Programming Structures

1
Programming Structures

• Sequence - program instructions are processed,
  one after another, in the order in which they
  appear in the program
• Selection - allows the program to make a
  decision/comparison and then select the
  appropriate action to take based on that
  decision/comparison
• Repetition - (looping or iteration) tells the
  computer to repeat one or more instructions
  either a specified number of times or until some
  condition is met

2
Forms of the Repetition Structure

• For Next
• Do While
• Do Until

3
For Next Loop

• Tells the computer to repeat one or more
  statements a specified number of times. Called a
  pretest loop because the loop is evaluated before
  the instructions are processed
• Syntax
• For counter startvalue To endvalue Step
  stepvalue
• instructions you want repeated
• Next counter
• counter is the name of a numeric variable and it
  keeps track of how many times the loop
  instructions are repeated
• startvalue, endvalue, and stepvalue must be
  numeric and they can be either positive or
  negative, integer or non-integer (default
  stepvalue is 1)

4
For Next Loop Tasks

• The loop initializes the counter to the
  startvalue (done only once, at the beginning of
  the loop)
• If the stepvalue is positive (negative), the loop
  checks if the value in counter is greater than
  (less than) the endvalue. If it is, the loop
  stops otherwise the instructions within the loop
  are processed and the next task is performed
• The loop adds the stepvalue to the counter. It
  then repeats the prior two steps

5
Flowchart and Pseudocode

• Hexagon

• Repeat for intCount 1 to 3 by 1
• Print intCount
• Next intCount

intCount
gt 3
1
1
Print intCount
6
Do While and Do Until Loops

• Do While loop repeats a block of instructions
  while a condition is true
• You also can use a Do While loop to repeat a
  block of instructions a specified number of times

• Do Until loop repeats a block of instructions
  until a condition becomes true
• You also can use a Do Until loop to repeat a
  block of instructions a specified number of times

7
Syntax of the Do While and Do Until Loops

• Do While condition
• loop instructions
• Loop

• Do
• loop instructions
• Loop Until condition

• condition must evaluate to true or false
• condition can contain variables, constants,
  properties, functions, mathematical operators,
  relational operators, and logical operators

8
Do While and Do Until Loops

• In the Do While loop, the instructions are
  processed only when the condition evaluates to
  true the loop stops when the condition evaluates
  to false
• Called a pretest loop

• In the Do Until loop, the instructions are
  processed only when the condition evaluates to
  false the loop stops when the condition
  evaluates to true
• Called a posttest loop

9
Do While Loop
intCount 1

• intCount 1
• Repeat while intCount lt 3
• Display intCount
• Add 1 to intCount
• End Repeat while intCount lt 3

intCount lt 3
F
T
Display intCount
intCount intCount 1
10
Do Until Loop
intCount 1

• intCount 1
• Repeat
• Display intCount
• Add 1 to intCount
• End Repeat until intCount gt 3

Display intCount
intCount intCount 1
intCount gt 3
T
F
11
Counters and Accumulators

• Used within a repetition structure to calculate
  subtotals, totals, and averages
• Initialized (usually to 0 or 1) outside the loop
  and updated within the loop
• A counter is a numeric variable used for counting
  something and is typically updated by 1
• An accumulator is a numeric variable used for
  accumulating (adding together) and is updated by
  an amount that varies

12
Calculating an Average

• Average accumulator / counter
• Be sure to verify that counter is greater than 0
  before calculating the average
• Use the IfThenElse selection structure

13
Control Arrays

• Group of controls of the same type that have the
  same name and share the same set of event
  procedures
• Each control in the array is identified by the
  arrays name and a unique index
• The first control in the array has an index of 0

14
Creating a Control Array

• To make existing controls into an array, simply
  assign the same name to each control. When you
  are asked if you want to create a control array,
  click the Yes button
• If the controls are not already on the form,
  place the first control on the form and set its
  name and other properties
• Copy the control to the clipboard, then paste the
  appropriate number of controls on the form
• When you are asked if you want to create a
  control array, click the Yes button

15
Control Array Code Window

• Index As Integer appears in the Code windows for
  a control array
• All controls in the array share the same set of
  Code windows
• Index contains an integer that represents the
  value stored in the Index property of the control
  receiving the event

