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Chapter 6

Chapter 6. Sub Procedures And Functions. Chapter 6 Introduction. A procedure is a collection of statements that performs a task. Chapter 6 Topics. A Sub procedure is a collection of statements that performs a task An abbreviation of the older term subroutine

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Chapter 6

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  1. Chapter 6 Sub Procedures And Functions

  2. Chapter 6Introduction A procedure is a collection of statements that performs a task.

  3. Chapter 6 Topics • A Sub procedure is a collection of statements that performs a task • An abbreviation of the older term subroutine • Event procedures are Sub procedures • A Function procedure is a collection of statements that performs a task and returns a value to the VB statement that executed it • Function procedures work like intrinsic functions, such as Val and IsNumeric • A method is a procedure declared in a class

  4. Section 6.1Sub Procedures You Can Write Your Own General Purpose Sub Procedures That Perform Specific Tasks General Purpose Sub Procedures Are Not Triggered by Events but Called From Statements in Other Procedures

  5. Sub Procedure Uses • May handle events such as a click event • Also used to simplify a program by • Breaking it into small, manageable pieces or • Performing a task that is needed repeatedly • Sub procedures help to modularize code • Divides a program into a set of logical tasks

  6. Sample Sub Procedure, Tutorial 6-1 Private Sub btnGo_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnGo.Click ' This procedure calls the DisplayMessage procedure. lstOutput.Items.Add("Hello from btnGo_Click procedure.") lstOutput.Items.Add("Calling the DisplayMessage " & _ "procedure.") DisplayMessage() lstOutput.Items.Add("Now I am back in the btnGo_Click procedure.") End Sub Returns to btnGo_Click Calls DisplayMessage procedure Sub DisplayMessage() 'A Sub procedure that displays a message. lstOutput.Items.Add("") lstOutput.Items.Add("Hello from DisplayMessage.") lstOutput.Items.Add("") End Sub

  7. Declaring a Sub Procedure • AccessSpecifier is optional and establishes accessibility to the program • Sub and End are keywords • ProcedureName used to refer to procedure • Use Pascal casing, capitalize 1st character of the name and each new word in the name • ParameterList is a list of variables or values being passed to the sub procedure [AccessSpecifier] Sub ProcedureName ([ParameterList]) [Statement(s)] End Sub

  8. More on Access Specifier • Private allows use only from that form or class • Public allows use from other forms or classes • If not specified, default is Public • Additional access specifiers: • Protected • Friend • Protected Friend • These will be discussed in later chapters • Access specifiers won’t be used for now • Practice writing procedures in Tutorial 6-2

  9. Procedures and Static Variables • Variables needed only in a Sub procedure, should be declared within the Sub procedure • Creates a local variable with scope only within the sub procedure where declared • Local variable values are not saved from one sub procedure call to the next • To save value between procedure calls, use Static keyword to create a static local variable • Static VariableName As DataType • Scope is only within the procedure • But variable exists for lifetime of procedure

  10. Section 6.2Passing Arguments to a Sub Procedure When calling a procedure, you can pass it values known as arguments

  11. Arguments • Argument – a value passed to a procedure • We’ve already done this with functions • Value = CInt(txtInput.Text) • Calls CInt function and passes txtInput.Text • A Sub must be declared so it accepts an argument

  12. Passing Arguments By Value DisplayValue(5) ‘calls DisplayValue procedure Sub DisplayValue(ByVal number As Integer) ' This procedure displays a value in a message box. MessageBox.Show(number.ToString) End Sub • Number declared as an integer argument • Storage location number created • A value, 5 in this case, must be supplied and is copied into the storage location for number • DisplayValue then executes • Tutorial 6-3 demonstrates passing arguments

  13. Passing Multiple Arguments • Multiple arguments separated by commas • Value of first argument is copied to first • Second to second, etc. ShowSum(5, 10) ‘calls ShowSum procedure Sub ShowSum(ByVal num1 As Integer, ByVal num2 As Integer) ' This procedure accepts two arguments, and prints ' their sum on the form. Dim sum As Integer sum = num1 + num2 MessageBox.Show("The sum is " & sum.ToString) End Sub

  14. Passing Arguments ByVal or ByRef • Arguments are usually passed ByVal • New storage location created for procedure • Storage location gets a copy of the value • Any changes in value are made to the copy • Calling procedure won’t “see” the changes • Arguments can also be passed ByRef • Procedure points to (references) argument’s original storage location • Any changes are made to the original value • Calling procedure “sees” the changes • Tutorial 6-4 demonstrates this difference

  15. Section 6.3Function Procedures A Function Procedure Returns a Value to the Part of the Program That Called the Function Procedure

  16. Declaring a Function Procedure • New keyword Function • Also new is As DataType which states the data type of the value to be returned • Return value is specified in a Return expression [AccessSpecifier] Function FunctionName ([ParameterList]) _ As DataType [Statements] End Function

  17. Function Call Example • value1 and value2 must be data type Single • Total must be declared as data type Single • Tutorial 6-5 demonstrates function use total = Sum(value1, value2) Function Sum(ByVal num1 As Single, ByVal num2 As Single) _ As Single Dim result As Single result = num1 + num2 Return result End Function

  18. Returning Non-Numeric Values Function FullName(ByVal first As String, ByVal last As String)_ As String Dim name As String name = last & ", " & first Return name End Function Function IsValid(num As Integer) As Boolean Dim status As Boolean If num >= 0 And num <= 100 Then status = True Else status = False End If Return status End Function

  19. Section 6.4More About Debugging Step Into Step Over Step Out

  20. Debugging Involving Procedures • Step Into - continue to debug by single-stepping through a procedure • Step Over - run procedure without single-stepping, continue single-step after the call • Step Out - end single-stepping in procedure, continue single-step after the call • Tutorial 6-6 provides examples

  21. Section 6.5Building the Bagel and Coffee Price Calculator Application Use Sub procedures and functions to calculate the total of a customer order.

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