Transcript Communication and Distributed Systems

Enterprise Development
Using Visual Basic 6.0
Autumn 2002
Tirgul #3
‘Tirgul’ # 3
Short Quiz
• Write a simple Function that receives a
String and returns its length
• Write a Simple Function that receive an
Integer, if it is negative, returns a String
with error message, otherwise returns
the Integer as a String
‘Tirgul’ # 3
Objectives
• Parameters passing
• Getting Deeper
– If – Then – Else
– Case
– Loops
• Arrays
– Dimension arrays correctly
– Recognize the default values of array elements
– Access array items by subscript or all elements in a loop
– Use parallel arrays for storage of related lists of items
– Use list boxes for displaying choices
– Use 2-D arrays
‘Tirgul’ # 3
Review: important concepts
• Remember to plan your programs
(algorithm) before starting to write the
code! (80:20)
• Use procedures and functions
– parameters must be carefully set
– Use ByRef and ByVal
• Fundamental Structures:
– Sequence, Selection, and Iteration
‘Tirgul’ # 3
Parameter Passing
• By default, variables are passed ByRef
– called procedure uses same memory location for
the variable
– assigns a new name for that location, uses that
name within the procedure
– contents of this location may be changed
‘Tirgul’ # 3
Parameter passing
• Parameters can also be passed ByVal
– called procedure sets up new memory
locations
– value of parameter is copied into the new
locations
– changes do not affect original variable
‘Tirgul’ # 3
Example
What will be printed out?
What if we called
Sample2 (b, a) ?
Nested IF statements
If condition1 Then
If condition2 Then
statement1
End If
Else
statement2
End if
‘Tirgul’ # 3
If
Example
Grade >= 90 Then
txtLetterGrade.Text = “A”
Else
If Grade >=80 Then
txtLetterGrade.Text = “B”
Else
txtLetterGrade.Text = “C”
End If
End If
‘Tirgul’ # 3
Example
If Income >= 40000 Then
If Status = “Single” Then
TaxRate = 0.33
ElseIf Status = “Married” Then
TaxRate = 0.25
End If
Else
sTaxRate = .15
End If
Less end if’s
‘Tirgul’ # 3
If statement summary
• VB is sensitive at design-time and runtime
• Indentation!
• Use if… Then
• Use end if
• Use elseif
‘Tirgul’ # 3
Select Case Structure
• Multiple IF statements can be replaced by more
readable Select CASE statements
Select Case selector
Case valueList1
action1
Case valueList2
action 2
…
Case Else
if no other match
End Select
 valueList
options:
Case 1
Case 2 to 5
Case 6, 9
Case “text”
Case Is >= 10
‘Tirgul’ # 3
Example
Select case AccessCode
case is < 1000
message = “Access Denied”
case 1465 To 1689
message = “Technical
Personal”
case 999898 , 10000006 To 10000008
message = “Scientific Personal“
case Else
message = “Access Denied”
End Select
‘Tirgul’ # 3
Case statement summary
• Use String/Integer type
• When using String pay attention to
content!
• Don’t forget default
‘Tirgul’ # 3
Do / Loops
Format 1:
Do {While | Until} Condition
loop body
Loop
Format 2:
Do
loop body
Loop {While | Until} Condition
•Loop body executes while the condition is true or until the
condition is true
•The first form uses a pretest, the second uses a posttest.
•With a pretest, loop may not execute at all.
•With a posttest, loop always executes at least once.
‘Tirgul’ # 3
Do While Example
Do While sTotal < 25
sNum = Val(Inputbox(“Enter a number”))
If sNum > 0 then
sTotal = sTotal + sqr(sNum)
End If
Loop
‘Tirgul’ # 3
Choosing Loop Type
• For..Next loop should be used when you know the number of
loops
• Do ..While is used when you do not know in advance the
number of iteration.
– Examples
• You want to get user input until she hits
the escape button
• You want your server to keep listening
for request until it receives a shut down
message.
‘Tirgul’ # 3
Danger
Loops summary
•
•
•
•
For – next
While – wend
Do – while
Common mistakes
– Stop condition
– Increment
• Exit loop
‘Tirgul’ # 3
Arrays
• Array – Set of elements of the same
type indexed in a data structure
• Each array object is called an element
• Each element is identified with an Index
‘Tirgul’ # 3
Why use arrays?
