Variables

1
Variables Data Types
Chapter 3
2
Overview

• Variables
• Data Types
• Expressions
• Type conversion
• VB IDE issues
• Dealing with long lines of code
• Comments

3
Types of Data

• Numeric
• Can contain only numbers
• Real numbers, integers, currency, dates
• Used for arithmetic calculations
• String (text)
• May contain any symbol
• No arithmetic calculations
• Append, insert, extract
• Boolean
• True or False, like the visible property

4
Variables - The Concept

• Variables are named containers of data
• They are not displayed on a form
• Their contents can change (vary) during runtime
• Variables are grouped by size and purpose
• Some types of data require more memory than
  others
• Each variable belongs to a type
• The type of a variable determines
• Size
• How operators will work on the variable
• Add, subtract, append, logical NOT

5
VB Variables

• Variables are not created using the tool box
• Instead, they are declared within the program
• Declaring a variable
• sets aside memory for the variable
• assigns a name to that space in memory
• describes what type of data can be stored there
• Variables dont work with the properties window
• Changes to a variable must be made in a program

6
Declaring a Variable

• A declaration begins with the Dim statement
• This alerts VB that a new variable is being
  defined
• A declaration contains a variable name and type
• Just as each control must have a (name), like
  lblInput, and type, like label
• The form of a declaration Dim ltnamegt as lttypegt
• For example
• Dim intCount as Integer

7
Numeric Variable Types

• There are several different numeric variable
  types, each with varying
• Precision
• Size
• Representation scheme
• Selecting the wrong type for variables can hurt
  the performance of a program

8
Counting Things

• Integer types
• BYTE ? Small range of values, 0 to 255
• INTEGER ? -32,768 to 32,767
• 2 bytes
• LONG ? -2,147,483,648 to 2,147,483,647
• 4 bytes

9
Measuring Things

• SINGLE
• Positive or negative
• As close to zero as 1.401298E-45
• As large as 3.402823E38
• 4 bytes
• DOUBLE
• Positive or negative
• As close to zero as 4.94065645841247E-327
• As large as 1.79769313486232E308
• 8 bytes

10
Highest Precision

• Scaled integers
• CURRENCY
• /-922,337,203,685,477.5807
• 8 bytes
• DECIMAL
• /-79,228,162,514,262,337,593,543,950,335
• /-7.9228162514262337593543950335
• no rounding, slow but exact to precision limit
• 14 bytes

11
Storage Space Prefixes

• BYTE 1 byte byt
• INTEGER 2 bytes int
• LONG 4 bytes lng
• SINGLE 4 bytes sng
• DOUBLE 8 bytes dbl
• BOOLEAN 2 bytes bln
• CURRENCY 8 bytes cur
• DECIMAL 14 bytes dec

12
Operators and Expressions

• As in algebra, VB variables can be connect with
  operators to form expressions
• An expression, like a variable, has a value and a
  type
• For exampleDim intValue as IntegerDim sngValue
  as SingleintValue 4sngValue
  1.25lblOuput.Caption 4intValue sngValue

Expression Type Single, Value 17.25
Assignment Statement
13
Assignment and Type

• It is possible for an assignment statement to
  have a variable of one type on the left of the
  equal sign, and an expression of another type on
  the right
• Also, sometimes an operator is used on 2
  variables of different type
• In both these cases, VB will convert the values
  to the most appropriate type
• Type conversion rules get pretty complex, so
  we'll focus on simple, common results

14
Automatic Type Conversion

• Dim intValue as integer
• intValue 12
• intValue "12"
• intValue 12.5
• intValue 11.51
• Due to automatic type conversion, all these
  statements result in the integer variable
  intValue being set to 12

15
Variable Scope

• A variable has a scope, the area it is accessible
  
• Variables can be declared inside of a sub
• These are said to be local variables
• They cannot be accessed outside the sub
• They are re-initialized each time the sub is
  called
• Variables declared outside of any procedure are
  global
• Can be used in any code on that form
• Values are preserved between button clicks

Dim intCount As Integer Private Sub
cmdOn_Click() Dim intSlot as Integer
lblOut.Caption _ intCount intSlot End Sub