procedures and modular design · •avoid logical errors •efficiency •debugging. chapter 4 - vb...

Chapter 4 - VB 2005 by Schneider 1

Procedures and Modular Design

September 20, 2006

Today…

• On to Chapter 4

Friday…

• Assignment 2!

Chapter 4 - GeneralProcedures

• 4.1 Sub Procedures, Part I

• 4.2 Sub Procedures, Part II

• 4.3 Function Procedures

• 4.4 Modular Design

Procedures are an important part ofprogramming in general…

4.1 Sub Procedures, Part I

• Sub Procedures

• Variables and Expressions as Arguments

• Calling Other Sub Procedures

Devices for modularity

• Why do we want to split things up?• Readability

• Avoid logical errors

• Efficiency

• Debugging

Devices for modularity

• Visual Basic has two devices forbreaking problems into smaller pieces:• Sub procedures

• Function procedures

Sub Procedures

• Perform one or more related tasks

• General syntax

Sub ProcedureName()

statements

End Sub

Calling a Sub procedure

• The statement that invokes a Subprocedure is also referred to as a callstatement.

• A call statement looks like this:ProcedureName()

Naming Sub procedures

• The rules for naming Sub procedures arethe same as the rules for namingvariables.

• You can choose whatever you like……but ultimately, the best choice is astrong description of what the proceduredoes…• E.g. AddTwoNums()

Sub ExplainPurpose()

ExamplelstBox.Items.Clear()ExplainPurpose()lstBox.Items.Add("")

lstBox.Items.Add("Program displays a sentence")

lstBox.Items.Add("identifying a sum.")

End Sub

Passing Values• You can send values to a Sub procedure Sum(2, 3)

Sub Sum(ByVal num1 As Double, ByVal num2 As Double) lstBox.Items.Add("The sum of " & num1 & " and " _ & num2 & " is " & (num1 + num2) & "."End Sub

• In the Sum Sub procedure, 2 will be stored in num1 and 3will be stored in num2

Arguments and Parameters

• Sum(2, 3)

Sub Sum(ByVal num1 As Double, ByVal num2 As Double)

arguments

parameters

displayedautomatically

Several Calling StatementsExplainPurpose()Sum(2, 3)Sum(4, 6)Sum(7, 8)

Output:Program displays a sentence identifying a sum.The sum of 2 and 3 is 5.The sum of 4 and 6 is 10The sum of 7 and 8 is 15.

Passing Strings and Numbers

• Demo("CA", 34)

Sub Demo(ByVal state As String, ByVal pop As Double) txtBox,Text = state & " has population " & pop & _ " million."End Sub

• Note: The statement Demo(34, "CA") would not be valid.The types of the arguments must be in the same order asthe types of the parameters.

Variables and Expressions asArguments

Dim s As String = "CA"Dim p As Double = 17Demo(s, 2 * p)

Sub Demo(ByVal state As String, ByVal pop As Double) txtBox.Text = state & " has population " & pop & _ " million."End Sub

• Note: The variable names in the arguments need not matchthe parameter names. For instance, s versus state..

CallingA Sub procedure can call another Sub procedure.

Private Sub btnAdd_Click(...) Handles btnAdd.Click Sum(2, 3) End Sub

Sub Sum(ByVal num1 As Double, ByVal num2 As Double) DisplayPurpose() lstBox.Items.Add("The sum of " & num1 & " and " _ & num2 & " is " & (num1 + num2) & "."End Sub

4.2 Sub Procedures, Part II

• Passing by Value

• Passing by Reference

• Local Variables

• Class-Level Variables

• Debugging

ByVal and ByRef

• Parameters in Sub procedure headersare proceeded by ByVal or ByRef

• ByVal stands for By Value

• ByRef stands for By Reference

Passing by Value

• When a variable argument is passed to aByVal parameter, just the value of theargument is passed.

• After the Sub procedure terminates, thevariable has its original value.

