Decision Statements in Flowcharts: Logic and Control

1. Chapter 6 Decisions and Conditions

2. Objectives • Read and create flowcharts that show the logic of a selection process. • Use if statements to control the flow of logic. • Understand and use nested ifs. • Evaluate conditions using the relational operators. • Combine conditions using the logical operators. • Perform validation on numeric fields. • Determine the event that caused the actionPerformed method to be called. • Understand the precedence and relationship of assignment, arithmetic, logical, and relational operators.

3. Decision Statements • The decision statements are one of the most powerful statements because it has the ability to make decisions and take alternate courses of action based on the outcome. • The decision made by the computer is formed as a question: Is the given condition true or false? If it is true do one thing and if it is false do something else.

4. The if Statement-General Form if (condition) statement; [else statement] if (condition) { statement(s); } [else { statement(s); }]

5. The if Statement-Examples if (fltHours > 40.0f) { CalculateOvertime(); } if (intGrade >= 70) { lblGrade.setText("Pass"); } else { lblGrade.setText("No Pass"); } if (intAge < 12) CalculateDiscount();

6. Decision Statements • The curly braces become important when you have more than one statement after the if or else statement. • Let us look at Examples: • intAge = 12 • Example 1: if( intAge < 12) • CalculateDiscount(); • DisplayDiscount(); • Example 2: if( intAge < 12) • { • CalculateDiscount(); • DisplayDiscount(); • }

7. Decision Statements Continued • It is good programming practice to have curly brackets regardless of the number of statements you have in the if or else clauses. • If you place a semicolon after the if statement. You will not get a syntax error but the if statement will be terminated after the condition.

8. Conditions • You can form conditions with numeric variables and constants, character (char) variables and constants, and arithmetic expressions. • Note that the relational operator for equality is = = signs, as one equal sign is an assignment operator. • If you use an assignment operator in a condition you will receive an error as the condition has to evaluate to a boolean.

9. Sample Comparison Section Sample Comparisons int intAlpha = 5; int intBravo = 4; int intCharlie = -5; `4

10. Comparing Data • The comparison of one character to another is based on the ASCII code. • The comparison of a character with or without single quotes produces different evaluations. • For example : chrSex = = ‘F’ // this will compare to the letter F. • For example : chrCode != ‘\0’ // this will compare with a null character and not 0. • For example : chrCode = = ‘9’ // this will compare to the digit 9.

11. Comparing Data Continued • For example : chrCode = = ‘9’ // this will compare to the ASCII code 9 which is a tab character. • For example : char chrQuestionMark = ‘?’, chrExclamation = ‘!’; • The condition (chrQuestionMark < chrExclamation) will evaluate to false. • chrQuestionMark has the ASCII value of 63 • chrExclamation has the ASCII value of 33 • If you wanted to compare individual characters in a string, you can by obtaining a single character with the charAt method. • For example : (strName.charAt(0) = = ‘A’)

12. Comparing Numeric Wrapper Classes • When using numeric wrapper classes you will have to use the equals method to compare the numeric wrapper objects.

13. Comparing Strings • You can use methods of the String class to compare String objects to String objects or string literals enclosed in quotes. • If you are testing for equality, you can use the equals method or equalsIgnoreCase method. • If you wish to compare strings in alphabetical order then you need to use the compareTo method. It will compare greater than or less than.

14. The Equals Method • The string class equals method returns true if the strings are the same and false if they differ. • As soon as a character in one string is not equal to the corresponding character in the second string, the comparison is terminated, and condition returns false. • Let’s look at an example : • String strName = new String(“Joan”); • String strName = new String(“John”); • (strName.equals(strName2)) will evaluate to false because a has a lower ranking than h in John.

15. The equalsIgnoreCase Method • Let’s look at an example: • String strName = new String(“joan”); • String strName2 = new String(“JOAN”); • (strName.equals(strName2)); //evaluates to false • (strName.equalsIgnoreCase(strName2)); // evaluates to true

16. The compareTo Method • The compareTo method returns an integer with one of three possible values. • If strString1 is greater than strString2, a positive value is returned. • If strString2 is greater than strString1, a negative value is returned. • To use this method effectively, you can set up a condition using the return value and relational operator.

17. The compareTo Method Continued • (strString1.compareTo(strString2) = = 0) //Are the strings equal • (strString1.compareTo(strString2) ! = 0) //Are the strings different • (strString1.compareTo(strString2) > 0) //Is strString1 greater than strString2 • What happens when one word is shorter than the other? How does it compare? • The shorter word is padded with blanks to have the same length and the comparison begins.

