Visual Basic VB: Procedures

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Visual Basic (VB)Procedures
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Procedures in VB

• VB provides two types of procedure
• Sub Procedures.
• Function Procedures.

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Sub Procedures

• Weve already met Sub procedures when we were
  handling events that were happening to form
  objects.
• Private Sub Button1_Click()
• stuff(a)
• End Sub
• However, you can write Subs that dont handle
  events.
• Sub stuff(ByVal i As Integer)
• Debug.WriteLine(Str(i))
• End Sub

Wheres the Dim? Wheres i declared?
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Cont. Sub Procedures

• Sub Definition
• Private Sub Method1(parameter list)
• body
• End Sub
• Sub call
• Method1(argument list)

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Function Procedures

• Function Procedures are different though
• They return a value
• Can be called anywhere an expression is used
• For example, on the right hand side of an
  assignment statement
• Weve seen some built-in ones before, eg CStr

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Cont. Function Procedures 1
Calling a Function Dim a As Integer Dim b As
Integer Dim c As Integer c add(a, b)
Function Definition Function add(ByVal i As
Integer, ByVal j As Integer) As Integer
add i j End Function
If a1 and b2, what does c become?
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Cont. Function Procedures 2

• Function definition
• Private Function Method1(parameter list)
  Return type
• body
• End Function
• Function call
• type x Method1(argument list)

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Sub V Function Procedures

• Functions
• Return a value before exiting the Function.
• This value must match the Function return type.
• Subs
• Do not return a value.
• Therefore, cannot be used in assignment
  statements.

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Why Using Procedures

• Both Subs and Functions useful to provide
  reusable chunks of code
• Can be called in different contexts
• Or even reused in different applications
• Reuse is important to RAD.
• If you ever write essentially the same piece of
  code twice, think of using a procedure.

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ByVal versus ByRef

• In VB, arguments in procedures can be passed by
  value or by reference.
• When passing by value, copies of the arguments
  are passed.
• When passing by reference, the address of
  arguments are passed.

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Swap Example Without Using a Procedure

• Dim a As Integer 1
• Dim b As Integer 2
• Dim temp As Integer
• temp a
• a b
• b temp

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Swap Example using a Procedure (call by Value)

• Dim a As Integer 1
• Dim b As Integer 2
• Private Sub swap(ByVal x As Integer, ByVal y As
  Integer)
• Dim temp As Integer
• temp x
• x y
• y temp
• End Sub
• .
• .
• Swap(a, b) a ? , b ?

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Swap Example using a Procedure (call by
Reference)

• Dim a As Integer 1
• Dim b As Integer 2
• Private Sub swap(ByRef x As Integer, ByRef y As
  Integer)
• Dim temp As Integer
• temp x
• x y
• y temp
• End Sub
• .
• .
• Swap(a, b) a ? , b ?

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The scope of a variable

• defines which parts of the code are aware of
  its existence.
• When we declare a variable within a procedure,
  only the code in that procedure can access or
  change its value
• The variable has a scope that is local to that
  procedure.
• Storage reserved on entry and destroyed at exit
  from procedure
• But sometimes we need to use a variable with a
  broader scope
• eg, available to all the procedures within the
  same module, or even to all the procedures in the
  application.
• VB allows you to specify the scope of a variable
  when you declare it.
• But how?

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Dim, Public and Private

• Dim
• Declares variables and allocates them storage
  space.
• Variables at the module level available to all
  procedures within the module.
• At the procedure level, available only within the
  procedure.
• Public
• Used at the module level to declare public
  variables and allocate storage space.
• Variables available to all procedures in all
  modules.
• Private
• Used at the module level to declare private
  variables and allocate storage space.
• Private variables are available only to the
  module in which they are declared.
• There are other related keywords, eg Protected,
  Friend, Protected Friend, Static that you may
  come across and can explore for yourself.

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Procedure Visibility

• As well as being able to specify the visibility
  of variables, we can do the same with procedures
• Public Sub/Function.
• Indicates that the Function or Sub procedure is
  accessible to all other procedures in all
  modules.
• Private Sub/Function.
• Indicates that the Function or Sub procedure is
  accessible only to other procedures in the module
  where it is declared.

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Things to Do

• In the meantime, your coursework should be well
  underway!
• Remember, you should be working outside of
  timetabled classes.
• Also remember, developing away from the labs can
  be problematic when we assess your work.
• Also, read and do the exercises in chapters 5, 6,
  10 and 11 of Bell and Parrs Visual Basic .NET
  for students in the next week.
• Or Sections 1.3, 7.1, 7.2, 7.3, 7.4 and
  exercises of Introduction to Programming Using
  Visual Basic" by Gary Bronson
• You might also want to try the exercises given on
  the second tutorial sheet available from the
  module Web pages.