• Often, all data must be read in and stored for
future use.
• Not always possible to get each value and process
it, it is better to get all data, store it in an array and
process it
• Use of data structures is preferred and
unavoidable.
‘Tirgul’ # 3
Declaration
• Array name - identifier
• Array type – type of elements in the
array
• Array range – how many elements
Dim
class(1 to 10) as students
class(1)
class(30)
‘Tirgul’ # 3
Subscripts
• Recall – Array starts at 1 BUT:
Dim GradeArr(MAX_STUDENTS) as Single
Dim GradeArr(0 to 29) as Single
• Array of size 25
• subscripts may be constants, variables,
or numeric expressions
• Use Constants!!!
‘Tirgul’ # 3
Array Value set
Dim GradeArr(1 to MAX_STUDENTS) as Single
GradeArr(1) = 95
for index = 2 to 10
GradeArr(index) = 100
next index
GradeArr(11) = (sGrade(1) + sGrade(2) )/2
GradeArr(30) = ?
‘Tirgul’ # 3
Array bounds
• Ubound, Lbound will return the array
declared size (NOT elements)
• To prevent errors, check Ubound for
highest index
• Common use to iterate the array:
for index = Lbound(GradeArr) to Ubound(GradeArr)
…
next index
‘Tirgul’ # 3
24
Declaring Dynamic Array
• An Array declaration at the module level – No
memory location yet:
Dim DBConnections() as Connection
Public DBRecordSet() as RecordSet
• Memory allocation at sub/function level:
Sub openConnection ()
. . .
ReDim DBConnections(1 To MAX_CONNECTIONS)
End sub
‘Tirgul’ # 3
25
Redim
• Resizing an existing array
• Procedure level allocation
• Deallocate memory (free memory)
Redim array(0)
• Array values are lost at Redim!
• Use preserve to keep old values
ReDim preserve newArray(MAX_ARRAY_SIZE + 1)
‘Tirgul’ # 3
26
For Each…Next Statement
• Use For Each…Next Iteration to iterate al
array elements
• Use variant type variable if you don’t
know element type.
Dim day as Variant
For each day in week
print day
Next day
‘Tirgul’ # 3
Split - Join
• Split – Namely split a String to the array
dim wordsArr()as String
wordsArr
dim Sentence as string
1
Good
Sentence = “good morning”
2
Morning
wordsArr = Split(Sentence,“ “ )
• Join – Namely create a String from array
Sentence = join(wordsArr,“ “)
Delimiter
‘Tirgul’ # 3
Sentence
“good morning”
Two-Dimensional Arrays
• Think of a table structure
• use two indexes
– row index
– column index
• all elements must be of the same type
‘Tirgul’ # 3
Examples
• Dim iArray (1 to 2, 1 to 5) as Integer
Row
Col
• array of 2 rows and 5 columns
‘Tirgul’ # 3
Accessing Array Elements
• iArray (1, 2) = 10
• iArray (2, 5) = iArray(1, 4) + iArray(1, 5)
• I=7 : J=5
sGrades (I, J) = 93
• x=1:y=2
iArray (1, x + y) = 15
‘Tirgul’ # 3
Practical - Combo Boxes
• Combo box represent a list of items to select
from.
• Items can be specified during Design/Run
time
• Uses:
– Inset Item
– Delete Item
– Get selected Item
‘Tirgul’ # 3
Code sample for Combo Box
• ListIndex represents the selected Index
• Text represents the item String value
Debug.print cmbDays.text
• Remove selected item
cmbDays.RemoveItem cmbDays.ListIndex
• Add items to list
cmbDays.AddItem “Sunday”
‘Tirgul’ # 3
Using Combo Boxes As an Array
• Combo box is an array controlled
by VB
• Count – combobox.ListCount
• Add/Remove operations
• Lists start at 0, array at 1
‘Tirgul’ # 3
3 Items
Fill
Clear
Count
Controls array
• In a from, you can group elements
– Tab, Option button…
• Useful when having lots of controls
• Notation:
Option(0), Option(1)
• Design time
– Copy/Paste in the form
– setting index
‘Tirgul’ # 3