Introduction
Þ
Visual Basic is a tool
that allows you to develop Windows (Graphic User Interface – GUI) applications. The
applications have a familiar appearance to the user.
Þ
Visual Basic is event-driven;
event driven meaning code remains idle until called upon to respond to some
event (button pressing, menu selection). Visual Basic is governed by an event
processor.
Þ
The original Visual Basic for DOS and Visual Basic for Windows were introduced
in 1991.
Steps in Developing Application
There
are three primary steps involved in building a Visual Basic application:
1.
Draw the user interface
2.
Assign properties to controls
3.
Attach code to controls
Rule for Variable
Variables
are used by Visual Basic to hold information.
Þ No more than 40 characters
Þ They may include letters, numbers, and
underscore (_)
Þ The first character must be a letter
Þ
You cannot use a reserved word (word needed by Visual Basic)
Visual Basic Data Types
Data
Type Suffix
Boolean None
Integer
%
Long
(Integer) &
Single
(Floating) !
Double
(Floating) #
Currency @
Date None
String
$
Variant None
Variable Declaration
There
are three ways for a variable to be typed (declared):
1.
Default
2.
Implicit
3.
Explicit
1.
Default variable in visual basic is variant,
by default the variant
data type is a special type used by Visual Basic that can contain numeric,
string, or date data.
2.
Implicit type a variable,
use the corresponding suffix shown above in the data type table.
For example,
TextValue$
= "This is a string" creates a string variable, while
Amount%
= 300 creates an integer variable.
3. Explicit type
a variable determines its scope. There are four levels of scope:
a. Procedure level
b. Global level
Within a procedure & for Global, variables
are declared using the Dim statement:
Dim x as Integer
Dim y as Double
Dim name as String
Visual Basic Operators
1. arithmetic operator with
order of precedence are:
Operator Operation
^ Exponentiation
* / Multiplication
and Division
Mod Modulus
+
- Addition and subtraction
2. Comparison operators in Visual Basic:
Operator Comparison
> Greater than
<
Less than
>=
Greater
than or equal to
<=
Less than
or equal to
=
Equal to
<> Not equal to
3. Logical operators
Operator Operation
Not Logical not
And
Logical and
Or Logical
or
Message box
The
Message box displays a message, optional icon, and selected set of command
buttons.
The statement form of the message
box returns no value (it simply displays the box):
MsgBox Message, Type, Title
where
Message Text
message to be displayed
Type Type
of message box (discussed in a bit)
Title
Text in title bar of message box
Value Meaning Symbolic
Constant
1
OK
button selected vbOK
2
Cancel
button selected vbCancel
3
Abort
button selected vbAbort
4 Retry
button selected vbRetry
5 Ignore
button selected vbIgnore
6 Yes
button selected vbYes
7 No
button selected vbNo
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