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The Decision Logic Structure

Chapter 6

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IntroductionOutside of Sequential Logic,Decision Logic is the most used in

solutionsDecision Logic allows the programto react to different situations inthe data at runtime.

Decisions are made based on acondition(s) being true or false.

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The Structure of Decision

Logic in Computer LanguagesIf (something is true)

then

do statements that should be done ifthe condition is

else

do statements that should be done if

the condition isOnly one section of the decisionstatement is ever run in one executionof the containing program

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The Condition ExpressionMay only return True or False

Uses the or

a combination of the relationaloperators and the logic operators

The allow theprogrammer to make more thanone condition to be satisfied beforethe true or false block is selected

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What to Test?The exact nature/values of thetest(s) in the conditional

expression(s) are determinedduring problem analysis.

In C++ the construct is used to implement

decisions in the implementation ofthe solution.

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Programs must often anticipate

a variety of situations.

For example, ATMs must servevalid bank customers. They mustalso reject invalid PINs. The codethat controls an ATM must permit

these different requests. Softwaredevelopers must implement codethat anticipates all possibletransactions.

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The if Statement

General form:

if ( Conditional-expression )

true-part ;

The (Conditional-expression) is any

expression that evaluates to

either true or false.CondExpression: Value:

int hours = 40;

is hours > 40 No/ False

is hours >= 40 Yes/True

is hours < 40 No/ Falseis hours

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Conditional/Logical

expressionsRelational Meaning

Operators

< Less Than

> Greater than

= Greater than or equal to

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== // Exactly equal to

!= // Not equal to

4 == 4 ___________ // true (1) or false (0)

4 != 4 ___________

4 = 60.0)

cout

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The if...else StatementThe if...else statement allows two alternate courses

of action.

The logical expression is evaluated to 0 (false) or a

nonzero (true) value. When true, the true-part is executedand the false-part is disregarded. When the logicalexpression is false, the false-part executes.

Logical Expression

True (non zero)

statement

statement

statement

statement

False (zero)

True-partFalse-part

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The if...else Statement

if (logical-expression)

{

true-part ;

}

else

{

false-part ;

}

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(miles > 24000)

cout

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The Compound Statement in C++

{statement-1 ;

statement-2 ; //The compound statement

// (block)...

statement-N ;

}

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The compound statement groups togethermany statements as one.

( PINFound == true && NameValid == true)// Note: This true-part is a compound statement

// Process a balance query

Balance= Balance - WithdrawAmount;

// Using { } with 1 statement is unnecessary, yetconsistent

cout

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Common Selection Statement Errors

if(GPA >= 3.5)

// The true-part is the cout only

cout

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Logical Operators in C++

Expression Result Expression Result Expression Result ! 0 1 1 || 1 1 1 && 1 1

! 1 0 1 || 0 1 1 && 0 0

0 || 1 1 0 && 1 0

0 || 0 0 0 && 0 0

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Example

if((test >= 0) && (test

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Evaluate expression with relational and logicaloperators. Is test between 0 and 100 inclusive (leftcolumn)? Is test outside the range of 0 and 100inclusive (right column)?

(test >= 0) && (test 100)

( 97 >= 0) && ( 97 100)

1 && 1 0 || 01 Yes/ 0 No/

Relational and Logical Operators Combined

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Short Circuit Evaluation

// If E1 is false, E2 is not

// evaluated// If E1 is true, E2 is not

// evaluated

// Could we get an error ____?//Would it be possible to take the sqrt of a

negative number in the evaluation ofthis expression?

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Straight-through LogicAll decisions are processed one ata time in sequence

No false sections are included

All the decisions are consideredand the true statements areexecuted when the condition istrue

When a condition is false theprogram goes on to the nextdecision.

if(Guess < Num)

cout Num)

cout

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Positive Logic

False conditionalexpressions lead toadditional decisionswhich if False, allowfor more decisions tobe made, until aTrue is encounteredwhich is a signal to

go on to the nextdecision in asequence ofdecisions.

if(NotInArea == true)

cout

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Negative Logic

True conditionalexpressions lead to

additional decisions which if

True, allow for more

decisions to be made, until

a False is encountered

which is a signal to go on to

the next decision(s) if they

exist in a sequence of

decisions.

if(Money > 20)

{

if(Gas == true)

{

if(CarStart == true)

{

if(FriendAvail == true)

cout

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Nested LogicNested logic is one control structure containing

another similar control structure. An if...elseinside another if...else. e.g. (the 2nd if is placed onthe same line as the 1st):

if(GPA < 3.5)

cout

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More Nested LogicGiven the following

scale:

Value of C Output

34 C Hot20 C < 34 Warm

12 C < 20 Mild

0 C < 12 Cold

C < 0 Freezing

Complete this nested

if..else to show the

appropriate message

if(c >= 34)cout = 20)cout

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Which Decision Logic to Use

Evaluate all three types and make a decisionMost readableRequires the fewest tests

Easiest to maintain

My experience is that the problem will hint atwhich is best. Example:

(Success == true) Do Lots of stuff like the rest of the program

This means the else will be at the bottom of the program

Put out a warning message.

(Success == false) Put out warning message

Put the rest of the program here.

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Logic ConversionThis means converting from positive tonegative logic or negative to positive logic

Change < to >=

Change > to = to 20) ---> may be T or F

(Gas in Car) --> may be T or F

(Friend Avail) --> may be T or F

There are 8 possible combinations of thesethree conditions

See Next Slide

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A Decision Table

Conditions Possible Combinations(Money > 20) T T T T F F F F

(Gas in Car) T T F F T T F F

(Friend Avail) T F T F T F T F

Dinner Out X

Go To Bank X X X X

Get Gas X X

Eat At Home X X X X X X X

The goal is to go out to dinner with a friend. Before one can do

this, I must have $20 or more, Gas in the car, and my friend must

be available. Otherwise, I must correct some problems and eat at

home.

A

CT

I

O

N

S

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Decision Logic for NowAll programs use some sort of decision logic.

Whether it is Positive or Negative decision logicis generally irrelevant

The important thing is to correctly capture allthe possible solutions decision situations andassociate them with the proper outcomes.

Proper Testing insures that all potentialsituations are accounted for!

In C++ the if()else construct is the primarydecision making facility.

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Some examplesThe following slides give you someexamples of C++ programs using if

logic

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// Filename: AGETEST.CPP

// Helps ensure age values are reasonable.

#include

using namespace::std;

void main()

{

int age;

cout > age;

if (age >=16)

{cout

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// Filename: IFELSE1.CPP

// Demonstrates if-else by printing whether an

// input value is greater than zero or not.#include

void main()

{

int num;

cout > num; // Get the user's number.

if (num > 0)

{

cout

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// Filename: SERV.CPP

// Prints a message depending on years of service.

#include

void main()

{

int yrs;

cout > yrs; // Determine the years they have worked.

if (yrs > 20)

{cout 10)

{

cout