© 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 ( ) is only executed if the expression is true –Format 2 if ( ) else is only executed if the expression is true 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 –

© 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 ( ) { case : break; case : break; default: }

© 2000 Scott S Albert Sample int nMenu; // System.out.println statements listing a menu cout<<“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