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Introduction to Programming Structure

Lesson 4

McManus

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Overview

• Guidelines• Modules & Functions• Cohesion & Coupling• Local & Global Variables• Parameters• Variable Names & Data Dictionaries• Three Logic Structures

McManus

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Guidelines on Program Structure

• Use Modules– Each part should have a particular

function• Use the three logic structures

– Sequential, Decision & Iteration• Don’t reinvent the wheel

– Avoid rewriting identical processes• Use techniques to improve

readabilityMcManus

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Cohesion & Coupling

Making Modules Better!

McManus

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Cohesion & Coupling

• Each module should– Be functionally independent– Perform one problem-related task

• Calculating IRS Withholding is one problem-related task, but may have multiple operations within the module

– When connected, use the smallest interface possible.

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Cohesion

• The degree of interaction within a module.– Each module should perform one

functionally-related task…not necessarily one assignment statement.

– Concentration is on what goes on within the module.

McManus

Term was coined by Larry Constantine in mid-1960’s

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Scale of Cohesion

• Stevens, Myers, Constantine, and Yourdon developed the Scale of Cohesion as a measure of the “black boxiness” of a module, and as a result, the maintainability of a module.

McManus

Type Measure Black BoxFunctional Best Black BoxInformational ** BestSequential CommunicationalProcedural Gray BoxTemporalLogicalCoincidental Worst Transparent

or White Box**Originally not part of Scale

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Coupling

• The degree of interaction between two modules.– Interaction is the

interface, or lack thereof, between two modules.• The interface is the

parameter list. – Create it, use it, lose it

McManus

Best (Lowest Interaction)

Worst (Highest Interaction)

Normal Data Stamp Control

Common

Content

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Effects?

• Which pieces affect Cohesion and which affect Coupling?

McManus

Private Sub Minimum(min As Long, y As Long, z As Long)

If y < min Then

min = y

End If

If z < min Then

min = z

End If

lblSmallest.Caption = "Smallest value is " & min

End Sub

What does this code do?

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What is the code doing?

• What are the parameters? Variables?

McManus

Private Sub Minimum(Scully As Long, Mulder As Long)

Dim Temp As Long

If Scully < Mulder Then

Temp = Scully

Scully = Mulder

Mulder = Temp

End If

End Sub

Global &Parameters

Local Variable

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Each Goal

• Cohesion’s Goal– To create a procedure that performs one

functionally-related task. • Coupling’s Goal

– To protect global data and local data from being used within a procedure without declaring it on the procedure’s header• Create as close to where you’re going to use

it as possible, use it, then lose it as soon as possibleMcManus

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Goal of Cohesion & Coupling?

• High Cohesion – Functional or Information

• Low Coupling– Data, Stamp, Control

McManus

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Modules, Procedures & Functions

The subparts to a Program

McManus

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Modules

• “A module is a lexically contiguous sequence of program statements, bounded by boundary elements, having an aggregate identifier.” Yourdon & Constantine (1979)– A part of a larger system– Written and tested separately– Combined with other modules to form a

complete system– Used in top-down programming– Procedures & Functions

McManus

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Procedures

• A smaller part of the main program.• 2 Advantages

1. Eliminates the need to program the same thing more than once.

2. Larger programs are easier to read when broken into procedures (and functions).

McManus

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Functions

• Functions– A subprogram that acts like a

mathematical function: • Given a particular set of argument values,

the function returns a unique result.• Big Difference Between Procedures &

Functions:– Use Return values that are associated with the

name of the function

McManus

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Function Examples

• PascalFUNCTION doublenum(b : Integer) : Integer; BEGIN doublenum := 2 * b END;

• Visual BasicPrivate Function Doublenum(b As Integer) As Integer Doublenum = 2 * bEnd Function

• C++Int doublenum ( int b){ return 2 * b;}

McManus

All three do the same thing,

just with different syntax

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What Do They Have In Common?