16
Index Property vs Index Argument

• The Index property stores a number that
  identifies each control in the array
• Each control in the array has a value in its
  Index property
• The Index property is like an address

• The Index argument found in the Code window is a
  local variable created by Visual Basic
• The Index argument contains the address of the
  control receiving the event

17
Parallel Arrays

• Arrays whose contents are related by their
  position in the arrays--in other words, by their
  index
• chkDone(0) txtScore(0)
• chkDone(1) txtScore(1)

18
Enabled Property

• An objects Enabled property determines whether
  the object can respond to user-generated events,
  such as clicking or entering information
• Can be set to True (default) or False
• It is customary in Windows applications to
  disable objects that dont apply to the current
  state of the application

19
GotFocus Event

• Occurs when a control receives the focus
• It is customary in Windows applications to
  highlight a text boxs existing text when the
  text box receives the focus. You do so by setting
  the text boxs SelStart and SelLength properties

20
Change Event

• Occurs when the contents of an object are changed
• You can use this event to clear the results of
  calculations from label controls when the
  contents of a text box has changed

21
Alignment Property

• You can use the Alignment property to align the
  contents of a label control
• You also can use the Alignment property to align
  the contents of a text box, but you must be sure
  to set the text boxs MultiLine property to True
  and also code its KeyPress event to prevent the
  text box from recognizing the Enter key

22
MsgBox Function

• Displays one of Visual Basics predefined dialog
  boxes, which contains a message, one or more
  command buttons, and an icon
• After displaying the dialog box, the MsgBox
  function waits for the user to choose a button,
  then returns a value that indicates which button
  the user selected

23
MsgBox Function

• Syntax
• MsgBox(prompt, buttons, title,helpfile,
  context)
• prompt is the message you want displayed
• buttons is a numeric expression that specifies
  the number and type of buttons, the icon, the
  default button, and the modality
• title is a string expression displayed in the
  title bar

24
Modality

• Application modal
• Default modality user must respond to the dialog
  boxs message before he or she can continue
  working in the current application the user
  still can access other applications
• System modal
• All applications are suspended until the user
  responds to the dialog boxs message

25
buttons Argument

• Constant Value Description
• vbOKOnly 0 Display OK button only.
• vbOKCancel 1 Display OK and Cancel buttons.
• vbAbortRetryIgnore 2 Display Abort, Retry, and
  Ignore buttons.
• vbYesNoCancel 3 Display Yes, No, and Cancel
  buttons.
• vbYesNo 4 Display Yes and No buttons.
• vbRetryCancel 5 Display Retry and Cancel
  buttons.
• vbCritical 16 Display Critical Message icon.
• vbQuestion 32 Display Warning Query icon.
• vbExclamation 48 Display Warning Message icon.
• vbInformation 64 Display Information Message
  icon.
• vbDefaultButton1 0 First button is default.
• vbDefaultButton2 256 Second button is default.
• vbDefaultButton3 512 Third button is default.
• vbDefaultButton4 768 Fourth button is default.
• vbApplicationModal 0 Application modal the user
  must respond to the message box
• before continuing work in the current
  application.
• vbSystemModal 4096 System modal all applications
  are suspended until the user
• responds to the message box.

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Return Values

• Constant Value Description
• vbOK 1 OK
• vbCancel 2 Cancel
• vbAbort 3 Abort
• vbRetry 4 Retry
• vbIgnore 5 Ignore
• vbYes 6 Yes
• vbNo 7 No

27
Unload Event

• Occurs when a form is about to be removed from
  the screen and memory
• Triggered when you use the Unload statement in
  code or when you click the Close button to close
  the form
• Use the Unload event to verify that the user
  wants to exit an application and also for code
  that saves and closes files

28
Unload Statement vs Unload Event

• The Unload statement tells Visual Basic to unload
  the form
• The Unload event is the procedure that occurs
  before the form is unloaded

29
Unload Event Code Window

• The Cancel argument is a local variable created
  by Visual Basic and it contains the value 0
• To prevent the form from being unloaded, set
  Cancel to a value other than 0

30
Debugging Technique

• You can use the KeyPress event to control what
  the user can enter into a text box