Chapter 3:Decision Structures

1. Chapter 3:Decision Structures

2. Chapter 3:Decision Structures • 3.1 The if Statement • 3.2 The if-else Statement • 3.3 The if-else-if Statement  • 3.4 Nested if Statements • 3.5 Logical Operators • 3.6 Comparing String Objects • 3.7 More About Variable Declaration and Scope • 3.9 The switch Statement • 3.10 Creating Objects with the DecimalFormat Class • 3.12 Common Errors to Avoid

3. The if Statement • The code in methods executes sequentially. • The if statement allows the programmer to make decisions on whether a section of code executes or not. • The if statement uses a boolean expression as an argument to decide if the next statement or block of statements executes. if(boolean expression is true) execute next statement.

4. Yes Is it cold outside? Wear a coat. Flowcharts • If statements can be modeled as a flow chart. if (coldOutside) wearCoat(); No

5. Yes Is it cold outside? Flowcharts • A block if statement may be modeled as: if (coldOutside){ wearCoat(); wearHat(); wearGloves(); } Wear a coat. Wear a hat. Wear gloves. Note the use of curly braces to block several statements together.

6. Relational Operators • In most cases, the boolean expression, used by the if statement, uses relational operators.

7. Boolean Expressions • A boolean expression is any variable or calculation that results in a true or false condition.

8. If Statements and Boolean Expressions if (x > y) System.out.println(“X is greater than Y”); if(x == y) System.out.println(“X is equal to Y”); if(x != y){ System.out.println(“X is not equal to Y”); x = y; System.out.println(“However, now it is.”); }

9. AverageScore.java double score1; double score2; double score3; double average; String input; input = JOptionPane.showInputDialog(“Enter ” + “ score #1:”); score1 = Double.parseDouble(input);

10. AverageScore.java input = JOptionPane.showInputDialog("Enter score #2:"); score2 = Double.parseDouble(input); input = JOptionPane.showInputDialog("Enter score #3:"); score3 = Double.parseDouble(input); average = (score1+score2+score3) / 3.0;

11. AverageScore.java JOptionPane.showMessageDialog(null, "The average is " + average); if (average > 95) JOptionPane.showMessageDialog(null, "That's a great score!"); } }

12. Programming Style and if Statements • If statements can span more than one line; however, they are still one statement. if(average > 95) System.out.println(“That’s a great score!”); • is functionally equivalent to if(average > 95) System.out.println(“That’s a” + “great score!”);

13. Programming Style and if Statements • Rules of thumb: • The conditionally executed statement should be on the line after the if condition. • The conditionally executed statement should be indented one level from the if condition. • If an if statement does not have the block curly braces, it is ended by the first semicolon encountered after the if condition. if(expression) statement; No semicolon here.Semicolon ends statement here.

14. Block if Statements • Conditionally executed statements can be grouped into a block by using curly braces {} to enclose them. • If curly braces are used to group conditionally executed statements, the if statement is ended by the closing curly brace. if(expression){ statement1; statement2; } Curly brace ends the statement.

15. Block if Statements • Remember that if the curly braces are not used, then only the next statement after the if condition will be executed conditionally. if(expression) statement1; statement2; statement3; Only this statement is conditionally executed.

16. Flags • A flag is a boolean variable that monitors some condition in a program. • When a condition is true, the flag is set to a true value. • The flag can be tested to see if the condition has changed. if(average > 95) highScore = true; • Later, this condition can be tested: if(highScore) System.out.println(“That’s a high score!);

17. Comparing Characters • Characters can be tested using the relational operators. • Characters are stored in the computer using the unicode character format. • Unicode is stored as a sixteen (16) bit number. • Characters are ordinal, meaning they have an order in the unicode character set. • Since characters are ordinal, they can be compared to each other. char c = ‘A’; if(c < ‘Z’) System.out.println(“A is less than Z”);

18. if-else Statements • The if-else statement adds the ability to conditionally execute code based if the expression of the if statement is false. if(expression) statementOrBlockIfTrue; else statementOrBlockIfFalse;

19. Division.java double number1, number2; // Division operands double quotient; // Result of division number1 = Double.parseDouble( JOptionPane.showInputDialog( “Enter a number: “)); number2 = Double.parseDouble( JOptionPane.showInputDialog( “Enter another number: “)); if (number2 == 0){ System.out.println("Division by zero is not possible."); System.out.println("Please run the program again and "); System.out.println("enter a number other than zero."); }

20. Division.java else{ quotient = number1 / number2; System.out.print("The quotient of " + number1); System.out.print(" divided by " + number2); System.out.println(" is " + quotient); }

21. Yes Is it cold outside? Wear shorts. Wear a coat. if-else Statement Flowcharts No

22. if-else-if Statements • if-else-if statements can become very complex. • Imagine the following decision set. if it is very cold, wear a heavy coat, else, if it is chilly, wear a light jacket, else, if it is windy wear a windbreaker, else, if it is hot, wear no jacket.

23. if-else-if Statements if (expression) statement or block else if (expression) statement or block // Put as many else ifs as needed here else statement or block • Care must be used since else statements match up with the immediately preceding unmatched if statement.

24. if-else-if Flowchart

25. TestResults.java int testScore; // Numeric test score String input; // To hold the user's input = JOptionPane.showInputDialog( "Enter your numeric test score and I” +“ will tell you the grade: "); testScore = Integer.parseInt(input);

26. TestResults.java if (testScore < 60) JOptionPane.showMessageDialog(null, "Your grade is F."); else if (testScore < 70) JOptionPane.showMessageDialog(null, "Your grade is D."); else if (testScore < 80) JOptionPane.showMessageDialog(null, "Your grade is C."); else if (testScore < 90) JOptionPane.showMessageDialog(null, "Your grade is B."); else if (testScore <= 100) JOptionPane.showMessageDialog(null, "Your grade is A."); else // Invalid score JOptionPane.showMessageDialog(null, "Invalid score.");

27. Nested if Statements • If an if statement appears inside of another if statement (single or block) it is called a nested if statement. • The nested if is only executed if the if statement it is in results in a true condition. • Nested if statements can get very complex, very quickly.

28. LoanQualifier.java double salary, yearsOnJob; String input; input = JOptionPane.showInputDialog("Enter your " + "annual salary."); salary = Double.parseDouble(input); input = JOptionPane.showInputDialog("Enter the number of " +"years at your current job."); yearsOnJob = Double.parseDouble(input);

29. LoanQualifier.java if (salary >= 30000){ if (yearsOnJob >= 2){ JOptionPane.showMessageDialog(null, "You qualify " + "for the loan."); } else { JOptionPane.showMessageDialog(null, "You must have " + "been on your current job for at least " + "two years to qualify."); } } else { JOptionPane.showMessageDialog(null, "You must earn at” +“least $30,000 per year to qualify."); }

30. No Yes Is it cold outside? Wear shorts. Is it snowing? Yes No Wear a jacket. Wear a parka. Nested if Statement Flowcharts

31. if-else Matching • Curly brace use is not required if there is only one statement to be conditionally executed. • However, sometimes curly braces can help make the program more readable. • Additionally, proper indentation makes it much easier to match up else statements with their corresponding if statement.

32. if-else Matching if (employed == 'y') { if (recentGrad == 'y') { System.out.println("You qualify for the special interest rate."); } else { System.out.println("You must be a recent college graduate to qualify."); } } else { System.out.println("You must be employed to qualify."); } This else matches with this if. This else matches with this if.