General Control Issues

8/8/2019 General Control Issues

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General control IssuesGeneral control Issues


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Contents.Contents.

y Boolean Expressions

y Compound statements

y

Null statementsy Taming Dangerously Deep Nesting

y A Programming Foundation: StructuredProgramming

y Control Structures and Complexity


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Boolean ExpressionsBoolean Expressions

y Use the identifiers

True or false

Dont use flags like 0 or 1.

x By using flags we may cant understand that which is

representing true and which one is representingfalse.


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Visual Basic Examples of Using Ambiguous FlagsVisual Basic Examples of Using Ambiguous Flagsfor BooleanValuesfor BooleanValues

Dim printerError As Integer

Dim reportSelected As Integer

Dim summarySelected As Integer

...If printerError = 0 Then InitializePrinter()

If printerError = 1 Then NotifyUserOfError()

If reportSelected = 1 Then PrintReport()If summarySelected = 1 Then PrintSummary()

If printerError = 0 Then CleanupPrinter()


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Visual Basic Examples of UsingVisual Basic Examples of Using TrueTrue andand FalseFalse for Tests Insteadfor Tests Insteadof NumericValuesof NumericValues

Dim printerError As Boolean

Dim reportSelected As ReportType

Dim summarySelected As Boolean

...If ( printerError = False ) Then InitializePrinter()

If ( printerError = True ) Then NotifyUserOfError()

If ( reportSelected = ReportType_First ) Then

PrintReport()If ( summarySelected = True ) Then PrintSummary()

If ( printerError = False ) Then CleanupPrinter()


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Making complicated expressionsMaking complicated expressions

simple.simple.

y Several steps to simplify complicated

expressions.

Break complicated tests into partial tests with

new boolean variables.

Move complicated expressions into booleanfunctions.

x If a test is repeated often.

x Distracts from the main flow of the program

x Make function of the code for test purpose.


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Visual Basic Example of a Complicated TestVisual Basic Example of a Complicated Test

If ( ( document.AtEndOfStream ) And ( Not inputError ) )

And _

( ( MIN_LINES


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Example of complicated code with newExample of complicated code with new

boolean variable to make it easy.boolean variable to make it easy.Function DocumentIsValid( _

ByRef documentToCheck As Document, _

lineCount As Integer, _

inputError As Boolean _

) As BooleanDim allDataRead As Boolean

Dim legalLineCount As Boolean

allDataRead = ( documentToCheck.AtEndOfStream ) And ( NotinputError )

legalLineCount = ( MIN_LINES


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Contd.Contd.

y In above example.

Error processing() is a boolean function thatindicates the processing status.


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Example of the Main Flow of the Code Without theExample of the Main Flow of the Code Without theComplicated TestComplicated Test

If ( DocumentIsValid( document, lineCount, inputError ) )

Then

do something or other

...

End If


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Forming boolean expressions positivelyForming boolean expressions positively

y Many people cant understand negative expressions.

y An example of negative IF ELSE code.

if ( !statusOK )

{

// do something

...

}

else {

// do something else

...

}


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Contd.Contd.

y The previous code is now changed to

positive expressions.if ( statusOK ) {

// do something else...

}

else {

// do something

...

}


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Apply DeMorgan Law.Apply DeMorgan Law.

y It converts doubly negated code into one

negation.

y E.gif ( !displayOK || !printerOK ) ...

y After applying DeMorgan law.

if ( !( displayOK && printerOK ) ) ...


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Transformation of logical expression underTransformation of logical expression under

DeMorgans theoremDeMorgans theorem

Initial Expression Equivalent Expression

Not A not B Not (A or B)

Not A and B Not (A or not B)

A and not B Not (not A or B)

A and B Not (not a or not B)

Not A or not B Not (A and B)

A or not B Not (not A and B)

A or B Not (not A and not B)


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Using Parentheses to Clarify BooleanUsing Parentheses to Clarify Boolean

ExpressionsExpressions

y When expressions are complex dont rely

on language evaluation order.

y User proper parentheses to makereadable code.

y Example of few parentheses.if ( a < b == c == d ) ...

y Example of better parentheses.if ( ( a < b ) == ( c == d ) ) ...


