, to delete it, in the Solution Explorer, right-click it and click Delete. You will receive a warning to confirm your intentions or to change your mind.
Access Modifiers
Introduction
As mentioned already, you can use more than one module in a project and you can declare variables in a module. This allows different modules to exchange information. For example, if you are planning to use the same variable in more than section of your application, you can declare the variable in one module and access that variable in any other module in the application.
The Friendly Members of a Module
A variable that is declared in one module and can be accessed from another module in the same application is referred to as a friend. Variables are not the only things that can benefit from this characteristic. We will see other types.
To declare a friendly variable, instead of Dim, you use the Friend keyword. Here is an example:
Module Exercise
Friend FullName As String
End Module
After declaring such a variable, you can access it from any module of the same application. Here is an example:
| File 1: Module1.vb |
Module Exercise
Friend FullName As String
End Module |
| File 2: Exercise.vb |
Module Exercise
Sub Main()
FullName = "Gertrude Monay"
Console.WriteLine("Full Name: " & FullName)
End Sub
End Module |
The Private Members of a Module
Instead of allowing a member of a module to be accessible outside the module, you may want to restrict this access. The Visual Basic language allows you to declare a variable that can be accessed only within the module that contains it. No code outside the module would be able to "see" such a member. A member inside a module and that is hidden from other modules is referred to as private.
To declare a private variable, instead of Dim or Friend, you use the Private keyword. Here is an example:
Module Exercise
Friend FullName As String
Private DateHired As Date
Sub Main()
FullName = "Gertrude Monay"
DateHired = #4/8/2008#
Dim Information As String
Information = "Full Name: " & FullName &
vbCrLf &
"Date Hired: " & DateHired
Console.WriteLine(Information)
End Sub
End Module
This would produce:
Practical Learning: Declaring Global Private Variables
Module CleaningOrder
Dim UnitPriceShirts As Double = 1.25
Dim UnitPricePants As Double = 1.95
Dim UnitPriceOtherItems As Double = 3.25
Dim TaxRate As Double = 5.75
Private CustomerName As String, CustomerPhone As String
Private OrderDate As Date, OrderTime As Date
' Unsigned numbers to represent cleaning items
Private NumberOfShirts As UInteger, NumberOfPants As UInteger
Private NumberOfOtherItems As UInteger
' Each of these sub totals will be used for cleaning items
Private SubTotalShirts As Double, SubTotalPants As Double
Private SubTotalOtherItems As Double
' Values used to process an order
Private TotalOrder As Double, TaxAmount As Double
Private NetTotal As Double
Private AmountTended As Double
Sub Main()
' Request order information from the user
CustomerName = InputBox("Enter Customer Name:")
. . . No Change
End Sub
End Module
| Customer Name: | Landry Kurtzmann |
| Customer Phone: | (202) 223-3325 |
| Order Date: | 04/12/2008 |
| Order Time | 07:35 |
| Number of Shirts: | 8 |
| Number of Pants: | 2 |
| Number of Other Items: | 0 |
The Public Members of a Module
When working on a project, you may want to create objects or declare variables that you want to be accessible from other applications. Such a member is referred to as public.
To declare a variable that can be accessed by the same modules of the same project and modules of other projects, declare it using the Public keyword. Here is an example:
Module Exercise
Friend FullName As String
Private DateHired As Date
Public HourlySalary As Double
Sub Main()
FullName = "Gertrude Monay"
DateHired = #4/8/2008#
HourlySalary = 36.75
Dim Information As String
Information = "Full Name: " & FullName & vbCrLf &
"Date Hired: " & DateHired & vbCrLf &
"Hourly Salary: " & HourlySalary
Console.WriteLine(Information)
End Sub
End Module
This would produce:
Access Modifiers and Modules
The Friend, Private, and Public keywords are called access modifiers because they control the level of access that a member of a module has. In previous sections, we saw how to control the members of a module. The level of access of a module itself can also be controlled. To control the level of access of a module, you can precede the Module keyword with the desired access modifier.
