Unit II Programming Fundamentals

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VB Programming Fundamentals


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Variables values that can change during a

program execution

Constants values that remain the fixed

throughout the program run.

Reserved words/ Keywords words with

predefined meaning. E.g.


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Data types in VB

Data types are means to identify the type of dataand associated operations on it.


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Data Type Description

Boolean True / false

Byte Allows positive integers in the

range 0-255

Currency Used to hold currency data (

up to 4 decimal places)

Date Used to hold date and time

Double Holds numeric values ( 14

digits of precision)

Integer Integers in range ( -32768 to

32767)


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Data Type Description

Long Integer / Long Holds integer values greater

than those supported by

integer data typeObject Used to hold and refer to

objects such as controls and

forms

Single Holds numeric values ( 6

digits of precision)String Holds multiple characters

textual data.

Variant Can hold data of any type

except strings of fixed length

or user defined data types


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A variable is a temporary data storage location in your program. Your code can use one or many variables,which can contain words, numbers, dates, properties, orobject references. Variables are useful because they let

you assign a short, easy-to-remember name to a piece ofdata you plan to work with. Variables can hold:

User information entered at run time.

The result of a specific calculation.

A piece of data you want to display on your form.

In short, variables are simple tools you can use to trackalmost any type of information whose value can change.


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Declaring a variable is the process of creating avariable in your Visual Basic program code. Youcan do this explicitly with the Dim keyword or

implicitly by typing a new variable name in aprogram statement.

Explicit Declaration

In Visual Basic, one of the ways to create avariable is to declare it explicitly using the Dimstatement.


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Dim < var name> [as ]

Where Dim is a keyword that tells VB that a variable is

being declared.

is the variable name

As is the keyword that tells VB the data type ofthe variable

is a legal data type in VB

[] brackets mean that the part is optional


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E.g.

Dim roll_no as Byte

Dim name as string

Dim amt as currency

Note : Default data type is Variant


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Implicit declaration

You can also declare a variable without the Dim

statement; this process is called implicitdeclaration.

To declare a variable implicitly, you simply use thevariable on its own and skip the Dim statement

altogether. E.g.I=34


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Variable naming conventions

1.

Variable name must be less than 255 characterslong.

2. Variable name cannot contain a space, period,

or a hyphen.

3. A variable must begin with a letter

4. It must be unique within the scope


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Strings

Can store multiple character data.

String length : no: of characters in string

Each character requires one byte for its storage.


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Strings can be :

Fixed length

Declaration: Dim s as string * 10

Variable length:

Declaration: Dim s as string


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Assigning values to variable

Dim I as integer

I= 24

Dim s as string

S= items selected


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Default values for variables:

Data type Default value

Integer 0Long 0

Single 0

Double 0

String (Blank string)


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Data type Default value

Boolean False

Variant Empty

Date 0

Currency 0


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Literals

The values assigned to variables are called

literals. A literal whose data type is notspecified is considered to be a variant

type. The following guidelines must be

followed to explicitly provide data type of

a literal.


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String literals are enclosed in quotation

marks. E.g. Deepa

Date and time values must be enclosed in

#s E.g. #22-Jan-1999#, #2:15pm#

Boolean literals are : True or False


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Constants

If a variable in your program contains a value that neverchanges, you should consider storing the value as aconstant instead of as a variable. A constant is ameaningful name that takes the place of a number or textstring that doesnt change (such as p, a fixedmathematical quantity.)

Advantages of using constants include:

Making program code more readable

Saving memory Making program -wide changes easier to accomplish.


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How Constants Work

Constants operate a lot like variables, but you cant modifytheir values at run time. You declare constants with theConst keyword, as shown in the following example:

Const Pi = 3.14159265

This statement creates a constant called Pi that you can usein place of the value of p in the program code. To createa constant that's available to procedures throughout aprogram, use the Public keyword to create the constant

in a standard module. For example:

Public Const Pi = 3.14159265


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Operators in VB

Arithmetic : + Addition

- Subtraction* Multiplication

/ Division

\ IntegerDivision

Mod Modulus

^ Exponentiation


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E.g.

2 + 5 =7

51-3=482*5=10

5/2=2.5

5\2=25 mod 2=1

3^2=9


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ConcatenationOperator ( &)

Is used to concatenate two strings.

E.g S1=Geeta

S2=SahiName= S1 & & S2

Msgbox( Name)

The above code will display a message box with : Geeta Sahi

If + is used with strings then also it concatenates two strings.


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RelationalOperators / Comparison operators

= Equal to ( Returns True if both operands are equal otherwise

false)< >Not equal to ( returns true if its both operands are not

equal otherwise returns false)

> Greater than ( returns true if first operand is greater than the

second operand otherwise returns false)

> = Greater than equal to ( returns true if first operand isgreater than or equal to the second operand otherwise

returns false)


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< Less than ( returns true if first operand is less than the

second operand otherwise returns false)


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Conditional Expression Result

10 < > 20 True (10 is not equal to 20)

Score < 20 True if Score is less than 20;

otherwise, FalseScore = Label1.Caption True if the Caption property of the

Label1 object contains the same value

as the Score variable; otherwise, False

Text1.Text = Bill True if the word Bill is in the first text

box; otherwise, False


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Logical operators

LogicalOperator MeaningAnd If both conditional expressions are

True, then the result is True.

Or If either conditional expression is

True, then the result is True.


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Not If the conditional expression is False, then the

result is True. If the conditional expression is

True, then the result is False.

Xor If one and only one of the conditional expressions

is True, then the result is True. If both are True or

both are False, then the result is False.

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Unit II Programming Fundamentals