Visual Basic 'Net AAPP00832 VBN

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Visual Basic .NetAAPP00-8-3-2 VBN
Sub Procedures Lecture 8
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Topic Structure of the lesson

• Introduction to Modular Design Concepts
• Write Sub Procedures
• Compare between Call by Value and Call by
  Reference

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Learning Outcomes

• At the end of this lecture you should be able to
  
• Declare a sub program
• Write a sub program and pass arguments to it
• Write a Call-By-Value and a Call By Reference sub
  procedure

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Key Terms you must be able to use

• If you have mastered this topic, you should be
  able to use the following terms correctly in your
  assignments and tests
• Call By Ref
• Call By Val

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Modular Design Concepts
Modularity refers to breaking a large problem
down into smaller self-contained modules. This
allows the programmer to isolate problems within
the program by looking at specific
areas. Breaking problems down into smaller
modules, will decrease program maintenance cost.
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Dividing code into procedures

• Divide Conquer Approach
• Makes program development more manageable.
• Software Reusability
• Using existing procedures as building blocks
  for new programs
• Avoid duplicating parts of programs

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Different Modular Techniques
Visual Basic provides us with the ability to
represent modules through the use of the
following

• Sub Procedures
• Function Procedures
• Event Procedures

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Sub Procedures
What are Sub Procedures?
Sub Procedures are subprograms or routines which
are self contained and are used to perform a
particular task.
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Breaking down a problem into smaller parts

• Private Sub btnDrawSnowman_Click()
• Dim small, medium, large, left, right as
  integer
• small 1
• medium 2
• large 3
• call drawCircle(small)
• call drawCircle(medium)
• call drawCircle(large)

call drawHat() left 1 right 2 call
drawArm(left) call drawArm(right) End Sub
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A Sub Program with No Arguments
MAIN PROGRAM Private Sub Command1_Click( )
Check the parentheses. There is no argument
Call No_arg_No_return( ) End Sub
No arguments
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No Arguments
SUB PROCEDURE Public Sub No_arg_No_return( )
Dim x, y, z As Integer x 200 y 400 z
x y Label1.Caption "TOTAL " z End
Sub
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OUTPUT
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A Sub Procedure with Arguments

• Private Sub cmdAdd_click()
• Call max(2,1)
• End sub
• Private Sub max(num1, num2 as integer)
• Dim max as integer
• if num1 gt num2 then
• max num1
• elseif num2 gt num1 then
• max num2
• Else
• MessageBox.Show (The numbers are equal)
• End If
• End Sub

parameters
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Exercise 1
Write a program to accept two numbers using
textboxes and pass the numbers to a subprogram
called product to multiply them. Display the
answer in the product procedure. Use a
MessageBox to display the answer
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Answer

• Private Sub cmdMultiply_Click()
• Dim a,b as integer
• a InputBox(enter 1st number)
• b InputBox(enter 2nd number)
• Call product (a, b)
• End Sub
• Private Sub product (num1 As integer, num2 As
  integer)
• MsgBox ("The product of num1 and
  num2 is
  num1 num2)
• End Sub

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Exercise 2

• Write a program that will accept three numbers
  and pass the numbers to a procedure minimum to
  determine the smallest number.
• Accept the numbers using an InputBox and display
  the smallest number in a Text Box.

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Solution

• Private Sub cmdSmallest_Click()
• Dim value1 As Long, value2 As Long, value3 As
  Long
• value1 txtOne.Text
• value2 txtTwo.Text
• value3 txtThree.Text
• Call Minimum(value1, value2, value3)
• End Sub
• Private Sub Minimum(x As Long, y As Long, z As
  Long)
• dim min as Long
• min x
• If y lt min Then
• min y
• End If
• If z lt min Then
• min z
• End If
• lblSmallest.Caption "Smallest value is "
  min
• End Sub

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EXIT Sub

• EXIT SUB will alter the flow of control in your
  program
• Executing EXIT SUB in a sub procedure causes an
  immediate exit from that procedure.

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EXIT SUB Example
Private Sub cmdBegin_Click() Dim x As Integer For
x 5 To -1 Step -1 Call PrintNumbers(x) Nex
t cmdBegin.Enabled False End Sub
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EXIT SUB Example

• Private Sub PrintNumbers(number As Integer)
• If number gt0 Then
• Print number
• Else
• Print "Exiting Sub with number "
  number
• Exit Sub
• End If
• End Sub

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Call By Reference

• If a variable retains the value it is given in a
  sub procedure, it is passed by reference
• By default Call By Reference is used in VB (FYI
  This is not true in C and Java.)

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Call By Reference - Procedure

• Private sub a()
• dim a as integer
• a 5
• call square(a)
• MessageBox.Show (Ans a a)
• end sub
• private sub square(num as integer)
• num num num
• end sub
• Ans a 25

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Call By Value

• When you pass a copy of the variable to a sub
  program - it does not affect the value in the
  main program

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Call By Value

• Private sub a()
• dim a as integer
• a 5
• call square(a)
• MessageBox.Show (Ans a a)
• end sub
• private sub square (byval num as integer)
• num num num
• end sub
• Ans a 5

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Exercise 3

• Write a program that accepts a student mark and
  passes the mark to a subprogram called grade
  which will determine the students grade.
• Mark Grade
• 75-100 A
• 60-74 B
• 50- 59 C
• 40-49 D
• 0 39 F

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Exercise 4

• Write a program that uses a Sub procedure Maximum
  to determine the largest of three Integers. The
  smallest value is displayed in a Label.

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Summary of Main Teaching Points

• Module Design
• Sub Procedures
• Function Procedures
• Event Procedures
• Sub Procedures
• Do not return a value
• Call By Value
• Call By Reference

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Next Session

• Function Procedures

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Question and Answer Session
Q A