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CVEV 118/698 Visual Basic

Learn about control flow statements (If...Then...Else, Select Case) and loop statements (Do...Loop, For...Next, While...Wend) in Visual Basic programming language.

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CVEV 118/698 Visual Basic

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  1. CVEV 118/698 Visual Basic Lecture 2 Prof. Mounir Mabsout Elsa Sulukdjian Walid El Asmar

  2. Control Flow Statements • An important feature in programming languages is the ability to examine external conditions and act accordingly. • VB provides three control flow (or decision) statements: • If … Then … End If • If … Then … Else … End If (Elseif) • Select Case

  3. If/Then Statements • Example 1: If intAnswer = 1 Then strMood = “happy” End If • Example 2: If intAnswer = 1 Then strMood = “happy” Else strMood = “sad” End If • Example 3: If intAnswer = 1 Then strMood = “happy” Elseif intAnswer = 2 Then strMood = “sad” Else strMood = “don’t care” End If

  4. Select Case Statement • Efficient method for multi-conditional cases. • Select Case WeekDay(intDate) • Case 1 • strDayName = “Monday” • strMessage = “Have a nice week” • Case 6 • strDayName = “Saturday” • strMessage = “Have a nice weekend” • Case 7 • strDayName = “Sunday” • strMessage = “Working on Sunday!!??” • Case Else • strMessage = “Welcome back” • End Select Select Case expression Case Value_1 Statement block_1 Case Value_2 Statement block_2 … Case Value_n Statement block_n Case Else Statement block End Select

  5. Select Case (cont’d) • The Select Case structure compares the results of one expression to several values, and if it matches one of them, the corresponding block of statement is executed. • If more than one Case value matches the expression, only the statement block associated with the first matching Case executes. • Disadvantage: The Select Case evaluates the expression before entering the Case statements, while the If/Then/Else can evaluate a different expression for each ElseIf statement.

  6. Select Case (cont’d) • Example 2 Select Case WeekDay(intDate) Case 1, 2, 3, 4, 5 strDayType = “Workday” strMessage = “Enjoy Work” Case 6, 7 strDayType = “Holiday” strMessage = “Enjoy your weekend” Case Else strDayType = “VBday” strMessage = “Goto your assignment!” End Select

  7. Loop Statements • Loop Statements allow to execute code repetitively. • VB provides three loop statements: • Do … Loop • For … Next • While … Wend

  8. Do … Loop • Syntax 1: Do While condition statement block Loop • VB evaluates the condition, if it is False the block is never executed. If it is True, VB executes the statement block, re-evaluates the condition, and repeats the statement block if it is True. • Example 1: Do While dblGrade <= 80 dblGrade = dblGrade + 1 MsgBox “Work Harder!” Loop

  9. Do … Loop (cont’d) • Syntax 2: Do statement block Loop While condition • The main difference with syntax 1 is that the statement block is executed the first time independently of the condition. The evaluation starts before the second loop. • Example 2: Do dblGrade = dblGrade + 1 MsgBox “Work Harder!” Loop While dblGrade <= 80

  10. For … Next • The loop executes depending on a preset number of times not on a conditional statement. • It uses a variable (counter) that increases or decreases in value each time the statement is executed. • Syntax: For counter = start To end [Step increment] statements Next [counter]

  11. For … Next (cont’d) • Example 1: For intNum = 1 to 10 intSquareNum(intNum) = intNum * intNum Next intNum • Unless specified, the incremental step of the counter (I.e. intNum) is 1 by default. • Example 2: For intNum = 10 to 1 Step -2 intSquareNum(intNum) = intNum * intNum Next intNum

  12. While … Wend • While…Wend executes a block of statements while a condition is true. • Syntax: While condition statement block Wend • Why all this trouble? Just use the Do While Loop 

  13. Functions & Subroutines • VB code is not a monolithic listing. An application in VB is made up of small self contained segments that are “event handler”. • Difference between a function & a subroutine: • A Function executes a series of commands and returns a value. • A Subroutine can be considered a special function since it does not return anything.

  14. Subroutines • A subroutine is a block of statements that carry out a well-defined task. Sub MyFirstSub () …… …… End Sub • All the event procedures in VB are coded as subroutines e.g.: the “click” event of a button.

  15. Functions • A function returns a result, accordingly it must have a type: Function MyFirstFunction () As Double …… …… End Function

  16. Arguments • Arguments are values passed to a procedure (a function or subroutine) and on which the procedure acts. • Example: Function dblCircleArea (r As Double) As Double dblCircleArea = pi * r * r End Function To call the above function: dblCArea1 = dblCircleArea (dblRadius) (where dblCArea1 & dblRadius are previously defined variables of type double)

  17. Arguments (cont’d) • The number of arguments passed must be the same as the number of arguments defined in the function or subroutine. • Example: Function dblRectArea (a, b As Double) As Double dblRectArea = a * b End Function ‘Error will be generated in the following code: dblAreaWrong = dblRectArea(dblLength) ‘The right piece of code is: dblAreaRight = dblRectArea(dblLength,dblWidth)

  18. Optional Arguments • You may need a function to handle in some cases one argument; in other cases two or more. • Then you need to use the “Optional argument”: Function dblRectArea (a As Double,Optional b As Double) As Double dblRectArea = a * a If IsMissing(b) Then Exit Function dblRectAtea = a * b End Function Both calls for the above function are valid: RArea1 = dblRectArea(dblLength) RArea2 = dblRectArea(dblLength,dblWidth)

  19. Forms • A form is the “container” which includes all the “controls” that make up the user interface. • Forms have a built-in functionality that is always available without any coding from you (title bar, resize, etc.). • Properties of a form exist for you to customize its appearance. Some of these properties are: • MinButton, MaxButton • BorderStyle • ControlMenu

  20. Forms (cont’d) Title Bar Minimize Maximize Control Menu Close

  21. Basic Controls • Controls are another key factors in the “event-driven” programming. • VB has a multitude of controls, and many third party companies are developing more. • You can create your own control.

  22. Frames and Option Boxes • Rectangular entity inside a form that groups one or more controls. • Frames are used for esthetics reasons and/or for proper functioning of option boxes: inside each frame, only one option box can be ticked

  23. Frames and Option Boxes: Example

  24. TextBox Properties • Multiline: set to true if you want to display correctly multiple lines. • PasswordChar: set to a character that gets displayed to hide the real characters. • ToolTipText: Yellowish text box that explains about a control. • ScrollBars: Controls the attachment of scroll bars, if the text exceeds the control dimensions. • MaxLength: Maximum number of characters contained. • TabStop: Controls the access to the control with the tab key. • Enabled: Controls the access to the control

  25. What’s Next • Form manipulation • Menu design • Other controls

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