Form load ,show and hide properties,VB Datatypes and variables declaration

8/7/2019 Form load ,show and hide properties�,VB Datatypes and variables declaration

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Form load ,show and hide properties

VB Datatypes and variablesdeclaration



Form (show and hide)

� Form an be loaded into memory using load <formname> orformname.show

� We can hide the form using formname.hide when we hide theform it remains in the memory

�When we unload <formname> then the form does not exists inthe memory even.

� There are two modes of the form <formname>.show <mode>

1.Modeless:It is the default mode that we can any form at runtime (we can specify 0) in mode.

2.Modal:In this mode the form which is selected takes the

control when it closed then only we can acess the other forms.Formname.show 1 or formname.show vbmodal



Datatypes In VB

� Boolean:this datatype take true or false value.

� Byte:this Datatype allows positive intergers formrange 0-255

� Currency:This datatype holds the values whichcan have four decimal places and 15 digits beforedecimal.

� Date:this datatype holds the date and time value.

�Double:hold numeric value it can hold the valuewith 14 digits after decimal.



� Integer:It hold the non fractional values integer in therange of -32768 to +32767.

� Long:If holds the integer value larger then integerdatatype.

� Object:special datatype that is used to hold and refertothe objects such as controls and forms.

� Single:another datatype for decimal values it hold upto 6decimal places.

� String:Datatype hold multiple characters it can have

numeric ,alphabets and other character.� Variant:it does not accepts string that is fixed length or

datatypes created by user.it is default datatype incase thedatatypes are not specified or unknown.



Declaration of variables

� A named Storage location whose contents can

be varied is called a variable.

�

A variable is associated with two things nameand its data type.

� Variable name refers to the value stored in it

and data type tells what type value can be

stored in the variable.� Dim variablename [as datatype][,variablename [as datatype]].



� Dim X ( a variable x is declared with data type

variant i.e it can hold value of any data type

except fixed length string and user defined

datatype)

� Dim y as integer ,z as integer

� Dim y,z as integer( wrong declaration)



Type declaration Symbols or

Characters

Dim <variablename> <type

declaration character>

Ex:dim a%

we need not use as clause wth

this method

Type Declaration

Symbol

Datatype

%

Integer

$String

@

currency

&

long

#

double

!

Single



Implicit and Explicit Variable

Declaration� Implicit declaration: The programmer just

need to type variable name into source code &

the variable is automatically created by the VB.

Eg. B=helloB contains String value hello & VB

automatically created B & selected a default

datatype .Disadvantage of implicit declaration is that

variant data type takes larger memory as

compare to other data types.



Explicit Declaration

� In this declaration it becomes mandatory to

declare the variable before there use .error

message is flashed incase variables are not

declared. In this case the programmer must

first name the variable & describe what type of

information will be contained within the

variable.



There are two methods :

1)By checking require variable declaration option in option dialogavailable tools menu.

2)By writing option explicit in general declaration.

Eg: Dim B As StringExplicit Declaration has 3 parts:

�The first part is a keyword that determines the SCOPE of

variable.

�The second part is the variable name that will be used in thesource code.

�The third part is the datatype of the variable that tells the

computer how much memory is required .



OPTION EXPLICIT1. To force variable declarations to be explicit i.e that each

varibale must be declared we use Option Explicit statement atthe module or form level in the general declaration section.

2. Is we don¶t use O.E all undeclared variables are of VARIANT

type unless the default type is otherwise specified.

3. It is used to avoid incorrectly typing the name of an existing

variable or to avoid confusion in code where the scope of the

variable is not clear.

Eg: Option Explicit

Dim R As integer

Private Sub Form_Load()

X=10

End Sub



SCOPE OF VARIABLE

1. Local

2. Static

3. Private

4. Public



Two method for option explicit



Fixed length and Variable Length

String

� Fixed length string declaration:

Dim firstname as string * 15

�Variable Length String Declaration:

Dim Firstname as string



Val() and Str () Function

� Val():val function converts a string value to

equivalent number.

�

Str():str function converts the function intoequivalent string.



Defining Named Constants

� Const <constname> = <value>

� Constants name should be written in upper

case characters.

Form load ,show and hide properties,VB Datatypes and variables declaration

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