• Each module – is an entity by itself– has a single purpose– should be easily read, modified and

maintained– Length is governed by function and

number of instructions contained within– Controls the order of processing

McManus

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An Example of Control

McManus

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Types of Modules/Procedures

• Control – Demonstrates

overall flow of data• Initialization &

Wrap-Up– Processes

instructions to be performed once (either at beginning or at the end of the program)

– Used typically in batch processing

• Process Data– Calculation– Print– Read and Validation

• Event– Used in OO and

some event driven programming languages

– More about these later

McManus

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Control Modules

• Most often called “Main”• All other modules, procedures and

functions are subordinate to the control module

Sub MainCall ProcedureA(X, Y)Call ProcedureB(A, B)

End Main

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Init Module

• Used in some languages to initialize variables or processes– Examples

• Opening files• Initializing variables• Printing report headings

Procedure BeginDim X, Y, Z As IntegersOpen Payroll file

End Procedure ‘BeginMcManus

Does this violate

Cohesion or

Coupling?

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Process Data Modules

• Calculation– Performs

• Arithmetic operations

• Accumulations• Sorting or Searching

Private Sub Double (X, Y)Dim Temp as IntegerTemp = X * YPrint Temp

End Sub

Read and Data Validation◦ Reads and validates

input data Usually separate

modules

McManus

Private Sub Verify(X)If X < 0 Or X > 10 Then

lblMessage.Text = “Data Error”End If

End Sub

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Wrap Up Module

• Used to close out processes– Examples

• Closing files• Printing reports• Returning updated data to database

Procedure LastProcedureClose Employee fileClose Payroll file

End Procedure ‘LastProcedure

McManus

Does this violate

Cohesion or

Coupling?

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Global or Local Variables

Scope!

McManus

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Scope

• The area of a program where an identifier (variable) is visible

• When an identifier has multiple declarations in different modules, the most local declaration is used each time that identifier is referenced. (overloading)

• Global or “non-local” variables subject to side effects.

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Side Effects

• Caused when the value of a global variable is changed within a procedure or function– Any effect of one module on another module

that is not a part of the explicitly defined interface between them

• Also caused when a variable name is used in globally and locally (causes overloading)

• A nasty effect that should be avoided!

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Global Scope

• Declared within the main program• Can be referenced anywhere in the

program– Is visible and accessible everywhere

McManus

X, Y, Z

A

CB

X, Y & Z are Global to modules

A, B & C

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Local Scope

• Declared within a module– Has no effect outside the procedure or

function in which it is declared• Can be referenced only within a

procedure or a function

McManus

X, Y, Z

A m

C pB n

Within A, Variable m is defined, but can also see X, Y, & Z

Within B, Variable n is defined , but can also see X, Y, & Z

Within C, Variable p is defined , but can also see X, Y, & Z

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Global or Local?

Private Sub Minimum(Scully As Long, Mulder As Long)

Dim Temp As Long

If Scully < Mulder Then

Temp = Scully

Scully = Mulder

Mulder = Temp

End If

End Sub

McManus

Scully & Mulder are what type

of variables?

What type of variable is Temp?

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Another Example

McManus

Scope of X, Y, Z, Procedure1

Scope of M, N, Me, X, You

program ShowScope; var X, Y, Z : Real; procedure Procedure1 (var M, N, Me : Real);

var X, You : Real; begin {Procedure1} ....... end; {Procedure 1}

begin {ShowScope} Procedure1(X, Y, Z)end. {ShowScope}

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Parameters

How we avoid side effects!

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Parameters

• Are the variables that are passed into and out of modules

• Use global parameters – (to the procedure or function)

• Pass values through the use of variables

• Actual and Formal parameters• Call-by-reference & Call-by-value

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Parameter Communication

• A measure of the quantity of data passing through a module’s interface.

• Is also a measure of the module’s coupling.

• The goal is to strive for a minimal amount of information being passed.

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How Parameters are Used

• Input Parameter – Information passed into a procedure, but not

returned or passed out of the procedure.• Output Parameter

– Information returned to the calling program from a procedure.

