© 2000 Scott S Albert

Selection Structures

Structured Programming 256

Chapter 4

© 2000 Scott S Albert

Flow of Control

• Computers execute statements sequentially

• Flow of control statements alter this sequential process– Branching statements choose one or another

branch depending on a test– Looping statements repeats one or more

statements multiple times

© 2000 Scott S Albert

C++ flow control statements

Sequence• the default

• C++ automatically executes the next instruction unless you use a branching statement

Branching: Selection

• if

• if-else

• if-else if-else if- … - else

• switch

© 2000 Scott S Albert

If Else Statement

• Branching statement– Format 1

• if ( <boolean expression> ) <statement>• <statement> is only executed if the expression is true

– Format 2• if ( <boolean expression> ) <statement1>

else <statement2>• <statement1> is only executed if the expression is true

<statement2> is only executed if the expression is false

© 2000 Scott S Albert

Boolean Expressions

• Boolean Expression– Expression that can simplifies to true or false– We have already used some Boolean

expressions and if statements in our sample programs

• if (cAnswer == ‘y’) cout<<“thanks”;

• Simple Expressions– <variable> <comparison operator> <variable or constant>

© 2000 Scott S Albert

Comparison Operators

equals = = nT*5/9-32 == 0

not equal ! = sChar != ‘S’

greater than > hours > 40

less than < pay < 0

greater or equal >= radius >= 5.2

less than or equal <= sales <= plan

© 2000 Scott S Albert

Boolean Expressions

• Complex arithmetic expressions can be used• rate*43 / 17 <= base * 8

• Simple expressions can be combined using– And -- &&

• Both expressions must be true for the result to be true

– Or -- || • If either expression is true the result is true

• Each simple expression should be in ( )

© 2000 Scott S Albert

Boolean Variables

• Boolean variables have the value true or false

• Boolean variables can be assigned– A boolean expression– The value true or false– Another boolean variable

• A boolean variable can be substituted for a boolean expression

© 2000 Scott S Albert

Comparing Objects

• Comparison operators only work with basic types (int, float, char etc.)

• To compare classes, like Strings you must– Use a class comparison method if one exists– Use a class method to retrieve the class data

into a basic type

© 2000 Scott S Albert

Grouping statements

• What if you need more than one statement to be executed if an if Boolean expression is true?– Statements are grouped by { }– Statement groups can be nested– All statements in a group are treated as a single

statement. This is called a statement block

© 2000 Scott S Albert

Example

int nHours;float nPay, nGross, nOvertime;

// Read in nHours, nPay, sOT (overtime paid)

if (nHours <= 0)cou<<"Error in hours input value is ”<<nHours;

else {if ( nHours > 40 )

nOvertime = (nHours - 40) * ( Pay * 1.5 );else

nOvertime = 0;}

© 2000 Scott S Albert

Matching Elses

– An else matches up to the nearest preceding if that has not already been spoken for by another else Use blocks to help document your code.

if (coffee==‘y’) if (donuts == ‘y’) cout<<“We have coffee and donuts.”; else cout<<“We have coffee, but not donuts.”;else if (tea==‘y’) cout<<“We have no coffee, but tea.”; else cout<<“We have no coffee, but tea.”;

© 2000 Scott S Albert

Switch Statement

– Multi-branch if statements such as the one used in the previous example are quite common in many programming projects

• Especially in menu systems– Enter 1 for New Messages

– Enter 2 for Greetings

– Enter 3 for Administrative options

– Enter 4 to Send a message

– Enter 5 to end

• Switch is easier to implement than many if statements

© 2000 Scott S Albert

Switch Statement Syntax

– switch ( <integer or char expression> ) { case <integer or char> :

<statement(s)> break; case <integer or char>:

<statement(s)>break;

default:<statement(s)>

}

© 2000 Scott S Albert

Sampleint nMenu;// System.out.println statements listing a menucout<<“Enter choice”;cin>>nMenu;switch( nMenu ) { case 1:

cout<<“Listening to messages”;break;

case 2:cout<<“Change greetings”;break;

case 3:cout<<“Administrative stuff”;break;

case 4:cout<<“Sending message”;break;

case 5:cout<<“Goodbye”;break;

default:cout<<“Sorry we did not understand you”;

}

© 2000 Scott S Albert

Switch notes

• Usually at least one case statement is needed

• The default statement is optional, but highly recommended

• If you omit the break statement, the flow of execution will continue into the next case