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Enterprise Development Using Visual Basic 6.0 Autumn 2002 Tirgul #3

This guide covers the fundamentals of Visual Basic 6.0 development for enterprise applications, including parameter passing, loops, arrays, and more.

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Enterprise Development Using Visual Basic 6.0 Autumn 2002 Tirgul #3

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  1. Enterprise Development Using Visual Basic 6.0 Autumn 2002Tirgul #3 ‘Tirgul’ # 3

  2. 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

  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

  4. 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

  5. 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 Sub Add_And_Change (ByRef x as Single, ByRef y as Single) x = x + y End Sub ‘Tirgul’ # 3

  6. 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 Sub Add_And_Print (ByVal x as Single, ByVal y as Single) x = x + y picOut.print x End Sub ‘Tirgul’ # 3

  7. Sub Sample1 (ByRef x as Single, • ByRef y as Single) • x = x - y • picOut x, y • End Sub • Sub Sample2 (ByVal x as Single, • ByVal y as Single) • x = x - y • picOut x, y • End Sub • Sub cmdCompute • Dim a, b as Single • a = 2 • b = 3 • Sample2 (a, b) • picOut1.Print a, b • Sample1 (a, b) • picOut.Print a, b • End Sub Example What will be printed out? What if we called Sample2 (b, a) ?

  8. Nested IF statements Ifcondition1Then Ifcondition2Then statement1 End If Else statement2 End if ‘Tirgul’ # 3

  9. Example If Grade >= 90 Then txtLetterGrade.Text = “A” Else If Grade >=80 Then txtLetterGrade.Text = “B” Else txtLetterGrade.Text = “C” End If End If ‘Tirgul’ # 3

  10. 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

  11. If statement summary • VB is sensitive at design-time and run-time • Indentation! • Use if… Then • Use end if • Use elseif Sub Sample_if () If a = b then … End if End Sub Sub Sample_if2 () If a = b then … elseif … End if End Sub Sub Sample_if2 () If a = b then … else if a > b then … end if End if End Sub ‘Tirgul’ # 3

  12. Select Case Structure • Multiple IF statements can be replaced by more readable Select CASE statements SelectCaseselector CasevalueList1 action1 CasevalueList2 action 2 … CaseElse if no other match EndSelect • valueList options: Case 1 Case 2 to 5 Case 6, 9 Case “text” CaseIs >= 10 ‘Tirgul’ # 3

  13. 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

  14. Case statement summary • Use String/Integer type • When using String pay attention to content! • Don’t forget default ‘Tirgul’ # 3

  15. Do / Loops • Format 1: • Do {While | Until} Condition • loop body • Loop • 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. Format 2: Do loop body Loop{While | Until} Condition ‘Tirgul’ # 3

  16. 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

  17. 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

  18. Danger Loops summary While true … index = index +1 Wend • For – next • While – wend • Do – while • Common mistakes • Stop condition • Increment • Exit loop Do For I = 1 To 1000 MyNum = Int(Rnd * 100) Select Case MyNum Case 7: Exit For Case 29: Exit Do Case 54: Exit Sub End Select Next I Loop ‘Tirgul’ # 3

  19. 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

  20. 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

  21. 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

  22. Subscripts • Recall – Array starts at 1 BUT: • Array of size 25 • subscripts may be constants, variables, or numeric expressions • Use Constants!!! Dim GradeArr(MAX_STUDENTS) as Single Dim GradeArr(0 to 29) as Single ‘Tirgul’ # 3

  23. 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

  24. 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

  25. Declaring Dynamic Array • An Array declaration at the module level – No memory location yet: • Memory allocation at sub/function level: Dim DBConnections() as Connection Public DBRecordSet() as RecordSet • Sub openConnection () • . . . • ReDim DBConnections(1 To MAX_CONNECTIONS) • End sub ‘Tirgul’ # 3

  26. Redim • Resizing an existing array • Procedure level allocation • Deallocate memory (free memory) • Array values are lost at Redim! • Use preserve to keep old values • Redim array(0) ReDim preserve newArray(MAX_ARRAY_SIZE + 1) ‘Tirgul’ # 3

  27. 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

  28. Split - Join dim wordsArr()as String dim Sentence as string Sentence = “good morning” wordsArr = Split(Sentence,““ ) • Split – Namely split a String to the array • Join – Namely create a String from array wordsArr 1 Good Morning 2 Sentence Sentence = join(wordsArr,““) “good morning” Delimiter ‘Tirgul’ # 3

  29. 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

  30. Examples • Dim iArray (1 to 2, 1 to 5) as Integer • array of 2 rows and 5 columns Row Col ‘Tirgul’ # 3

  31. 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

  32. 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

  33. Code sample for Combo Box • ListIndex represents the selected Index • Text represents the item String value • Remove selected item • Add items to list • Debug.print cmbDays.text • cmbDays.RemoveItem cmbDays.ListIndex • cmbDays.AddItem “Sunday” ‘Tirgul’ # 3

  34. 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

  35. Controls array • In a from, you can group elements • Tab, Option button… • Useful when having lots of controls • Notation: • Design time • Copy/Paste in the form • setting index • Option(0), Option(1) ‘Tirgul’ # 3

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