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Subroutines and Functions

Chapter 6 in Deitel, Deitel and Nieto

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Resolution

Two definitions of “resolution” A solution … of a problem The act … of separating into constituent or

elementary parts (Webster’s New Universal Unabridged Dictionary)

One of the primary techniques for solving complex problems is “divide and conquer” Break the problem into manageable pieces Solve the pieces Reassemble the pieces into a complete solution

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Modules

Modules are one of the “constituent parts” into which a programmer breaks Visual Basic code

Another “unit” of programming is the object

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Divide and Conquer Example

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Divide and Conquer Example

Option Explicit

Dim EmployeeName As StringDim Hours As IntegerDim Wage As DoubleDim Salary As Double

Private Sub cmdCalculate_Click() GetInfo CalculateWeeklySalary PrintResultEnd Sub

Problem broken into three modules

They’re getting rid of the currency type so use double

These are “calls”

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The GetInfo Module

Private Sub GetInfo()

EmployeeName = txtFirstName.Text & _ " " & txtLastName.Text

GetHours

GetWage

End Sub

This module is further broken down into more modules

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GetHours Module

Private Sub GetHours() If IsNumeric(txtHours.Text) Then Hours = CInt(txtHours.Text) Else MsgBox ("Please enter a number (e.g. 34)” _

& “for the Hours.") txtHours.Text = "" txtHours.SetFocus End IfEnd Sub

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GetWage Module

Private Sub GetWage() If IsNumeric(txtWage.Text) Then Wage = CDbl(txtWage.Text) Else MsgBox ("Please enter a number “ _

& “ (e.g. 7.95) for the Wage.") txtWage.Text = "" txtWage.SetFocus End IfEnd Sub

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CalculateWeeklySalary Module

Private Sub CalculateWeeklySalary()

If Hours > 40 Then

Salary = (Hours - 40) * 1.5 * Wage + _

40 * Wage

Else

Salary = Hours * Wage

End If

End Sub

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PrintResult Module

Private Sub PrintResult()

lblSalary.Caption = EmployeeName & _ " earned "

lblSalary.Caption = lblSalary.Caption & _ Format$(Salary, "currency")

lblSalary.Caption = lblSalary.Caption & _

" the week of " & Date & "."

End Sub

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Code maintenance

Modularization makes the code easier to maintain If the way the data is obtained changes, we

need only change the GetInfo module If we alter the overtime rules, we need only

change the CalculateWeeklySalary module If we decide to cut an actual check, we

need only change the PrintResult module

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Reduce Repeated Code

Another possible benefit of modules is the reduction of repeated code

A given module can be called more than once from more than one location in the code

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Select a Color Revisited

Recall we must change the form’s backcolor property as well as the backcolor property of all the optionbuttons, and we must do that in click method of each of the optionbuttons

To prevent a lot of repetition, we will use a subroutine

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Add Procedure

Does not return anything so sub

Used only by this form so private

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Add Procedure Result

Or just type this; actually VB supplies the End Sub automatically

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Calling Subroutines

Private Sub optBlue_Click() Color = vbBlue Call ColorFormEnd Sub

Private Sub optGreen_Click() Color = vbGreen ColorFormEnd Sub

Call ColorForm subroutine using keyword Call

Call ColorForm subroutine without keyword Call

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Subroutine ColorForm

Private Sub ColorForm() frmSelectColor.BackColor = Color optRed.BackColor = Color optBlue.BackColor = Color optGreen.BackColor = Color optYellow.BackColor = Color optCyan.BackColor = Color optMagenta.BackColor = ColorEnd Sub

Open parenthesis; close parenthesis

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Scope

If a variable is declared at the top of the module, it is referred to as global and is available to all of the modules belonging to the form

If a variable is declared within a module, it is referred to as local and is available only within that module

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What’s the difference?

