What is a Variable?

Variable

Computer memory locations where programmers can temporarily store data. The data may be entered by the user at the keyboard, or it may be read from a file, or it may be the result of a calculation made by the computer.

The memory locations are called variables because the contents of the locations can change as the application is running.

Every varialbe has a name, data type, scope, and lifetime.

Data Type for a Variable

See Figure 3.1 page 72

Boolean

True, False

Byte

0 to 255

Char

One Unicode character

A, B, C, ...

Date

Thu, 01 Jan 1970 00:00:00 GMT

Decimal

16 bytes

fixed-point number, no rounding

33.3333333

Double

8 bytes

Integer

4 bytes

1,2,3,-10

Long

8 bytes

1,2,3,-10

Object

Default data type to a variable

Short

-32,768 to 32,767

Single

11.56, -12.99

String

"Hello, World"

Naming Variables

See Figure 3.2

Start with a 3 character id

intScore, intRegionWest

m can be used to stand for module scope, mintScore

names must begin with a letter

names can not contain punctuation

names can contain numbers, but the number can not be at beginning, dec2002Sales

Declaring a Variable

Syntax: accessibility variablename [As datatype][= initialvalue]

Dim decPrice As Decimal

Private mblnDataOk As Boolena = True

Assigning Data To an Existing Variable

				
Dim dblRate As Double 
dblRate = .5

				
Dim decPrice As Decimal 
decPrice = .03D]

Literal Type Charecters

				
shrAge = 35S
intHours = 4I
lngPopulation = 20500L
decRate = .03D
sngRate = .03F
dblSales = 2356R
charInitial = "A"C

Convert

					
Convert.ToDouble (value)
Convert.ToInt16 (value)
Convert.ToInt32 (value)
Convert.ToSingle (value)
Convert.ToString (value)

Writing Arithmetic Expressions

^ (exponentiation, 4^2 = 4 * 4 = 16)
- (negation)
*, / (multiplication & division)
\ (integer division)
Mod (modulus arithmetic)
+, -

				
intAge=intAge + 1

Scope & Lifetime of a Variable

Scope: Where in the application's code the variable can be used
Lifetime: How long the variable remains in the computers internal memory.
Procedure-Level Variable: Has procedure scope becuase only that procedure can use the variable. See Figure 3.10
Module-Level Variable: Has module scope because it can be used by all of the procedures in the form. See Figure 3.11
Block-Level Variables: Has block scope because it can be used by block of code If...THen...Else or For...Next

Named Constants

Syntax: Const constantname [As datatype][= expression]

Const decPI As Decimal = 3.141593D

Const intMAX_HOURS As Integer = 40

Internally Documenting the Program Code

Place an apostrophe (') before the text you want to treat as a comment.

Option Explicit & Option Strict

By default Visual Basic .NET creates undeclared variables for you

Option Explicit On

Variables must be declared

Option Strict On

Strings will not implicitly convert to numbers.
Numeric data types will be implicity converted to less restrictive types.
Implicit conversion to a more restrictive type will cause a syntax error.

Pseudocode

Use short phrases to describe the steps a procedure needs to take to accomplish its goal

Flowcharts

See Figure 3.19 page 90

Assign a Value to a Property

				
Me.txtName.Text = ""
Me.lblBonus.Text = Convert.ToString(decSales * .05D)

Using the Focus Method

				
Me.txtname.Focus()

Testing and Debugging

Valid Data

Expected Data - ???

Invalid Valid Data

Unexpected Data - ???

Debugging

Locating errors in a program

Syntax Errors

Typing Errors

Logic Errors

You enter instructions that don't give the expected results.

				
decAverage = decNum1 + decNum2 / 2

Formatting Numeric Output

				
'Currency Format					
Me.lblCommission.Text = decCommission.ToString("C")
					
'2 Decimal Didgets					
Me.lblTotal.Text = decTotal.ToString("N2")
					
'Percent
Me.lblRate.Text = decRate.ToString("P")

Variables, Constants, Methods, & Calculations (Chapter 3)

Variables