VB Functions and Subroutines

1
VB Functions and Subroutines

• VB makes a distinction between functions and
  subroutines
• Functions return a single value, subroutines do
  not return any value
• Functions may be used within an expression,
  subroutines cannot
• Either may have zero to many parameters

2
Subroutines

• FormatSub name( ByVal ByRef var1 as Type1
  , ) statement(s)End Sub
• ByVal passes a copy of a value to the subroutine
  - the value is unchanged when the subroutine
  finishes

3
Subroutines (Cont'd)

• ByRef passes a reference to a value to the
  subroutine - the value can be changed by the
  subroutine
• Class level variables can be accessed directly
  and need not be passed as parameters - ex event
  procedures can directly access all elements on a
  form

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Example Subroutine

• Takes an error message as a value parameter and
  prints itSub printError( ByVal err As String )
  MsgBox( "Error " err )End Sub

5
Example Subroutine

• Takes an error message as a value parameter and a
  string as a reference parameter and changes the
  latterSub setError( ByVal err As String, ByRef
  str As String ) str "Error " errEnd Sub

6
Example Subroutine

• Sets a label to a calculated valuePrivate Sub
  btnArea_Click( ) Handles btnArea.Click
  lblArea.Text CStr( pi txtRadius2 )End
  Sub
• The label and text box are class-level variables

7
Example Subroutine

• Sets an area value by referenceSub setArea(
  ByVal dblRadius As Double, ByRef dblArea As
  Double ) dblArea pi dblRadius2End Sub

8
Example Subroutine Calls

• Calls printErrorprintError( "Value lt 0" )
• Calls setErrorsetError( strError )(argument
  must be a variable)
• Calls setAreasetArea( dblRadius, dblArea )

9
Functions

• FormatFunction name ( ByVal var1 as Type1 ,
  ) As DataType statement(s) Return
  expressionEnd Function
• expression must be of DataType
• parameters are local to the function

10
Functions (Cont'd)

• Functions can have parameters passed by
  reference, but this is bad programming practice
  as function should return a single value rather
  than change values

11
Example Function

• Takes an error message as a value parameter and
  returns a stringFunction returnError( ByVal err
  As String ) As String Return "Error "
  errEnd Function

12
Example Function

• Returns an area given a radiusFunction
  circleArea( ByVal dblR As Double ) As Double
  Return pi dblR2End Function

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Example Function Calls

• Calls returnErrorstrNewError returnError(
  "Value lt 0" )
• Calls circleAreadblArea circleArea( dblRadius
  )