Chapter 3
Variables (變數),
Constants (常數) and
Calculations (計算)
Programming In
Visual Basic.NET
Prepared by
Johnny Tsui, CIM@IVE
Your work - Reminder
• How can you assign (給予) values to
the following two text boxes?
Textbox1
Textbox2
YES
NO
• How can you swap (互換) the values
in these two text boxes?
Textbox1
Textbox2
NO
YES
2
Suggested Solution
• How can you assign values to the
following two text boxes?
Textbox1
Textbox2
YES
NO
• Textbox1.text = “YES”
• Textbox2.text = “NO”
3
Suggested Solution
• How can you swap the values in
these two text boxes?
Textbox1
2
YES
Textbox2
NO
1
strTemp
3
YES
• Dim strTemp = Textbox1.text
• Textbox1.text = Textbox2.text
• Textbox2.text = strTemp
4
Variables &
Constants
• A temporary (暫時) location for
storage (儲存) and calculation (計算)
• Variable
– data that can be changed (可改變)
– Ex: study hours (溫習小時) 45 hr
• Constant
– data that cannot be changed (不能改變)
– Ex: service charge (服務費) 10%
5
Declaration
• Variables and Constants must be
declared before being used
6
Naming Conventions
(命名法則)
• May consist of letters (文字), digits
(數字) and underscores (底線)
• Begin with a letter
• Cannot contain any spaces (空位) or
periods (點)
• Cannot be reserved words (專用字)
• Not case sensitive (不分大小寫)
• Should be meaningful (有意思)
7
Your work – Valid?
1.
2.
3.
4.
5.
6.
7.
8.
omitted
int#Sold
int Number Sold
int.Number.Sold
sng$Amount
sub
strSub
text
9. conMaximum
10. MinimumRate
11. decMaxCheck
12. strCompanyName
13. room###123
14. Price$112
8
Declaration Statements
• DIM used to declare Variables
• CONST used to declare Constants
• Declaration
– DIM VariableName As DataType = Value
– CONST ConstantName As DataType = Value
9
Data Types (資料類別)
•
•
•
•
•
•
Boolean (True, False)
Char (“A”, “B” )
Date (“11/9/2004”)
String (“Hello”)
Decimal (123.45)
Integer (1, 23, 45)
10
Data Types – Prefixes (字頭)
•
•
•
•
•
•
Boolean – bln
Char – chr
Date – dat
String – str
Decimal – dec
Integer – int
11
Your work – Declaration?
Dim strName
Dim decPayRate
Dim datHireDate
Dim blnInsured
Dim chrLetter
Const decRATE
As String =
As Decimal =
As Date =
As Boolean =
As Char =
As Decimal =
12
Variables – Scope
• Global
– Available to all modules and procedures of Project
– Initialized at start of Project
• Local
– Available only to the procedure it is declared in
– Initialized every time the Procedure runs
13
Calculations
• Do NOT use Strings in calculations
• Ex: intX = “ABC” + 3
• Values from Text property of Text
Boxes
– Are Strings, even if they contain
numeric data
– Must be converted to a Numeric Data
Type
14
Conversion Functions
Function
CInt
CDec
CStr
Convert To
Integer
Decimal
String
15
Conversion Examples
intQuantity
decPrice
intWholeNumber
decDollars
strValue
=
=
=
=
=
CInt(txtQuantity.Text)
CDec(txtPrice.Text)
CInt(decFractionalValue)
CDec(intDollars)
CStr(decValue)
Function Name
Argument (參數)
16
Your work - Conversion
String
CInt
CDec
123.45
$100
1,000.00
-5
0.01
1.5
A123
17
Mathematical Operators
Operator
+
–
*
/
\
Mod
^
Operation
Addition
Subtraction
Multiplication (乘)
Division (除)
Integer Division
Modulus (餘數)
Exponentiation (次方)
18
Order of Operations
• from left to right in the following
orders
1.
2.
3.
4.
5.
6.
Parentheses - ()
Exponentiation - ^
Multiplication & Division - */
Integer Division - \
Modulus - Mod
Addition & Subtraction - +19
Mathematical Examples
3+4*2 = 11 Multiply then add
(3+4)*2 = 14 Parentheses control: add then multiply
8/4*2 = 4
Same level, left to right: divide then multiply
20
Calculation Examples
• 10
• 10
• 10
• 10
• 10
• 10
• 10
+2=
–2=
*2=
/2=
\4=
mod 3 =
^2=
21
Your work – Calculation?