18. ASCII Code

19. The Logical Operators • You can test multiple conditions using logical operators to combine conditions. Logical Operators

20. Precedence of Logical Operators in Compound Conditions • && operator has higher precedence than the || operator and the conditions are read left to right. • To alter the order of evaluation, use parentheses. • Beware of smart compilers, once they have determined that the condition is false, they will not evaluate the rest of the condition.

21. A. First Method: if (intTemp <= 32) { lblComment.setText("Freezing"); } else { if (intTemp > 80) { lblComment.setText("Hot"); } else { lblComment.setText("Moderate); } } B. Second Method: if (intTemp <= 32) { lblComment.setText("Freezing"); } else if (intTemp > 80) { lblComment.setText("Hot"); } else { lblComment.setText("Moderate); } A. Let us look at example of nested if: if(intAge < 21) { if(chrSex = = “M”) strCategory = “BOY”; else(chrSex = = “F”) strCategory = “GIRL”; } else { strCategory = “ADULT”; } Nested If Statements

22. True False intTemp>80? lblComment.Caption = “Freezing” True False lblComment.Caption = “Moderate” Flowcharting a Nested if Statement intTemp>32? lblComment.Caption = “Hot”

23. Conditional Operator • You can write decisions using Java’s conditional operator which is the shortcut version of if then else.

24. The Conditional Operator – General Format (condition) ? TrueResult : FalseResult

25. The Conditional Operator -Examples fltRate = (fltSales < 10000f) ? .05f : .1f; fltCommission = (fltSales < 10000f) ? fltSales * .05f : fltSales * .1f;

26. Validating User Input • Checking to verify the data entered is correct is called validation. • Validation may include making sure that data is numeric, checking for specific values, checking a range of values, or making sure that the required item is entered. • We separate the user interface code form the code to handle processing. • We check for the input data in the applet class and the business rules in the processing class.

27. Checking for Business Rules • Often you must check an input value to make sure that it follows business rules. • Business rules may be a rate of pay, a check amount, or the amount of hours worked do not exceed a certain limit.

28. Validating in a Class • So for invalid data the instance variables are set to negative 1. • This is one way of indicating bad data, the other is checking with a boolean variable. • In the applet that instantiates the processing class, you can check for the instance variables if they passed the validation, to continue processing or not. • Using this technique, you properly separate the UI components for the business rules and calculations.

29. Passing Variables as Arguments of a Method • When you set up a class, you need to decide whether to assign class instance variables in the class constructor or pass the variables as arguments in a method. • One drawback is cannot send any values back through a constructor. • One of the reasons setting the instance variables to –1 and the get methods to see the value of the variables.

30. Passing Variables as Arguments of a Method Continued • You can also use boolean variables to indicate bad data. The boolean variable indicates if the input values passed business rules validation. • For example: • if (myPayroll.blnInvalidRate) //did not pass validation if • { //condition is true • txtRate.selectAll(); • txtRate.selectAll(); • showStatus(“Invalid Data”); • }

31. Checking for Numeric Values • The best way to check for non-numeric data is by catching exceptions. • You can also use the isNaN method (is not a number) of the Double and Float wrapper classes.

32. Programming with Multiple Buttons • For any component that has an actionListener assigned triggers the same actionPerformed method. • You need to determine which object triggered the actionPerformed. • Java passes an ActionEvent object as an argument to the method, in which you give the argument a name(evt) - actionPerformed (ActionEvent evt). • The getSource method on the object will give the name of the object that triggered the event.

33. Programming with Multiple Buttons Continued • For example: Object objSource = evt.getSource(); • The next step is to use a condition to compare the object with the names of your components. • For example : if(objSource = = btnCalculate • || objSource = = txtHours • || objSource = = txtRate) • { • CalculatePay(); • } • else • { • ClearTextFields(); • }

34. Disabling and Enabling Buttons • You can disable a button by graying it out or you can hide a button. It not advisable to hide a button as it frustrates the user. • You use the setEnabled method for the enabling and disabling and setVisible method to show and hide.

35. The setEnabled Method--General Format object.setEnabled(booleanValue);

36. The setEnabled Method--Examples btnCalculate.setEnabled(true) ; btnClear.setEnabled(false) ;

37. Precedence of Operators * Increment and Decrement operators read right to left