Example

Dim n As Double = 4 Triple(n) txtBox.Text = CStr(n)End Sub

Sub Triple(ByVal num As Double) num = 3 * numEnd Sub

Output: 4

Same Example: n num

Dim num As Double = 4 Triple(num) txtBox.Text = CStr(num)End Sub

Sub Triple(ByVal num As Double) num = 3 * numEnd Sub

Output: 4

Passing by Reference

• When a variable argument is passed to aByRef parameter, the parameter is giventhe same memory location as theargument.

• After the Sub procedure terminates, thevariable has the value of the parameter.

ExamplePublic Sub btnOne_Click (...) Handles _ btnOne.Click Dim num As Double = 4 Triple(num) txtBox.Text = CStr(num)End Sub

Sub Triple(ByRef num As Double) num = 3 * numEnd Sub

Output: 12

Example: num nPrivate Sub btnOne_Click(...) Handles _ btnOne_Click Dim n As Double = 4 Triple(n) txtBox.Text = CStr(n)End Sub

Sub Triple(ByRef num As Double) num = 3 * numEnd Sub

Output: 12

ByVal and ByRef

Memory Location 1

ByVal Procedure Parameter

ByRef ProcedureParameter

Memory Location 116

This refers toanything stored in

this memory location

This refers to the value ofwhatever is in this

memory location…not thelocation itself

Any changes to this, donot effect what is storedin the memory location

1…it’s just a copy

Any changes to this doeffect what is stored in

the memory location 1…it’s not a copy

Local Variable

• A variable declared inside a Subprocedure with a Dim statement

• Space is reserved in memory for thatvariable until the End Sub…• …then the variable ceases to exist

Local VariablesPrivate Sub btnOne_Click(...) Handles btnOne_Click Dim num As Integer = 4 SetFive() txtBox.Text = CStr(num)End Sub

Sub SetFive() Dim num As Integer = 5End Sub

Output: 4

Local VariablesPrivate Sub btnOne_Click(...) Handles btnOne_Click Dim num As Integer = 4 SetFive() txtBox.Text = CStr(num)End Sub

Sub SetFive() Dim num As Integer = 5End Sub

Output: 4

num only existswithin this sub

procedure

this num iscompletely

unrelated to theone above

Class-Level Variables

• Visible to every procedure in a form’scode without being passed

• Dim statements for class-level variablesare placed• Outside all procedures

• At the top of the program region

Class-Level VariablesDim num As Integer = 4

Private Sub btnOne_Click(...) Handles btnOne_ClicktxtBox.Text = CStr(num)

SetFive() txtBox.Text &= CStr(num)End Sub

Sub SetFive() num = 5End Sub

Output: 45

• The scope of a variable is the portion ofthe program that can refer to it.

• In other words, the parts of the programthat can access that variable…

• …a variable is out of scope if it cannot bereferred to in that particular codesegment

• Class-level variables have class-levelscope and are available to allprocedures in the class.

• Variables declared inside a procedurehave local scope and are only availableto the procedure in which they aredeclared.

Debugging

• Programs with Sub procedures areeasier to debug

• Each Sub procedure can be checkedindividually before being placed into theprogram• This is sometimes called modularity

4.3 Function Procedures

• User-Defined Functions Having SeveralParameters

• Comparing Function Procedures withSub Procedures

• Collapsing a Procedure with a RegionDirective

Some Built-In FunctionsFunction Example Input Output

Int Int(2.6) is 2 number number

Math.Round Math.Round(1.23,1)is 1.2

number, number number

FormatPercent FormatPercent(.12)is 12.00%

number string

FormatNumber FormatNumber(12345.628, 1) is12,345.6

number, number string

Function Procedures

• Function procedures (aka user-definedfunctions) always return one value

• Syntax:Function FunctionName(ByVal var1 As Type1, _ ByVal var2 As Type2, _ …) As dataType statement(s) Return expressionEnd Function

Example: Form

txtFullName

txtFirstName

Example: CodePrivate Sub btnDetermine_Click(...) _ Handles btnDetermine.Click Dim name As String name = txtFullName.Text txtFirstName.Text = FirstName(name)End Sub

Function FirstName(ByVal name As String) As String Dim firstSpace As Integer firstSpace = name.IndexOf(" ") Return name.Substring(0, firstSpace)End Function

