Computer Programming with Java

1. Computer Programming with Java Chapter 3 Flow of Control

2. Contents • Overview • Branching Statements • Loop Statements • Programming with Loops

3. Overview • Flow of control • the order in which actions are performed by your program. • Branching statements • choose one action from a list of two or more possible actions • Loop statements • repeats some action again and again until some stopping condition is met

4. Branching Statements • The if-else- statement if (Boolean_Expression) statement_1 else statement_2 If the Boolean_Expression is true, then Statement_1 is executed; otherwise, Statement_2 is executed. if (Boolean_Expression) statement_1

5. public class BankBalance { public static final double OVER_DRAWN_PENALTY = 8.00; public static final double INTEREST_RATE = 0.02;//2% annually public static void main(String[] args) { double balance; System.out.print("Enter your checking account balance: $"); balance = SavitchIn.readLineDouble(); System.out.println("Original balance $" + balance); if (balance >= 0) balance = balance + (INTEREST_RATE * balance)/12; else balance = balance - OVER_DRAWN_PENALTY; System.out.println("After adjusting for one month"); System.out.println("of interest and penalties,"); System.out.println("your new balance is $" + balance); } } Display 3.1: A Program using if-else

6. Branching Statements (1) • Comparison Operators

7. public class StringEqualityDemo{ public static void main(String[] args){ String s1, s2; System.out.println("Enter two lines of text:"); s1 = SavitchIn.readLine(); s2 = SavitchIn.readLine(); if (s1.equals(s2)) System.out.println("The two lines are equal."); else System.out.println("The two lines are not equal."); if (s2.equals(s1)) System.out.println("The two lines are equal."); else System.out.println("The two lines are not equal."); if (s1.equalsIgnoreCase(s2)) System.out.println("But, the lines are equal ignoring case."); else System.out.println("Lines are not equal even ignoring case."); } } Display 3.3: Testing Strings for Equality

8. Branching Statements (2) • Boolean Expression • an expression that is either true or false. • “and”: && • When you form a larger boolean expression by connecting two smaller expression with &&, the entire larger expression is trueprovided that both of the smaller expressions are true. • (sub_expression1)&& (sub_expression2) • e.g.) if ((pressure > min) && (pressure < max)) System.out.println(“Pressure is OK.”); else System.out.println(“Warning: Pressure is out of range.”);

9. Branching Statements (3) • “or” • the meaning is essentially the same as the word “or” • (sub_expression1)|| (sub_expression2) • e.g.) if ((salary > expenses) || (savings > expenses)) System.out.println(“Solvent.”); else System.out.println(“Bankrupt.”);

10. && (and) || (or) ! (not) Display 3.12: Truth tables for boolean operators

11. Branching Statements (4) • Nested statements • an if-else statement contains two smaller statements within it. • Compound statements • these statements formed by enclosing a list of statements within curly brackets. • Rule of thumb • To eliminate some confusion in the nested if-else statements, you had better use curly brackets to group things.

12. Nested statements if (balance >= 0) if (INTEREST_RATE >= 0) balance = balance + (INTEREST_RATE * balance)/12; else System.out.println(“Cannot have a negative interest.”); else balance = balance - OVER_DRAWN_PENALTY; Compound statements if(balance >= 0){ System.out.println(“Good for you. You earned interest.”); balance = balance + (INTEREST_RATE * balance) /12; }else{ System.out.println(“You will be charged a penalty.”); balance = balance - OVER_DRAWN_PENALTY; }

13. Branching Statements (4) • Multibranch if-else statements • you can use nested if-else statements to produce multiway branches that branch into any number of possibilities. If ( Boolean_Expression1) Action1 else if ( Boolean_Expression2) Action2 … else if ( Boolean_Expression n) Action n else Default_Action if (balance >= 0) System.out.println(“Positive balance.”); else if (balance < 0) System.out.println(“Negative balance.”); else if (balance == 0) System.out.println(“Zero balance.”);

14. public class Grader{ public static void main(String[] args){ int score; char grade; System.out.println("Enter your score: "); score = SavitchIn.readLineInt(); if (score >= 90) grade = 'A'; else if (score >= 80) grade = 'B'; else if (score >= 70) grade = 'C'; else if (score >= 60) grade = 'D'; else grade = 'F'; System.out.println("Score = " + score); System.out.println("Grade = " + grade); } } Display 3.4: Multibranch if-else-Statement

15. Branching Statements (5) • The switch statement • a multiway branch that makes its decision on which way to branch based on the value of an integer or character expression switch (Controlling_Expression){ case Case_Label: statement; … break; case Case_Label: statement; … break; … default: statement; … break; }

16. public class MultipleBirths{ public static void main(String[] args){ int numberOfBabies; System.out.print("Enter number of babies: "); numberOfBabies = SavitchIn.readLineInt(); switch (numberOfBabies){ case 1: System.out.println("Congratulations."); break; case 2: System.out.println("Wow. Twins."); break; case 3: System.out.println("Wow. Triplets."); break; case 4: case 5: System.out.println("Unbelieveable."); System.out.println(numberOfBabies + " babies"); break; default: System.out.println("I don't believe you."); break; } } } Display 3.5: A switch-Statement

17. Loop Statements • Program often need to repeat some action. • Terminology • loop: A portion of a program that repeats a statement or group of statements • body: the statement (or group of statements) to be repeated in a loop • iteration: each repetition of the loop body • infinite loop: A loop that iterates its body repeatedly without ever ending • see text p116.