• Input/Output Parameter – Information passed into a procedure, perhaps

modified, and a new value returned.

McManus

Programmers now determine how their parameters are used.

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Parameters

• Provide the communication links between the main program and its modules.

• Make procedures and functions more versatile.– Different data can be manipulated each time

the module is called.• Come in two types:

– Actual– Formal

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Actual Parameters

– Are substituted for the formal parameter at the time the procedure is called.• Parameters used in the call statement

– Statements that transfer control to a procedure.• Data types must be assignment compatible

with its corresponding formal parameter– If they don’t, your program will blow up!

• Can be a variable, constant or an expression• Can be call-by-value or call-by-reference

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Formal Parameters

– Is a list of “place marker” names used in the procedure’s declaration.

– Parameters declared in the procedure header

– Can include the data type of the valued parameters.

– Must be a variable– Can be call-by-value or call-by-reference

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Parameter Correspondence Rules

• Determined by position in respective parameter lists

• Lists must be the same size, although the names may differ

• Data Types of corresponding actual and formal parameters must be identical

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Data Areas After a Call

McManus

8.0

10.0

8.0

10.0

? ?

Formal Parameters

Local Variables

Sum Average

Num1

Num2

Actual Parameters Var1

Var2

Main program data area Procedure data area

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Valued & Variable Parameters

• By Reference (Call-by-Reference)– Passing a variable to a procedure is called

passing an argument by reference, because a variable can be modified by a procedure and returned to the calling module.

– You can use it AND you CAN change it!• By Value (Call-by-Value)

– Passing a literal value (such as a string in quotation marks) to a procedure is called passing an argument by value, because a value cannot be modified by a procedure.

– You can use it BUT you CAN’T change it!

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Call-by-Reference

• The default for parameter passing• Gives access to the contents of the

storage area where values are stored• Giving the called procedure the

ability to directly access the caller’s data

• Allowing changes in the data

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Call-by-Value

• Protects the data being passed• Accomplished by creating a copy of

the value– without affecting the original value of

the variable• Thus…

– Called procedure is unable to change the values stored in the variable’s storage area

• Helps avoid Side Effects!McManus

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Parameter Relationships

Call Statements

Actual Parameters• Valued• Variable

McManus

Procedure Header Parameters

Formal ParametersValuedVariable

Parameter Interface using Global Variables

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Another Look at Variables

Names & the Data Dictionary

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Variable Names-Reminder

• Use mnemonic terms– Use a variable name that relates the name of

the variable to its usage• Contributes to self-documenting code

– Which reduces the amount of commenting required

– Z = X * Y What is it doing (besides multiplication?)

– SalesTax = SalesTaxRate * Cost (this you know)

• Examples– SalesTax, SalesRate, Sales_Rate, PayRate,

Temp

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The Data Dictionary

• Defines all of the variables used within a program

• Lists:– Item Name– Variable Names – Domain (range of possible values)– Data type– Location defined & accessed– Test Data (or error checking)

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Required!

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DD Example

Item Name

(required)

Variable Name

(required)

Data Type

(required)

Modules Domain(Range)(required)

Scope

Hours worked

Hours Numeric-real

GetHoursCalcGrossPay 0 <= n <=168

GlobalGlobal

Gross Pay

GrossPay Numeric-real

CalcGrossPayCalcDeductionsCalcNetPayPrintPayChecks

0 <= n <=1 million

GlobalGlobalGlobalGlobal

Net Pay NetPay Numeric-real

CalcNetPayPrintPayChecks 0 <= n <=1 million

LocalGlobal

McManus

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The Three Logic Structures

• Sequential– One statement follows another

• Selection (Decision)– Allows choices based on the data– IfThenElse, Nested If’s, Case, Switch

• Iteration (Looping or Repetition)– Allows statements to be repeated a specified

number of times– While, Do, For, Do Until, Repeat

McManus

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