Object-Oriented Programming Using C++Third Edition

Chapter 3Making Decisions

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Objectives

• Use the if and if-else statements

• Use nested if statements

• Avoid common pitfalls with if statements

• Use the switch statement

• Use the conditional operator

• Use the logical AND and the logical OR

• Make decisions with structure fields

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Using the if Statement

• The primary C++ selection structure statement used to perform a single-alternative selection is an if statementif (Boolean expression)

action if true;

• There is also a dual-alternative ifif (Boolean expression)

action if true;

else

action if false;

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Using the if Statement (continued)

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Using the if Statement (continued)

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Using the if Statement (continued)

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Using the if Statement (continued)• Performing multiple tasks when a condition is

met

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Using the if Statement (continued)• Don’t forget the curly braces

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The Dual-Alternative if

• Also called the if-else structure

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Using a Nested if

• Nested if: if structure that rests entirely within another if structure—within either the if or the else clause

• Consider the code in the previous slide– If the user enters ‘T’, it will display “Male”– You can use a nested if to make the code more

sophisticated

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Also called an if-else-if structure

Using a Nested if (continued)

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Avoiding Common Pitfalls with if Statements

• Forgetting that C++ is case sensitive

• Assuming that indentation has a logical purpose

• Adding an unwanted semicolon

• Forgetting curly braces

• Using = instead of ==

• Making unnecessary comparisons

• Creating unreachable code

See Figure 3-6

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Pitfall: Forgetting that C++ is Case Sensitive

• Code in Figure 3-10 displays “Invalid code” if a user enters ‘f’

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Pitfall: Adding an Unwanted Semicolon

Indentation has no logical purpose

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Pitfall: Using = instead of ==

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Pitfall: Making Unnecessary Comparisons

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Pitfall: Making Unnecessary Comparisons (continued)

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Pitfall: Creating Unreachable Code

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Pitfall: Creating Unreachable Code (continued)

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Using the switch Statement

• When you want to create different outcomes depending on specific values, you can use a series of ifs

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Using the switch Statement (continued)

• However, as an alternative to the long string of ifs, you can use the switch statement

Removing break changes behavior of statement

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Using the Conditional Operator

• The conditional operator is represented by a question mark

• Provides a concise way to express two alternatives– Alternative one:

if(driverAge < 26) insurancePremium = 250;else insurancePremium = 185;

– Alternative two:driverAge < 26 ? insurancePremium = 250 : insurancePremium = 185;

• cout<<(a > b ? a : b)<<“ is greater”<<endl;

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Using the Logical AND

• Consider the following example

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Using the Logical AND (continued)

• You can use a single if statement that contains a logical AND, which you create by typing two ampersands (&&) between two Boolean expressions:

• Remember to include a complete Boolean expression on each side of the &&

if(payRate >= 6.00 && <= 12.00) // ERROR!cout<<"Valid "<<endl;

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Using the Logical AND (continued)

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Using the Logical OR

• Consider the following example

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Using the Logical OR (continued)

• You can use a single if statement that contains a logical OR, which you create by typing two pipes (||) between two Boolean expressions

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Using the Logical OR (continued)

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Pitfall: Using OR when you mean AND

• Write a program given the following instructions: “The user should enter 1 or 2. If the user doesn’t enter 1 or 2, issue an error message.”

• Incorrect:if(userResponse != 1 || userResponse != 2)

cout<<"Incorrect entry"<<endl;

• Correct:if(userResponse != 1 && userResponse != 2)

cout<<"Incorrect entry"<<endl;

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Combining AND and OR Selections

• AND takes precedence over OR– AND is evaluated first

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Making Decisions with Structure Fields

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You Do It: Using a Single-Alternative if

cout<<"On average, how many hours are you gone from home each day? ";

cin>>hoursGone;

if(hoursGone > MANY_HOURS)

cout<<"You should consider a cat"<<endl;

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Using a Dual-Alternative if

else

cout<<"You can consider a dog"<<endl;

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Using a Compound Condition and Nested ifs

…

…

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Summary

• if statement is the primary selection structure statement to perform a single-alternative selection

• Dual-alternative if takes one action when its Boolean expression is evaluated as true, and uses an else clause to define the actions to take when the expression is evaluated as false

• A nested if rests entirely within another if structure• Easy to make several types of mistakes

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Summary (continued)

• Use a series of ifs or use the switch statement when you want to create different outcomes depending on specific values of a variable

• Conditional operator (?) provides a concise way to express two alternatives

• The logical AND is created by typing two ampersands (&&)

• Use a logical AND to create a compound Boolean expression in which two conditions must be true for the entire expression to evaluate as true

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Summary (continued)

• Logical OR is created by typing two pipes (||)• Use a logical OR to create a compound Boolean

expression in which at least one of two conditions must be true for the expression to evaluate as true

• When you combine an AND and an OR in the same expression, the AND takes precedence

• When you define a structure, and subsequently create objects that are instantiations of the structure, you use the individual structure’s fields the same way you use variables of the same type