Global variables should be fundamental to the problem and needed by several modules

Local variables are those that are needed in one or two modules only

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An Example

For example the i in For i=1 To n

is only needed within the for loop within one module, so it should be declared locallyThis way i cannot be confused with other counters in the problem (even if they are also called i) Duplicate variable names can be a big problem in longer programs, proper use of scope limits the difficulty

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Passing a Parameter

To get local information from one module to another, one “passes” the information

The information that is passed is placed in the parentheses

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Passing a Parameter

Option Explicit

Private Sub Form_Load() Call optRed_ClickEnd Sub

Private Sub optBlue_Click() Call ColorForm(vbBlue)End Sub

Look Ma, no global variables

Passing a parameter

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Local ColorForm

Private Sub ColorForm(Color As Long) frmSelectColor.BackColor = Color optRed.BackColor = Color optBlue.BackColor = Color optGreen.BackColor = Color optYellow.BackColor = Color optCyan.BackColor = Color optMagenta.BackColor = ColorEnd Sub

Passed variable and its type

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What has been gained?

First, the variable color is now local to ColorForm meaning that the variable color can be used elsewhwere in the program without problem

Second, the variable color (which corresponds to a memory location) exits only for the duration of ColorForm, so memory is freed up

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Information Hiding

“The process of hiding details of an object or function. Information hiding is a powerful programming technique because it reduces complexity. …. The programmer can then focus on the new object without worrying about the hidden details.”“Information hiding is also used to prevent programmers from changing --- intentionally or unintentionally -- parts of a program.” (http://www.webopedia.com)

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Multiple programmers

Most code is written by teams of codersOne should be able to use a module without detailed knowledge of how it works (its implementation) If a module uses global variables, then someone using module must declare these variables And if two modules use the same global variables, there can be conflicts

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To and Fro

We have seen how to get local information to a module, now we must consider how to get it back

VB distinguishes between subroutines and functions; the difference is that function return a value (send back some information) to whatever modules called it

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Weekly Salary Revisited

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New cmdCalculate_Click

Option Explicit

Private Sub cmdCalculate_Click() Dim EmployeeName As String Dim Hours As Integer Dim Wage As Double Dim Salary As Double

All variables local now

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New cmdCalculate_Click (Cont.)

EmployeeName = GetName()

Hours = GetHours()

Debug.Print Hours

Wage = GetWage()

Salary = CalculateWeeklySalary(Hours, Wage)

Call PrintResult(EmployeeName, Salary)

End Sub

Three functions return name, hours and wage respectively, note they are part of assignment statement

CalculateWeeklySalary now a function

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GetName Function

Private Function GetName() As String

GetName = txtFirstName.Text & " " _

& txtLastName.Text

End Function

Type of thing that gets returned

Whatever you want returned assign to the function’s name

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GetHours Function

Private Function GetHours() As Integer If IsNumeric(txtHours.Text) Then GetHours = CInt(txtHours.Text) Else MsgBox ("There was an error in the hours.") txtHours.Text = "" txtHours.SetFocus GetHours = 0 End IfEnd Function

Need to return something even if there was a mistake, better to test on validate method

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GetWage Function

Private Function GetWage() As Double If IsNumeric(txtWage.Text) Then GetWage = CDbl(txtWage.Text) Else MsgBox ("There was an error in the Wage.") txtWage.Text = "" txtWage.SetFocus GetWage = 0 End IfEnd Function

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CalculateWeeklySalary Function

Private Function CalculateWeeklySalary(Hours As Integer, _ Wage As Double) As Double

If Hours > 40 Then CalculateWeeklySalary = (Hours - 40) * _

1.5 * Wage + 40 * Wage Else CalculateWeeklySalary = Hours * Wage End IfEnd Function

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PrintResult Subroutine

Private Sub PrintResult(EmployeeName As _ String, Salary As Double)

lblSalary.Caption = EmployeeName & _ " earned "

lblSalary.Caption = lblSalary.Caption & _ Format$(Salary, "currency")

lblSalary.Caption = lblSalary.Caption & _ " the week of " & Date & "."

End Sub

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VB Functions

VB has some built in functions such an Abs(x) - return the absolute value of xExp(x) - return the exponential of x Int(x) - return the integer part of xSgn(x) - return the sign of the number xRnd() - return a “pseudo-random”

number between 0 and 1

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Random Numbers

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Random Numbers

Option Explicit

Private Sub cmdRandom_Click() txtRandomSingle.Text = Rnd()End Sub

Private Sub cmdRandomInteger_Click() txtRandomInteger.Text = Int(Rnd() * 10 + 1)End Sub

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Random numbers

Private Sub Command1_Click()

txtRandomAB.Text = Int(Rnd() * (txtB.Text - txtA.Text + 1) + txtA.Text)

End Sub