1. Chapter 5 Conditionals and Loops

2. © 2004 Pearson Addison-Wesley. All rights reserved5-2 The switch Statement The switch statement provides another way to decide which statement to execute next The switch statement evaluates an expression, then attempts to match the result to one of several possible cases Each case contains a value and a list of statements The flow of control transfers to statement associated with the first case value that matches

3. © 2004 Pearson Addison-Wesley. All rights reserved5-3 The switch Statement The general syntax of a switch statement is: switch ( expression ) { case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case... } switch and case are reserved words If expression matches value2, control jumps to here

4. © 2004 Pearson Addison-Wesley. All rights reserved5-4 The switch Statement eachOften a break statement is used as the last statement in each case's statement list A break statement causes control to transfer to the end of the switch statement If a break statement is not used, the flow of control will continue into the next case Sometimes this may be appropriate, but often we want to execute only the statements associated with one case

5. © 2004 Pearson Addison-Wesley. All rights reserved5-5 The switch Statement switch (option) { case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break; } An example of a switch statement: Assumes we have a character variable named option that has a character value… as in char option = ‘C’;

6. © 2004 Pearson Addison-Wesley. All rights reserved5-6 The switch Statement A switch statement can have an optional default case The default case has no associated value and simply uses the reserved word default  If the default case is present, control will transfer to it if no other case value matches  if there is no default case, and no other value matches, control falls through to the statement after the switch

7. © 2004 Pearson Addison-Wesley. All rights reserved5-7 The switch Statement  The expression of a switch statement must result in an integral type, meaning an int or a char  It cannot be a boolean value, a floating point value ( float or double ), or another integer type The implicit boolean condition in a switch statement is equality You cannot perform relational checks with a switch statement See GradeReport.java (page 225)GradeReport.java

8. © 2004 Pearson Addison-Wesley. All rights reserved5-8 public class GradeReport { //------------------------------ Again, I omitted lines so I could fit this on one slide. public static void main (String[] args) { int grade, category; Scanner scan = new Scanner (System.in); // we use buffered reader…. System.out.print ("Enter a numeric grade (0 to 100): "); grade = scan.nextInt(); // we use Wrapper classes to convert the String grade to an int grade. category = grade / 10; // What does this do? System.out.print ("That grade is "); switch (category) { case 10: System.out.println ("a perfect score. Well done."); break; case 9: System.out.println ("well above average. Excellent."); break; case 8: System.out.println ("above average. Nice job."); break; case 7: System.out.println ("average."); break; case 6: System.out.println ("below average. You should see the"); System.out.println ("instructor to clarify the material " + "presented in class."); break; default: System.out.println ("not passing."); } // end switch } // end main() } // end class

9. © 2004 Pearson Addison-Wesley. All rights reserved5-9 Outline The if Statement and Conditions Other Conditional Statements Comparing Data The while Statement Iterators Other Repetition Statements Decisions and Graphics More Components

10. © 2004 Pearson Addison-Wesley. All rights reserved5-10 Comparing Data – Very Important!! When comparing data using boolean expressions, it's important to understand the nuances of certain data types Let's examine some key situations:  Comparing floating point values for equality  Comparing characters  Comparing strings (alphabetical order)  Comparing object vs. comparing object references

11. © 2004 Pearson Addison-Wesley. All rights reserved5-11 Float Values Comparing Float Values  You should rarely use the equality operator ( == ) when comparing two floating point values ( float or double ) Two floating point values are equal only if their underlying binary representations match exactly Computations often result in slight differences that may be irrelevant In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal

12. © 2004 Pearson Addison-Wesley. All rights reserved5-12 Comparing Float Values To determine the equality of two floats, you may want to use the following technique: if (Math.abs(f1 - f2) < TOLERANCE) System.out.println ("Essentially equal"); If the difference between the two floating point values is less than the tolerance, they are considered to be equal The tolerance could be set to any appropriate level, such as 0.000001

13. © 2004 Pearson Addison-Wesley. All rights reserved5-13 Characters Comparing Characters As we've discussed, Java character data is based on the Unicode character set Unicode establishes a particular numeric value for each character, and therefore an ordering We can use relational operators on character data based on this ordering  For example, the character '+' is less than the character ' J' because it comes before it in the Unicode character set  Appendix C provides an overview of Unicode  Look at this Appendix!!

14. © 2004 Pearson Addison-Wesley. All rights reserved5-14 Comparing Characters In Unicode, the digit characters (0-9) are contiguous and in order Likewise, the uppercase letters (A-Z) and lowercase letters (a-z) are contiguous and in order CharactersUnicode Values 0 – 948 through 57 A – Z65 through 90 a – z97 through 122

15. © 2004 Pearson Addison-Wesley. All rights reserved5-15 Strings Comparing Strings objectRemember that in Java a character string is an object equals methodThe equals method can be called with strings to determine if two strings contain exactly the same characters in the same order The equals method returns a boolean result Assuming name1 and name2 are objects of type String and already have values, we can say: if (name1.equals(name2)) System.out.println ("Same name"); Note the syntax!!! This is how you do it.

