Two VB Programs

1
Two VB Programs

• "Hello World" in a console
• "Hello World" in a form

2
The Source Code - Console

• "Hello World" console

Module Module1 Sub Main()
Console.WriteLine("Hello World")
Console.ReadLine() End Sub End Module
3
The Source Code - Form

• We can't see the source code
• We can see the label properties with a dialog box
• We can add source code to act on an event to a
  widget or form

4
Comments

• Comments may appear at the end of an executable
  line or on their own line
• Comment early, comment often

' Print the message. Console.WriteLine( "Hello
World" ) Console.ReadLine() ' Wait for input
before closing window
5
Data Types

• VB is a typed language, so all variables,
  constants, and return values must be assigned a
  type
• Some primitive types areInteger, Long, Single,
  Double, Char, String, Boolean, Date, Decimal

6
Data Type Values

• Boolean (True or False)
• Char (one Unicode character)
• Date (date and time - m/d/y hms format)
• Decimal (for monetary calculations)
• Integer (32 bit integer)
• Long (64 bit integer)
• Single (32 bit float)
• Double (64 bit float)
• String (up to 2 billion Unicode characers)

7
Declaring Variables

• Variables are defined with the Dim (Dimension)
  keyword
• Variables must be assigned a type when declared
• Multiple variables may be assigned at once
• Numbers have default value of 0, strings to the
  empty string, booleans to False

8
Declaring Variables (Cont'd)

• Names must start with a letter or the underscore
• The rest of the name may have letters, digits, or
  the underscore
• Names cannot contain spaces or periods
• Names cannot be a VB keyword
• Variables must be declared before they can be used

9
Sample Variable Declarations

• Dim x As Integer
• Dim a, b, c As Double
• Dim s As String
• Dim d as Date
• Dim yn as Boolean

10
Assigning Values

• Variables may be assigned values only after they
  are declared
• A single variable may be assigned a value upon
  declaration
• The assigned value must be of the proper type

11
Sample Value Assigning

• Dim x As Integerx 5
• Dim q As Double 5.0
• Dim s As String "Hello World"
• Dim ch As Char "X"c('c' denotes a character)
• Dim today As Date 5/1/2008
• Dim time As Date 14500 PM

12
Constants

• Constants are declared with the Const keyword
• Constants must be assigned a value when they are
  declared
• Constants should stand out from variables, and
  are usually given names in all caps

13
Sample Constants

• Const Q As Integer 5
• Const RATE As Double 5.5
• Const ERR As String "Error"
• Const YES As Boolean True

14
Naming Prefixes

• Variables and constants can use any name that is
  not a keyword
• Some books recommend the use of naming prefix - a
  prefix that makes clear the type of variable
• Their usage is optional, but can be helpful

15
Sample Naming Prefixes

• Boolean bln - blnFlag
• Char chr - chrInputLetter
• Date dat or dtm - datToday
• Decimal dec - decCash
• Integer int - intLength
• Long lng - lngBigValue
• Single sng - sngWeight
• Double dbl - dblDistance
• String str - strName

16
Variable Scope

• Variables declared inside a method are local
  variables and are visible only inside that
  procedure
• Variables declared inside a class but outside a
  method are class-level variables and can be used
  anywhere in the class
• Variables declared outside of a class or method
  are global variables