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Topics. Data Types for variables Constants to replace literals Creating variables Variable scoping Naming variables User-defined type example. Integer Integer Long Floating point Single Double Currency String Variant User Defined Multi-element variable. -32,768 – 32,767

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topics
Topics
  • Data Types for variables
  • Constants to replace literals
  • Creating variables
  • Variable scoping
  • Naming variables
  • User-defined type example

Jeffrey P. Landry, University of South Alabama

data types
Integer

Integer

Long

Floating point

Single

Double

Currency

String

Variant

User Defined

Multi-element variable

-32,768 – 32,767

-2.147 billion – 2.147 billion

1.4012e-45 –3.4028e38

4.94065e-324 – 1.79769e308

922,337 trillion

Up to 65,500 characters

Any value above plus dates

Composed of collections of individual data elements

Data Types

Jeffrey P. Landry, University of South Alabama

constants
Used in place of literal values

Declare in the general section of modules

Good programming practice to use constants

literals tell the programmer nothing

changing in more than one place

Public Const RptHead$ = “Payroll Report”

Const MinRate As Currency = 5.50

Const MaxRate As Currency = 75.50

‘* Get employee rate from user

If (EmpRate < MinRate) Or (EmpRate > MaxRate) Then

MsgBox “Invalid pay rate”,...

Constants

Jeffrey P. Landry, University of South Alabama

variables
Variables
  • VB supports several types of variables
    • may be implicitly or explicitly declared
  • Three aspects of variables to focus on
    • Data type
    • Scope
    • Name
      • Keywords
      • Object names
      • Scope overlap

Jeffrey P. Landry, University of South Alabama

creating variables
Creating Variables
  • Introduce variable (bad!)
    • Let Num = 23
  • Declare, or dimension variables (good!)
    • with Dim, Public, Private, Static
      • Dim RecNum As Integer
      • Dim RecNum%
  • Option Explicit statement (very good!)
    • Placed in declarations section of form or module
    • Forces declaration (dimensioning) of all variables before use
    • Causes compile error if violated

Jeffrey P. Landry, University of South Alabama

variable scoping
Variable Scoping
  • Three levels
    • local - Variables first used or declared in a procedure are local to that procedure
    • module-level - Variables declared with Dim or Private keyword in the General section of a form or code module are available to any procedure in that module
    • global - Variables declared with Public keyword in General section are accessible throughout the application

Public FileName As String ‘* Database file name

Jeffrey P. Landry, University of South Alabama

variable scoping cont
Variable Scoping (cont.)
  • Declaring a variable at a local (procedure) level will “mask” a broader variable of the same name
  • Failure to declare local variables might produce unintended consequences
    • such as changing the value of a global or module-level variable
  • Local variables lose value when procedure terminates
    • unless declared as static with Static statement
    • Module and global variables always retain value throughout program

Jeffrey P. Landry, University of South Alabama

variable naming
Variable Naming

For I% = 1 to Count%M

Next I%

  • Same rules as for naming objects
    • up to 40 characters, beginning with letter
    • letters, numbers, underscore
    • no keywords
  • Competition for names with objects and keywords
  • Naming conventions
    • I, J, K, X, Y, Z counters
  • Editor auto-adjusts upper/lower case

Jeffrey P. Landry, University of South Alabama

user defined type example
Type ProductType

Name As String * 20

Price As Currency

Cost As Currency

OnHand As Integer

End Type

Dim Table As ProductType

Let Table.Name = “Tables”

Let Table.Price = 149.50

Let Table.Cost = 95.00

Let Table.OnHand = 75

User-Defined Type Example

Jeffrey P. Landry, University of South Alabama