16. © 2004 Pearson Addison-Wesley. All rights reserved5-16 Comparing Strings  We cannot use the relational operators to compare strings comes beforeThe String class contains a method called compareTo to determine if one string comes before another  (like if we are sorting or alphabetizing…) A call to name1.compareTo(name2)  returns zero if name1 and name2 are equal (contain the same characters)  returns a negative value if name1 is less than name2  returns a positive value if name1 is greater than name2

17. © 2004 Pearson Addison-Wesley. All rights reserved5-17 Comparing Strings – VIP Example!! if (name1.compareTo(name2) < 0) System.out.println (name1 + "comes first"); else if (name1.compareTo(name2) == 0) System.out.println ("Same name"); else System.out.println (name2 + "comes first"); Because comparing characters and strings is based on a character set, it is called a lexicographic ordering

18. © 2004 Pearson Addison-Wesley. All rights reserved5-18 Lexicographic Ordering not strictly alphabetical  Lexicographic ordering is not strictly alphabetical when uppercase and lowercase characters are mixed For example, the string "Great" comes before the string "fantastic" because all of the uppercase letters come before all of the lowercase letters in Unicode Also, short strings come before longer strings with the same prefix (lexicographically) Therefore "book" comes before "bookcase“  Characters are compared one-by-one until a difference is discovered!!

19. © 2004 Pearson Addison-Wesley. All rights reserved5-19 Comparing Objects The == operator can be applied to objects – it returns true if the two references are aliases of each other  What does this mean to you??? The equals method is defined for all objects, but unless we redefine it when we write a class, it has the same semantics as the == operator has beenIt has been redefined in the String class to compare the characters in the two strings When you write a class, you can redefine the equals method to return true under whatever conditions are appropriate. More later….

20. © 2004 Pearson Addison-Wesley. All rights reserved5-20 Outline The if Statement and Conditions Other Conditional Statements Comparing Data The while Statement Iterators Other Repetition Statements Decisions and Graphics More Components

21. © 2004 Pearson Addison-Wesley. All rights reserved5-21 Repetition Statements Repetition statements allow us to execute a statement multiple times Often they are referred to as loops Like conditional statements, they are controlled by boolean expressions  Java has three kinds of repetition statements:  the while loop  the do loop  the for loop  The programmer should choose the right kind of loop for the situation

22. © 2004 Pearson Addison-Wesley. All rights reserved5-22 The while Statement A while statement has the following syntax: while ( condition ) statement; If the condition is true, the statement is executed Then the condition is evaluated again, and if it is still true, the statement is executed again The statement is executed repeatedly until the condition becomes false

23. © 2004 Pearson Addison-Wesley. All rights reserved5-23 Logic of a while Loop statement true false condition evaluated Note: there is a ‘pretest.’ What does this mean??

24. © 2004 Pearson Addison-Wesley. All rights reserved5-24 The while Statement An example of a while statement: int count = 1; while (count <= 5) { System.out.println (count); count++; } If the condition of a while loop is false initially, the statement is never executed Therefore, the body of a while loop will execute zero or more times

25. © 2004 Pearson Addison-Wesley. All rights reserved5-25 The while Statement Let's look at some examples of loop processing A loop can be used to maintain a running sum A sentinel value is a special input value that represents the end of input See Average.java (page 229)Average.java A loop can also be used for input validation, making a program more robust See WinPercentage.java (page 231)WinPercentage.java Look at these two routines. You may see them again! If you have any questions, please let me know!!

26. © 2004 Pearson Addison-Wesley. All rights reserved5-26 Infinite Loops The body of a while loop eventually must make the condition false If not, it is called an infinite loop, which will execute until the user interrupts the program  This is a common logical error You should always double check the logic of a program to ensure that your loops will terminate normally

27. © 2004 Pearson Addison-Wesley. All rights reserved5-27 Infinite Loops An example of an infinite loop: int count = 1; while (count <= 25) { System.out.println (count); count = count - 1; } This loop will continue executing until interrupted (Control-C) or until an underflow error occurs

28. © 2004 Pearson Addison-Wesley. All rights reserved5-28 Nested Loops Similar to nested if statements, loops can be nested as well That is, the body of a loop can contain another loop For each iteration of the outer loop, the inner loop iterates completely

29. © 2004 Pearson Addison-Wesley. All rights reserved5-29 Nested Loops – Very Important e.g. How many times will the string "Here" be printed? count1 = 1; while (count1 <= 10) { count2 = 1; while (count2 <= 20) { System.out.println ("Here"); count2++; }// end inner while count1++; } // end out while