18. General structure of a Loop • initializing statements • loop body • mechanism for ending the loop • Rule of thumb • When you design a loop, you need to determine what action the body of the loop will take and you need to determine a mechanism for deciding when the loop should stop repeating the loop body. • How do you decide whether to use one of loop statements? • See text p 124: Choosing a Loop statement

19. Loop Statements (1) • while statements • repeats its action again and again until a controlling boolean expression becomes false. • Note that the body of a while-loop can be executed zero times. while(Boolean_Controlling_Expression){ Body } The Body may be either a simple statement or more likely, a compound statement consisting of a list of statements enclosed in curly brackets {}.

20. public class WhileDemo { public static void main(String[] args) { int count, number; System.out.println("Enter a number"); number = SavitchIn.readLineInt(); count = 1; while (count <= number) { System.out.print(count + ", "); count++; } System.out.println(); System.out.println("Buckle my shoe."); } } Display 3.6: A while-Loop

21. Loop Statements (2) • do-while statement • very similar to the while-statement, but the loop body of do-while statement is always executed at least once. do { Body }while(Boolean_Controlling_Expression); The Body may be either a simple statement or more likely, a compound statement consisting of a list of statements enclosed in curly brackets {}.

22. public class DoWhileDemo { public static void main(String[] args) { int count, number; System.out.println("Enter a number"); number = SavitchIn.readLineInt(); count = 1; do { System.out.print(count + ", "); count++; }while (count <= number); System.out.println(); System.out.println("Buckle my shoe."); } } Display 3.7: A do-while-Loop

23. public class BugTrouble{ public static final double GROWTH_RATE = 0.95;//95% per week public static final double ONE_BUG_VOLUME = 0.002;//cubic feet public static void main(String[] args){ System.out.println("Enter the total volume of your house"); System.out.print("in cubic feet: "); double houseVolume = SavitchIn.readLineDouble(); System.out.println("Enter the estimated number of"); System.out.print("roaches in your house:"); int startPopulation = SavitchIn.readLineInt(); int countWeeks = 0; double population = startPopulation; double totalBugVolume = population*ONE_BUG_VOLUME; while (totalBugVolume < houseVolume){ population = population + (GROWTH_RATE*population); totalBugVolume = population*ONE_BUG_VOLUME; countWeeks++; } Display 3.8: Roach Population Program

24. System.out.println("Starting with a roach population of ” + startPopulation); System.out.println("and a house with a volume of " + houseVolume + " cubic feet,"); System.out.println("after " + countWeeks + " weeks,"); System.out.println("the house will be filled"); System.out.println("floor to ceiling with roaches."); System.out.println("There will be " + (int)population + " roaches."); System.out.println("They will fill a volume of " + (int)totalBugVolume + " cubic feet"); System.out.println("Better call Debugging Experts Inc."); } }

25. Loop Statements (3) • for-statement • a specialized loop statement that allows you to easily convert pseudocode (see text p. 129) for(Initializing_Action; Boolean_Expression; Action_After_Each_Iteration){ Body } The Body may be either a simple statement or more likely, a compound statement consisting of a list of statements enclosed in curly brackets Do the following for each value of count from 1 to 3: System.out.println(count); System.out.println(“Go”);

27. public class ForDemo { public static void main(String[] args) { int countDown; for (countDown = 3; countDown >= 0; countDown--) { System.out.println(countDown); System.out.println("and counting."); } System.out.println("Blast off!"); } } Display 3.9: A for-statement

28. Loop Statements (4) • break-statement • if you want to end a loop in the middle of the loop body • exit Method • an invocation of the exit method ends the program. • 0: a normal termination • 1: abnormal termination • e.g.) System.exit(0);

29. public class BreakDemo{ public static void main(String[] args) { int itemNumber; double amount, total; System.out.println("You may buy ten items, but"); System.out.println("the total price must not exceed $100."); total = 0; for (itemNumber = 1; itemNumber <= 10; itemNumber++) { System.out.print("Enter cost of item #" + itemNumber + ": $"); amount = SavitchIn.readLineDouble(); total = total + amount; if (total >= 100) { System.out.println("You spent all your money."); break; } System.out.println("Your total so far is $" + total); System.out.println("You may purchase up to "+ (10 - itemNumber) + "more items."); } System.out.println("You spent $" + total); } } Display 3.10: Ending a loop with a break-statement