1. Making Interactive Programs with Visual Basic .NET

2. Quick Links Using Variables Built-in data types Option Explicit
Syntax Errors
The Toolbox Control

3. Using Variables
A variable is a named memory location that stores a value
Example: Imagine we are creating a program that takes in a user’s age as input and then displays the following messages depending on the age:
If the user’s age < 18, display “Too young to vote”
If the user’s age >= 18, display, “You can vote”
If we are creating this program, we need to get the input and store it in a variable (memory)

4. Using Variables
To get this input, we use a control object such as a TextBox control. Variables come into play when we want to store this information in memory for later use
We do this in VB .NET by first declaring a variable. This is done with the following syntax:
Dim age As Integer
“Reserve a space in memory called ‘age’ and save enough space for an integer”
Variable name

5. Variable Assignment
Now that we have declared a variable, we can start using it to store Integer values using assignment
age = ‘Now age stores the value 27
age = txtMyTextBox.Text ‘Now age stores the value ‘that the user entered into ‘the textbox
This is wrong, however:
27 = age
Good programming style to give variables meaningful names and to declare them at the beginning of the procedure

6. Some Variable Examples
A variable can only store one value at any given time
An expression can also be used on the right side of an assignment statement. Remember that the expression on the right side of an assignment is evaluated first and then assigned to the variable on the left side of the = sign.
Dim ageInYears As Integer
Dim ageInDays As Integer
ageInYears = 27
ageInDays = ageInYears * 365 ‘Not including leap years

7. Built-in Data Types
A data type defines the format of data and how it is stored on a computer
VB .NET has several built-in data types:
Type
Used to Represent
Single
Numbers possibly containing a decimal
Double
Integer
Integers (no decimals)
Long
String
A set of characters
Boolean
True or False

8. Variable Naming Conventions
Variable identifiers should have descriptive names and begin with an appropriate prefix as listed below
Type
Prefix
Single
sgl
Double
dbl
Integer
int
Long
lng
String
str
Boolean
bln

9. Naming Convention Examples
Dim strLastName As String ‘ User’s last name
Dim intAge As Integer ‘ User’s age
Dim dblHeight As Double ‘ User’s height in cm
Dim blnVote As Boolean ‘ True if user is old ‘ enough to vote; ‘ false otherwise
intAge = 17.4 ‘ ok, but doesn’t work as ‘ expected. intAge stores 17
dblHeight = ‘ dblHeight stores

10. Variable Declarations
A single Dim statement can be used to declare multiple variables of different types
Dim strName As String, intAge As Integer
VB .NET initializes variables when they are declared
Numeric types (Double, Integer…) are initialized to 0
String variables are initialized to “” (empty string)
Boolean variables are initialized to False

11. Option Explicit Dim intAge, intAgeInDays As Integer
intAg = 20 ‘ Misspelled variable – we never ‘ declared intAg
intAgeInDays = intAge * 365 ‘ Gives 0 since ‘intAge was never set and was initialized to 0 by ‘VB .NET
The Option Explicit statement can be added to the general section of the form module. Option Explicit displays an error message at run time when a variable is used before it is declared.

12. Syntax Errors
A statement that violates the rules of VB .NET is said to contain a syntax error
Example:
Dim intAge As Integer = 12 ‘Syntax error!
This is a syntax error because assignment is illegal in a variable declaration
Syntax errors are detected by VB .NET and the error is underlined in blue in the code window
Other types of errors are not detected until the program is run, at which time, a run-time error is generated

13. The TextBox Control
Text boxes are used to get textual input from the user. Examples:
User Name, height, age…
Textbox

14. The TextBox Control TextBoxes have the properties:
Name: identifies the object and is used by the programmer. Good style to prefix with txt.
Text: The text that is displayed in the text box. The user can change this text.
Alignment: Sets the alignment of text relative to the text box.

15. A TextBox Control Example
TextBox to get age in Years
txtAgeInYears
Calculates age in days
btnCalc
Label that will display the result to the user
lblAgeInDays

16. A TextBox Control Example
The following code goes in the event procedure for the button
Dim intAgeInYears, intAgeInDays As Integer
intAgeInYears = Me.txtAgeInYears.Text ‘ Get user input from ‘ text box
intAgeInDays = intAgeInYears * ‘ Calc age in days
Me.lblAgeInDays.Text = intAgeInDays ‘ Display the result in ‘ the label

17. Integer Division
In addition to (+, -, /, *, ^), VB .NET includes integer division (\)
The \ operator truncates the decimal portion of the quotient
Example
Dim intX As Integer
intX = 20 \ 7 ‘ intX is assigned 2

18. Modulus
In addition to (+, -, /, *, ^, \), VB .NET includes the Modulus operator
Modulus division returns the remainder resulting from division
Example
Dim intX As Integer
intX = 20 Mod 7 ‘ intX is assigned 6
You can use long division to find the remainder of 20 divided by 7 which is 6

19. New Order of Operations
Brackets ( () )
Exponents ( ^ )
Division ( / )
Multiplication ( * )
Integer Division ( \ )
Modulus Division ( Mod )
Addition ( + )
Subtraction ( - )

20. Automatic Type Conversion
In assignment, VB .NET automatically converts data to match the type of the variable it is being assigned to
Examples
Dim intX As Integer
intX = ‘ intX is assigned 7
intAgeInYears = Me.txtAgeInYears.Text
‘ Assigns Text that user entered (which is ‘hopefully a number) to an Integer variable

21. Using Named Constants
A constant is a named memory location which stores a value that cannot be changed at runtime from its initial assignment
Example
Const dblPi As Double = 3.14
dblPi = 29 ‘ This is not allowed
‘ dblPi cannot be changed
‘ at runtime