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Writing Numeric Expressions in numberWriting Numeric Expressions in number--

line orederline oreder

y Organize numeric tests so that they

follow the points on a number line.

y The idea is to order the elements left toright, from smallest to largest.

Example:MIN_ELEMENTS


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Contd.Contd.

y testing i against MIN_ELEMENTS only, position of ivaries

depending on where i is when the test is successful. If i is

supposed to be smaller.

while ( i < MIN_ELEMENTS ) ...

y But ifi is supposed to be larger, youll have a test like

while ( MIN_ELEMENTS < i ) ...

y This approach is clearer than tests like

( i > MIN_ELEMENTS ) and ( i < MAX_ELEMENTS )


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Comparisons to 0.Comparisons to 0.

y Programming languages use 0 for several

purposes.

Null terminator for string.

Lowest address for a pointer.

Value of first item in an enumeration.

False in logical expression.


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Contd.Contd.

y Compare numbers to 0

Example:

while(balance !=0)

rather than

while(balance)

y Compare characters to null terminator

explicitly.while ( *charPtr != '\0' ) ...

rather than

while ( *charPtr ) ...


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Contd.Contd.

y Compare pointers to NULL.

while ( bufferPtr != NULL ) ...

rather than

while ( bufferPtr ) ...


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Common problems with BooleanCommon problems with Boolean

expressionsexpressions

y In C and C++, put constants on the left

side of comparisons

C++ poses some special problems with

boolean expressions. Interchanging bitwise operators with logical

operators is common problem.

Example:if ( MIN_ELEMENTS = i ) ... (Error)

if ( i = MIN_ELEMENTS ) ... (No error)


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Contd.Contd.

y In Java, difference between a==b and

a.equals(b)

a==b tests for whether a and b refer to the same

object

a.equals(b) tests for whether the objects have the

same logical value


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2.Compound statements.2.Compound statements.

y Compound statement or block is a

collection of statements that are treatedas a single statement

y Compound statements are created by

writing { and } in C,C++,C# and java.


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Guidelines for using compound statements.Guidelines for using compound statements.

y Write pair of braces together.

Complaint about to match pairs of braces.

How to know begin and end braces.

Unnecessary problem


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Guideline to solve braces problemGuideline to solve braces problem

y Write this first: for ( i =0; i < maxLines;i++ )

y Write this next: for ( i =0; i 0.0 ) {

// do something

...

}

else {

// do something else

...

}


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Contd.Contd.

// selection in a case statement

switch ( commandShortcutLetter ) {

case 'a':

PrintAnnualReport();

break;case 'q':

PrintQuarterlyReport();

break;

case 's':

PrintSummaryReport();

break;

default:

DisplayInternalError( "Internal Error 905:Call customer support." );

}


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Contd.Contd.

y Iteration:

Iteration causes to a group of statements tobe executed multiple times.

Iteration is used as commonly in loop. For, while,


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Visual basic example of Iteration.Visual basic example of Iteration.

' example of iteration using a For loopFor index = firstTo last

DoSomething( index )

Next

' example of iteration using a while loop

index = firstWhile ( index


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Control structure and complexity.Control structure and complexity.

y Control structure have a big contribution

in overall program complexity.

Poor use increases complexity.

Good use decreases complexity.


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How to measure complexity.How to measure complexity.

y Researchers gave many techniques to

measure complexity.

y The most influential of the numeric

techniques is Tom McCabes. Measure complexity by counting number of

decision points.


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Contd.Contd.

y 1. Start with 1 for the straight path through the routine.

y 2. Add 1 for each of the following keywords, or theirequivalents: if while repeat for and or

y 3. Add 1 for each case in a case statement.

Example:

if ( ( (status = Success) and done ) or

( not done and ( numLines >= maxLines ) ) ) then ...

In this fragment, you count 1 to start; 2 for the if; 3 for theand; 4 for the or; and5 for the and. Thus, this fragment

contains a total of five decision points.


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What to do with complexity measurement.What to do with complexity measurement.

y Number to analyze routines.05 The routine is probably fine.

610 Start to think about ways to simplify the routine.

10+ Break part of the routine into a second routineand call it from the first routine.


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