Slide 1

Variables, Constants and Data Types

Slide 2

Variables

Three components define a variable:– Name (memory location)– Type– Information (value)

Variable name must:– Start with a letter– No spaces, periods nor punctuation characters– Unique (within its scope)– 255 characters in length– Not a VB reserved word

Slide 3

Variables

Variable naming conventions:– Descriptive (code is easy to read)– Short as possible (easy to type)– Use a prefix for type (easy for programmer to know its

type) String s sName Integer n nAge (table 8.1, page 161)

Slide 4

Types of Variables

Integer 2 bytes -32768 to +32767 Long 4 +/-2 billion Single 4 +/-1E-45 to 4E38 Double 8 +/-5E-324 to 1.8E308 Currency 8 +/- 9E14 String1B/char 65000 fixed, 2 billion dynamic Byte 1 0 to 255 Boolean 2 True or False Date 8 1/1/100 to 12/31/999 Object 4 Varient 16B+1B/Char

Slide 5

Variable Declaration

If a variable is not declared, it is declared by default by VB as Variant– Waste memory resources– Type might be invalid for use with functions

It is good programming practice to declare variables

Slide 6

Explicit Declaration

Explicit declaration uses statements to define the names and types of variables– Dim sFname as String, sLname as String– Private sFname as String, sLname as String– Static sFname as String, sLname as String– Public sFname as String, sLname as String

Multiple variables with multiple types If type is not indicated, Variant will be used

Slide 7

Implicit Declaration

VB defines the variable the first time it is used Variable name is preceded with a special character:

– Integer %– Long &– Single !– Double #– Currency @– String $

Example:– nNumVal% = 0– sFirstName$ = “Ali”

Slide 8

Fixed-Length Strings

Strings in general are variable-length Fixed length strings are declared like:

– Dim sName As String * 25

If a shorter string is assigned, it is appended with blanks

If a longer string is assigned, it is truncated

Slide 9

Variable Array

Array: is a group of variables of the same type, sharing the same name

Example: Dim nScores (1 to 35) As Integer Use for loop with array

For nCounter = 1 to 20

nScores(nCounter) = 0

Next nCounter

Slide 10

Scope

By default variables are local to the procedures where they are created in (local variable)

Public variables: can be accessed anywhere at the code

Example:Public sUserName As String

(Defined in the General Declarations Section)

Using Public variables:– Hard to debug– Bad use of resources

Slide 11

Static Variables

Variables declared local to a procedure are discarded afterwards

To preserve a variable in side a procedure use Static

Static nPages As Integer Using Static with functions or sub treats all

variables inside as Static

Slide 12

Option Explicit

Setting “REqire Variable Declaration” option (Tools – Options – Editor)

It places Option Explicit in the general section has no effect on Forms/Modules created before

setting it

Slide 13

Constants

Cannot be modified Easy to remember then it value Avoid typing long strings Minimize program modifications

Examples:– Const vbActibeTitleBar = -2147483646– Const PI = 3.141592654 – Public Const PI = 3.141592654

Slide 14

Converting Data Types

VB provides several conversion functions you can use to convert values into a specific data type.

To convert a value to Currency, for example, you use the CCur function:

PayPerWeek = CCur(hours * hourlyPay)

Slide 15

The Empty Value

Sometimes you need to know if a value has ever been assigned to a created variable. A Variant variable has the Empty value before it is assigned a value. The Empty value is a special value different from 0, a zero-length string (""), or the Null value. You can test for the Empty value with the IsEmpty function:– If IsEmpty(Z) Then Z = 0

When a Variant contains the Empty value, you can use it in expressions; it is treated as either 0 or a zero-length string, depending on the expression.

The Empty value disappears as soon as any value (including 0, a zero-length string, or Null) is assigned to a Variant variable. You can set a Variant variable back to Empty by assigning the keyword Empty to the Variant.

Slide 16

The Null Value

The Variant data type can contain another special value: Null.

Null is commonly used in database applications to indicate unknown or missing data. Because of the way it is used in databases, Null has some unique characteristics:

Expressions involving Null always result in Null. Thus, Null is said to "propagate" through expressions; if any part of the expression evaluates to Null, the entire expression evaluates to Null.

Passing Null, a Variant containing Null, or an expression that evaluates to Null as an argument to most functions causes the function to return Null.

Slide 17

The Null Value

You can also assign Null with the Null keyword:– Z = Null

You can use the IsNull function to test if a Variant variable contains Null:

If IsNull(X) And IsNull(Y) Then Z = Null

Else Z = 0

End If

Slide 18

Built-in Constants

Intrinsic Constants Colors, keycodes, shapes Described in help Object Browser