•
1.
2.
3.
4.
5.
6.
7.
8.
Assume (假設) that intX=2, intY=4,
intZ=3, what is the value of intA
intA = intX + intY ^ 2
intA = 8 / intY / intX
intA = intX * (intX + 1)
intA = intX * intX + 1
intA = intY ^ intX + intZ * 2
intA = intY ^ (intX + intZ) * 2
intA =(intY ^ intX) + intZ) * 2
intA =((intY ^ intX) + intZ) * 2
22
Option Explicit (明確)
• ON by default
• If turned off
– Variables can be used without first
being declared
• To turn off
– Option Explicit Off
23
Option Strict (嚴格)
• OFF by default
• If turned on
– VB becomes strongly typed language
– Will not allow implicit conversions
• To turn on
– Option Strict On
24
Your work – Explicit?
• What errors will you get if Option Explicit
Off has been defined?
– intA = 3
– Dim intB As Integer
– IntB = intA +5
• What errors will you get if Option Explicit
On has been defined?
– intA = 3
– Dim intB As Integer
– IntB = intA +5
25
Your work – Strict?
• What errors will you get if Option Strict Off
has been defined?
– Dim intA as Integer, intB As Decimal
– intA = 2.5
– IntB = intA +5
• What errors will you get if Option Strict On
has been defined?
– Dim intA as Integer, intB As Decimal
– intA = 2.5
– IntB = intA +5
26
Handling Exceptions
(Errors)
• Exceptions occur
– User enters nonnumeric data in Text
Box and code attempts to run a Numeric
Conversion Function
– User enters data that results in division
by zero
27
Try/Catch Blocks
• Handle exceptions in a Try/Catch
Block
– If an error occurs, control is
transferred to the Catch Block
– Include a Finally statement to indicate
code that should execute last whether
or not an exception occurred
28
Try Block - General Form
Try
statements that may cause error
Catch [VariableName as ExceptionType]
statements for action when an exception occurs
Finally
statements that always execute before exit of Try block
End Try
29
Try Block - Example 1
Catches All Exceptions
Try
intQuantity=CInt(txtQuantity.Text)
lblQuantity.Text=CStr(intQuantity)
Catch
lblMessage.Text="Error in input data."
End Try
30
Try Block - Example 2
Catches Specific Exception
Try
intQuantity=CInt(txtQuantity.Text)
lblQuantity.Text=CStr(intQuantity)
Catch MyErr as InvalidCastException
lblMessage.Text="Error in input data."
End Try
Conversion exception, usually caused
by nonnumeric or blank data
31
Try Block - Example 3
Catches Multiple Specific
Exceptions
Try
statements that may cause errors
Catch MyErr as InvalidCastException (轉型錯誤)
error messages and statements for nonnumeric data
Catch MyErr as ArithmeticException (計算錯誤)
error messages and statements for calculation problems
Catch MyErr as Exception (其他錯誤)
error messages and statements for any other exception
End Try
32
Your work – Exception?
• How can you handle exceptions for the
following coding?
:::
Dim strA as String
Dim intB as Integer
:::
strA = “$100.0”
intB = CInt(strA)
lblMessage.text = intB
:::
:::
33
MessageBox
• Use to display messages in a
special type of window
• Arguments of Show method
1.
2.
3.
4.
Message to display (內容)
Title Bar Caption (標題)
Button(s) (按鈕)
Icon (圖案)
34
MessageBox Syntax
MessageBox.Show (TextMessage, TitlebarText, _
MessageBoxButtons, MesssageBoxIcon)
• Example:
MessageBox.Show(“Good Morning”, “Greeting”, _
OK, Exclamation)
35
MessageBoxButtons
Constants
• OK
• OKCancel
• RetryCancel
• YesNo
• YesNoCancel
• AbortRetryIgnore
36
MessageBoxIcon Constants
•
•
•
•
•
Asterisk
Error
Exclamation
Hand
Information
•
•
•
•
None
Question
Stop
Warning
37
Your work - MessageBox
• Please write the
following messages
on the screen:
Message
Title
Button
Icon
Nice to meet you! Greeting
OK
Exclamation
You are wrong!
Attention
OK
Warning
Please press
ENTER!
Information
OKCancel
Information
38