The access modifier of a module can only be either Friend or Public. Here are examples:
| File 1: Module1.vb |
Friend Module Exercise
End Module
|
| File 2: Module2.vb |
Public Module Exercise
End Module
|
Practical
Learning: Access Modifying a Module
Public Module CleaningOrder
. . . No Change
End Module
| Customer Name: | William Nessif |
| Customer Phone: | (301) 220-3737 |
| Order Date: | 04/12/2008 |
| Order Time | 08:26 |
| Number of Shirts: | 3 |
| Number of Pants: | 3 |
| Number of Other Items: | 3 |
Details on Declaring Variables
Declaring a Series of Variables
Because a program can use different variables, you can declare each variable on its own line. Here are examples:
Module Exercise
Sub Main()
Dim NumberOfPages As Integer
Dim TownName As String
Dim MagazinePrice As Double
End Sub
End Module
It is important to know that different variables can be declared with the same data type as in the following example:
Module Exercise
Sub Main()
Dim NumberOfPages As Integer
Dim Category As Integer
Dim MagazinePrice As Double
End Sub
End Module
When two variables use the same data type, instead of declaring each on its own line, you can declare two or more of these variables on the same line. There are two techniques you can use:
Module Exercise
Sub Main()
Dim NumberOfPages, Category As Integer
Dim MagazinePrice As Double
End Sub
End Module
In this case, both variables NumberOfPages and Category, are the same type, Integer
Module Exercise
Sub Main()
Dim NumberOfPages, Category As Integer
Dim CountryName As String, MagazinePrice As Double
End Sub
End Module
In this case, CountryName is a string variable while MagazinePrice is a Double variable
You can use the same techniques when declaring many global variables. Here are examples:
Module Exercise
Friend FirstName As String, LastName As String
Private DateHired As Date, HourlySalary As Double
Sub Main()
End Sub
End Module
After declaring the variables, you can initialize and use them as you see fit.
We have indicated that when a variable is declared, it receives a default initialization unless you decide to specify its value. Whether such a variable has been initialized or not, at any time, you can change its value by reassigning it a new one. Here is an example:
Module Exercise
Sub Main()
' Initializing a variable when declaring it
Dim Number As Double = 155.82
Console.WriteLine("Number: " & Number)
' Changing the value of a variable after using it
Number = 46008.39
Console.WriteLine("Number: " & Number)
End Sub
End Module
This would produce:
In the same way, we saw that you could declare a variable at module scope outside of Main and then initialize or change its value when necessary. Here is an example:
Module Exercise
Private UnitPrice As Double
Private DateOfBirth As Date = #12/5/1974#
Private Number As Double
Sub Main()
' Initializing a variable
Number = 155.82
Console.WriteLine("Number: " & Number)
' Changing the value of a variable after using it
Number = 46008.39
Console.WriteLine("Number: " & Number)
End Sub
End Module
When declaring a variable, as the programmer, you should have an idea of how you want to use the variable and what type of values the variable should hold. In some cases, you may want the variable to hold a constant value and not be able to change it. We saw earlier that such a variable could be declared as a constant. An alternative is to declare it with the ReadOnly keyword. While a constant variable can be declared locally, a ReadOnly variable cannot. It must be declared globally.
As done for a constant, when declaring a ReadOnly variable, you should initialize it. If you do not, the compiler would assign the default value based on its type. For example, a number-based variable would be initialized with 0 and a String variable would be initialized with an empty string. As done so far, to initialize the variable, use the assignment operator followed by the desired value.
Like a constant, after declaring and optionally initializing a ReadOnly variable, you cannot change its value. Based on this, the following code would produce an error:
Module Exercise
Dim UnitPrice As Double
Dim DateOfBirth As Date = #12/5/1974#
ReadOnly Number As Double = 155.82
Sub Main()
' Initializing a variable
Number = 155.82
Console.WriteLine("Number: " & Number)
' Changing the value of a variable after using it
Number = 46008.39 ' Error: You cannot assign a value to
' a ReadOnly variable after initializing it
Console.WriteLine("Number: " & Number)
End Sub
End Module
In the Microsoft Visual Basic 2010, the parser would signal the errors by underlining the read-only variable when you try changing its value. A window named Error List would also point out the problems:
This means that a ReadOnly variable must be assigned a value once, when initializing it.
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