Introduction to VB.Net 2003
Variables, Constants
&
Computation
©Copyright 1997-2006 by Ronald P. Kessler, Ph.D.
Why Use Variables?
•They allow us to use math computations
•They make our programs generic to many solutions
What Is a Variable?
•
Just as in algebra, variables in VB are used to
represent values of things that can change.
•I can use a variable to compute sales tax:
TaxRate= .0775
SalesTax= Price * TaxRate
SalesTax, Price, and TaxRate are variables
How Does VB Know What Each Variable Is?
•
When we have been entering items & prices in our
data entry screens without specifically telling VB what is
text and what should be a number.
•Items that we type in (text) are Strings
•Prices are numbers and should be treated like Decimal.
PROBLEM:
I want to compute the total cost of items purchased.
•Let’s say I buy 3 dozen mouse traps at 16.95 per
dozen.
•I can tell VB to compute the total:
•My Total will be ( $16.95 * 3) or $50.85
In code it is:
Total = 16.95 * 3
I have defined the variable Total simply by typing it
in my formula.
Data Types for .Net
(See Page 99)
Short (Integer)
+32,768
2 bytes (i or int)
Integer
+ 2,147,483,648
4 bytes (i or int)
Long Integer
+9,223,372,036,854,775,808
8 bytes (lng)
Single Precision 6 digits of accuracy
4 bytes (sng)
Double Precision 14 digits of accuracy
8 bytes (dbl)
Decimal
16 bytes (dec)
More $$ than we can count
String
Boolean
Size of Str.
True/False
(s or str)
2 bytes (bln)
In VB 3,4,5,& 6 we used Currency instead of decimal & it was 8 bytes
Math Operators and
Computations
Price = 23.90
Tax = .0775
Total = Price * Tax
lblTotal.Text=Total
Math Operators
Use the +, -, * , and “/” for the basic math operations.
Integer Division uses a “\” (backslash) so that:
5\2 = 2
Use variables in your computations:
Dim Sales as Decimal
Dim SalesTax as Decimal
Dim TotalSale as Decimal
Sales= 765.90
SalesTax= .0775
TotalSale= Sales * SalesTax
lblTotal.Text = TotalSale
Operation Hierarchy
How to solve/setup formulas:
64 / 8 * 3 / ( 6 + 6 ) - 2 =
64 /8 * 3 / 12 -2 =
8 * 3 / 12 -2 =
24 / 12 -2 =
2-2=0
1.
Always do stuff in () first
2.
Exponentiation (raise things to powers)
3.
* and /
4.
+ and -
Always go left to right and handle * and / or + and - as they come
Difference Between Assignment and Math
In the formula: Income= HoursWorked * PayRate we are
computing a math operation.
In the formula: btnPrint.Text = “Print this junk” we are assigning
data from the right side to the left.
When we say : NumCustomers = NumCustomers + 1 we are
assigning, not doing math. In mathematics, this statement cannot
exist.
In VB, the equal sign (=) doubles as an operator and an assignment
character
Assignments take the stuff on the right and assign it to the variable
on the left and stores that value in memory.
Working with Data Entry...
‘----assign data from data entry screen and use conversion functions
‘
Procedure= txtProc.Text
Charge= CDec(txtChrg.Text)
‘Convert text to decimal type
Paid= CDec(txtPaid.Text)
Adjustment= CDec(txtAdj.Text)
CurrentBal= Charge - Paid – Discount
lblResult.Text = FormatNumber(CurrentBal, 2)
‘2 decimals
OR….
lblResult.Text = FormatCurrency(CurrentBal, 2)
FormatNumber & Val are built-in functions in VB
‘shows $
Formatting Your Numbers...
To format Currency (Decimal data type) we can say:
lblTax.Text = CDec(txtPrice.Text) * 0.08
'Make VB treat text as a number using the CDec (Convert to Decimal) function and we
decide how to format it.
lblTax.Text =FormatNumber(CDec(txtPrice.Text * 0.08), 2)
lblBalDue.Text = Format(CDec(txtPrice.Text) + CDec(lblTax.Text), "###.00")
If you want a $ to appear before the amount, use:
lblBalDue.Text = FormatCurrency(CDec(txtPrice.Text) + CDec(lblTax.Text), 2")
VB also has FormatPercent & FormatdateTime
Defining Variables
the
Correct Way . . .
Why Define Variables
Ourselves?
•This makes much better use of memory (RAM)
•It makes our programs run faster
•It will catch our spelling errors
Where Do I Tell VB to Force Me to Define
Variables?
In the GENERAL section of the main form of your program
use the term:
Option Explicit On
You can force VB to make you declare variables from every
project automatically if you want. Or you can type in Option
Explicit On each time.
I want you to start
using this from now on!
Use Tools|Options|Projects
to set it in Visual Studio.
How to Define Variables
Ourselves
•We use the command Dim (dimension) to define
the type of data we are wanting to use.
•If I want the total cost of an item, then I tell VB
this before I start using the variable:
•You usually Dimension variables in the
subroutines (event procedures) where you are
going to use them...
Define Variables In Subs. . .
Private Sub btnPrint_Click ()
‘---Declare variables here
Dim strItem as String
Dim decPrice as Decimal
Dim decTotal as Decimal
Dim intQuantity as Integer
strItem= txtItem.Text
decPrice= CDec(txtPrice.Text)
intQuantity= CInt(txtQuantity.Text) ‘use CInt or CShort
decTotal = decPrice * intQuantity
lblReceipt.Text = "Total===>" & Format(CDec(txtPrice.Text) + CDec(lblTax.Text), "###.00")
End Sub
Using Constants...
Using Constants...
In the section of the form, just after “Windows Form designer generated code”, use
this to define a constant :
Const SALESTAX As Decimal = 0.08D
The “D” makes it a decimal…I know it looks redundant!
Get in the habit of typing constants all in capitals.
Now we can use the value assigned to SALESTAXRATE any where
in our program. This value never changes as long as the program
is running.
Using Your Own Constants...
‘---Inside the form code
Const SALESTAX As Decimal = 0.08D
_________________________________________
Sub btnPrint_Click ()
Dim Tax as Decimal
Dim Price as Decimal
Price= CDec(txtPrice.Text)
Tax= Price * SALESTAXRATE
End Sub
All Done….for now at least.