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Introduction to Programming Structure

Lesson 4

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Overview

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

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Pointers 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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Modules, Procedures & Functions

The subparts to a Program

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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

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Procedures

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

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

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

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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.• Use Return values that are associated with

the name of the function

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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;}

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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

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An Example of Control

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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

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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

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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 * Y

End Sub

McManus

Read and Data Validation

Reads and validates input data

• Usually separate modules

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 databases

Procedure EndClose Employee fileClose Payroll file

End Procedure ‘End

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Global or Local Variables

Scope!

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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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Cohesion & Coupling

Making Modules Better!

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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.

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

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What is the code doing?

• What are the parameters?

McManus

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

Dim Temp As Long

If Scully < Mulder Then

Temp = Scully

Scully = Mulder

Mulder = Temp

End If

End Sub

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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

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Goal of Cohesion & Coupling?

• High Cohesion – Functional or Information

• Low Coupling– Data, Stamp, Control

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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.

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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 & Formal Parameters

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

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Actual & Formal Parameters

• Formal Parameters– Parameters declared in the procedure

header– Is a list of “place marker” names used in

the procedure’s declaration. – 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

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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.

• 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.McManus

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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 Parameters

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

• 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, PayRate, Temp

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The Data Dictionary

• Defines all of the variables used within a program

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

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DD Example

Item Name(no

spaces)

Data Type

Modules Domain(Range)

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

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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

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