Chapter 4 Decision Making

1. Chapter 4Decision Making Lecture Slides to Accompany An Introduction to Computer Science Using Java (2nd Edition) by S.N. Kamin, D. Mickunas, E. Reingold

2. Chapter Preview In this chapter we will: • discuss the use of decision making in computer programming • describe the use of the Java if and switch statements • describe the use of Boolean expressions in Java if statements • discuss the use of integer selector variables in Java switch statements

3. if Statement • Ensures that a statement is executed only when a condition is true • Conditions typically involve comparison of variables or quantities for equality or inequality • Example: if (age >= 18) out.println(“You are eligible to vote.”);

5. Relational Operators

6. if Statement with Optional else • An if statement may have an optional else clause that will only be executed when the condition is false • Example: if (wages <= 57600) tax = 0.124 * wages; else tax = 0.124 * 57600;

8. Compound Alternatives • To execute more than one statement conditionally, you may use { } to define a compound statement for either (or both) conditional alternatives • Example: if (firstNumber <= secondNumber) { quotient = secondNumber / firstNumber; remainder = secondNumber % firstNumber; } else { quotient = firstNumber / secondNumber; remainder = firstNumber % secondNumber; }

9. Example: if (condition-1) statement-1; else if (condition-2) statement-2; else statement-3; Another format: if (condition-1) statement-1; else if (condition-2) statement-2; else statement-3; Cascading if-else Statements

11. Code written: if (condition-1) if (condition-2) statement-1; else statement-2; Interpreted as: if (condition-1) if (condition-2) statement-1; else statement-2; Dangling else

14. Code written: if (condition-1) { if (condition-2) statement-1; } else statement-2; Interpreted as: if (not condition-1) statement-2; else if (condition-2) statement-1; Correcting the Problem

15. Boolean Operators • Logical “and” (conjunction) • true only when both expressions are true (MINIMUM_WAGE <= wages) && (wages <= MAXIMUM_WAGE) • Logical inclusive “or” (disjunction) • true when either or both expressions are true (MINIMUM_WAGE < wages) || (MINIMUM_WAGE == wages) • Logical exclusive “or” • true when exactly one of the expressions is true (MINIMUM_WAGE < wages) ^ (MINIMUM_WAGE == wages) • Logical “not” (negation) (MINIMUM_WAGE != wages)

16. Boolean Operator Truth Table

17. Operator Precedence Rules 1. ! - (unary) 2. * / % 3. + - 4. < <= > >= 5. == != 6. ^ 7. && 8. ||

18. Complicated Boolean Expressions boolean isLeapYear = ((year % 4) == 0) && ((year % 100) != 0) || ((year % 400) == 0); // Assume all months have 31 days dayNumber = (month - 1) * 31 + day; // Correct for months beyond February if (month > 2) { dayNumber = dayNumber - ((4 * month +23) / 10); if (isLeapYear) // Correct for leap year dayNumber = dayNumber + 1; }

21. Comparing Strings • Comparison is done by comparing strings character-by-character left to right, the first character that differs dictates which string is smaller lexicographically • What if one string is a prefix of the other? • The prefix is smaller than the longer string • How do upper- and lower-case characters compare? • If a case-sensitive comparison is used lower-case is always less than upper case • How do special characters (e.g. %) compare? • Decided by the character ASCII representation

22. Java String Class Comparison Methods

23. switch Statement • Used to accomplish multi-way branching based on the value of an integer selector variable • Example: switch (numberOfPassengers) { case 0: out.println(“The Harley”); break; case 1: out.println(“The Dune Buggy”); break; default: out.println(“The Humvee”); }

25. Why is break used in switch statements? • Consider the code fragment below int i = 1; switch (i) { case 0: out.println(“0”); case 1: out.println(“1”); case 2: out.println(“2”); case 3: out.println(“3”); } out.println( ); • Without breaks the output is: 123

26. Symbolic Constants in switch Statements final int SUNDAY = 1, MONDAY = 2, TUESDAY = 3, WEDNESDAY = 4, THURSDAY = 5, FRIDAY = 6, SATURDAY = 7; ind d; ... switch (d) { case SUNDAY: out.print(“Sunday”); break; case MONDAY: out.print(“Monday”); break; case TUESDAY: out.print(“Tuesday”); break; case WEDNESDAY: out.print(“Wednesday”); break; case THURSDAY: out.print(“Thursday”); break; case FRIDAY: out.print(“Friday”); break; case SUNDAY: out.print(“Sunday”); break; }

27. Multiple case Labels in switch Statements switch (d) { case MONDAY: case WEDNESDAY: case FRIDAY: out.println(“C.S. meets at 9:00 today”); out.println(“Math meets at 10:00 today”); break; case TUESDAY: case THURSDAY: out.println(“English meets at 9:00 today”); out.println(“Chemistry meets at 10:00 today”); break; case SUNDAY: case Saturday out.println(“Enjoy the weekend”); }

28. switch statement switch (expression) { case value-1: statement-1; break; case value-2: statement-2; break; … case value-i: statement-i; break; default: statement-(i+1); } if equivalent switchValue = expression; if (switchValue == value-1) statement-1; else if (switchValue == value-2) statement-2; … else if (switchValue == value-i) statement-i; else statement-(i+1); Comparing switch and if statements

29. Building Classes with Multiple Methods public class Classname { // Author, date, explanation declarations of public variables public void methodName ( parameters ) { declarations of “local” variables executable statements with relevant comments } public void methodName ( parameters ) { declarations of “local” variables executable statements with relevant comments } … }