Functioncall

Returnstatement

Example: CodePrivate Sub btnDetermine_Click(...) _ Handles btnDetermine.Click Dim name As String name = txtFullName.Text txtFirstName.Text = FirstName(name)End Sub

Function FirstName(ByVal name As String) As String Dim firstSpace As Integer firstSpace = name.IndexOf(" ") Return name.Substring(0, firstSpace)End Function

the functioncall

"becomes"the value it

returns

Example: Form

txtSideOne

txtSideTwo

txtHyp

Example: CodePrivate Sub btnCalculate_Click(...) _ Handles btnCalculate.Click Dim a, b As Double a = CDbl(txtSideOne.Text) b = CDbl(txtSideTwo.Text) txtHyp.Text = CStr(Hypotenuse(a, b))End Sub

Function Hypotenuse(ByVal a As Double, _ ByVal b As Double) As Double Return Math.Sqrt(a ^ 2 + b ^ 2)End Function

User-Defined Function HavingNo Parameters

Private Sub btnDisplay_Click(...) _ Handles btnDisplay.Click txtBox.Text = Saying()End Sub

Function Saying() As String Dim strVar As String strVar = InputBox("What is your" _ & " favorite saying?") Return strVarEnd Function

Comparing Function Procedureswith Sub Procedures

• Subs are accessed using a callstatement

• Functions are called where you wouldexpect to find a literal or expression

• For example:• result = functionCall

• lstBox.Items.Add (functionCall)

• For example:• result = 5

• lstBox.Items.Add (“some string”)

• For example:• result = functionCall

• lstBox.Items.Add (functionCall)

Functions vs. Procedures

• Both can perform similar tasks

• Both can call other subs and functions

• Use a function when you want to returnone and only one value

Collapsing a Procedure with aRegion Directive

• A procedure can be collapsed behind acaptioned rectangle

• This task is carried out with a Region directive.• To specify a region, precede the code to be

collapsed with a line of the form#Region "Text to be displayed in the box."

• and follow the code with the line#End Region

• Makes code more readable

Region Directives

Collapsed Regions

4.4 Modular Design

• Top-Down Design

• Structured Programming

• Advantages of Structured Programming

Design Terminology

• Large programs can be broken downinto smaller problems

• "divide-and-conquer" approach called"stepwise refinement"

• Stepwise refinement is part of top-downdesign methodology

Top-Down Design

• General problems are at the top of thedesign

• Specific tasks are near the end of thedesign

• Top-down design and structuredprogramming are techniques to enhanceprogrammers' productivity

Top-Down Design Criteria1. The design should be easily readable and

emphasize small module size.2. Modules proceed from general to specific as

you read down the chart.3. The modules, as much as possible, should be

single minded. That is, they should onlyperform a single well-defined task.

4. Modules should be as independent of eachother as possible, and any relationshipsamong modules should be specified.

Top-Level Design HierarchyChart

Detailed Hierarchy Chart

Structured Programming• Control structures in structured programming:• Sequences: Statements are executed one

after another.• Decisions: One of two blocks of program code

is executed based on a test for somecondition.

• Loops (iteration): One or more statementsare executed repeatedly as long as a specifiedcondition is true.

Advantages of StructuredProgramming

• Goal to create correct programs that areeasier to• write

• understand

• modify

• "GOTO –less" programming

Comparison of Flow Charts

Easy to Write

• Allows programmer to first focus on thebig picture and take care of the detailslater

• Several programmers can work on thesame program at the same time

• Code that can be used in manyprograms is said to be reusable

Easy to Debug

• Procedures can be checked individually

• A driver program can be set up to testmodules individually before the completeprogram is ready

• Using a driver program to test modules(or stubs) is known as stub testing

Easy to Understand

• Interconnections of the procedures reveal themodular design of the program.

• The meaningful procedure names, along withrelevant comments, identify the tasksperformed by the modules.

• The meaningful variable names help theprogrammer to recall the purpose of eachvariable.

Easy to Change

• Because a structured program is self-documenting, it can easily be decipheredby another programmer.

Object-Oriented Programming

• an encapsulation of data and code thatoperates on the data

• objects have properties, respond tomethods, and raise events.

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