1. Loops and Iteration for Statements, while Statements and do-while Statements

2. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 2 Loops Sometimes we want to execute a statement or a group of statements repeatedly Java structures for loops: for statement while statement do-while statement

3. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 3 Example applyInterest() method in BankAccount (takes an intRate parameter) public class BankAccount { double balance; // make balance double so it can handle centavos... public void applyInterest( double intRate ) { double interest = balance*intRate/100; balance += interest; System.out.println( “Interest:” + interest +”, balance is now” + balance ); }... }

4. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 4 Compounding Interest Suppose interest is computed monthly but compounded quarterly public void applyQtrlyInterest( double intRate ) { double interest; interest = balance*intRate/100; balance += interest; System.out.println( “Month 1 - interest:” + interest +”, balance:” + balance ); interest = balance*intRate/100; balance += interest; System.out.println( “Month 2 - interest:” + interest +”, balance:” + balance ); interest = balance*intRate/100; balance += interest; System.out.println( “Month 3 - interest:” + interest +”, balance:” + balance ); }

5. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 5 Repeated Execution We want a way to indicate that the following should be executed three times: interest = balance*intRate/100; balance += interest; System.out.println( “Monthly-interest:” + interest +”, balance:” + balance ); Better still, we want the following executed where i takes the value 1, 2, 3: interest = balance*intRate/100; balance += interest; System.out.print( “Month ” + i + “- interest: “+ interest ); System.out.println( “, balance:” + balance );

6. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 6 The for Statement Syntax for ( expr1; expr2; expr3 ) statement Notes expr1: initialization or setup (e.g., i = 1 ) expr2: condition (e.g., i <= 3 ) expr3: increment (e.g., i++) statement means any valid statement in Java (including blocks)

7. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 7 Compounding Interest public void applyQtrlyInterest( double intRate ) { double interest; int i; for( i = 1; i <= 3; i++) { interest = balance*intRate/100; balance += interest; System.out.print( “Month ” + i + “- interest: “+ interest ); System.out.println( “, balance:” + balance ); }

8. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 8 Another Example: Factorial Given an integer n, compute n! We want: result = 1*2*3*…*n; Repeated operation(s) multiply a number i to result increment the number i Do n times starting with i = 1, result = 1: result = result * i; i = i + 1; Looping code aka “Iterative” code one round of the loop is called an “iteration” properly pronounced “ih-teration” not “eye-teration” (think of how you say “reiterate”)

9. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 9 The while Statement public int factorial( int n ) { int result = 1; int i = 1; while ( i <= n ) { result = result * i; i = i + 1; } return result; } Condition Loop Body Setup Increment / Go to next step Syntax: while ( condition ) statement

10. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 10 The do-while Statement public int factorial( int n ) { int result = 1; int i = 1; do { result = result * i; i = i + 1; } while ( i <= n ); return result; } Condition (at the end of loop) Loop Body Setup Increment / Go to next step Syntax: do statement while ( condition );

11. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 11 Components of a Loop Setup/Initialization Terminating/Continuing condition Incrementing step not necessarily an increment, but something that moves the program further on – i.e., to a state possibly closer to a terminating condition e.g., decrementing, dividing, multiplitying, getting another input, etc. Loop body

12. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 12 Using a for loop for factorial public int factorial( int n ) { int result = 1; int i; for ( i = 1; i <= n; i++ ) { result = result * i; } return result; } Condition Loop Body SetupIncrement / Go to next step

13. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 13 for loop (version 2) public int factorial( int n ) { int result; result = 1; for ( int i = 1; i <= n; i++ ) { result = result * i; } return result; } You can declare the “counter” variable inside the for scope is within the loop’s block good when i is not used outside of the loop

14. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 14 for loop (version 3) public int result( int n ) { int i, result; for ( result = 1, i = 1; i <= n; i++ ) { result = result * i; } return result; } You can have multiple statements in “setup” part of for separated by commas need to declare the variables before the for, since we can’t have declarations here also works for increment part NOT RECOMMENDED! generally bad style to put several statements on one line if possible, choose a single counter variable, and just use that

15. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 15 for loop (version 3b) public int factorial( int n ) { int i, result; for ( result = 1, i = 1; i <= n; result *= i, i++ ) { } return result; } This is legal, but BAD! cryptic the for loop has no body!

16. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 16 for loop (version 3w) public int factorial( int n ) { int i, result; for ( result = 1, i = 1; i <= n; result *= i++ ) { } return result; } (“w” for worse!) even more cryptic Some C programmers actually like writing like this! DON’T!

17. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 17 for and while are equivalent! public int factorial( int n ) { int result; result = 1; { int i = 1; while ( i <= n ) { result = result * i; i++; } return result; } public int factorial( int n ) { int result; result = 1; for ( int i = 1; i <= n; i++ ) { result = result * i; } return result; } Setup Condition Increment Braces here are shown because in Java (not in C), scope of vars declared in setup of for is limited

18. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 18 Deciding which statement to use Using a for statement most appropriate when the number of iterations is known (e.g., factorial of n) Difference between while and do-while loop condition is performed at the top (while) or at the bottom (do-while) of the loop in do-while, body is executed at least once in while, we check first before executing

19. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 19 Nested Loops It is possible to have a loop within a loop Example (What gets printed out?) for ( int i = 0; i < 5; i++ ) { System.out.println( i ); for ( int j = 0; j < 5; j++ ) { System.out.println( j ); }

20. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 20 Nested Loops The condition can vary depending on the outer loop Example (What gets printed out?) for ( int i = 0; i < 5; i++ ) { System.out.println( i ); for ( int j = 0; j < i; j++ ) { System.out.print( j ); } System.out.println( “” ); }

21. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 21 Loop Pitfall # 1 Infinite Loops Both loops will not terminate because the boolean expressions will never become false. Infinite Loops Both loops will not terminate because the boolean expressions will never become false. int count = 1; while ( count != 10 ) { count = count + 2; } 2 2 int product = 0; while ( product < 500000 ) { product = product * 5; } 1 1 examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill

22. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 22 Loop Pitfall # 2 Using Real Numbers Loop 2 terminates, but Loop 1 does not because a float or double is only an approximation of a real number. Operations are not necessarily exact. Using Real Numbers Loop 2 terminates, but Loop 1 does not because a float or double is only an approximation of a real number. Operations are not necessarily exact. float count = 0.0f; while ( count != 1.0f ) { count = count + 0.33333333f; } //eight 3s 2 2 float count = 0.0f; while ( count != 1.0f ) { count = count + 0.3333333f; }//seven 3s 1 1 examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill

23. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 23 Loop Pitfall - 3 Goal: Execute the loop body 10 times. count = 1; while ( count < 10 ) {... count++; } 1 1 count = 0; while ( count <= 10 ) {... count++; } 3 3 count = 1; while ( count <= 10 ) {... count++; } 2 2 count = 0; while ( count < 10 ) {... count++; } 4 4 1 1 3 3 andexhibit off-by-one error (OBOE). examples and slide adapted from Introduction to Object Oriented Programming with Java by C. Thomas Wu, Copyright 2000, McGraw-Hill

24. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 24 Avoiding Pitfalls Infinite Loop loop body or increment statement should contain a statement that eventually leads to termination Real Numbers Avoid using == or != when using real numbers Use = depending on direction of loop OBOE (Off-By-One-Error) aka “Fencepost error” To execute the loop body N times … if counter starts at 0, check for counter < N if counter starts at 1, check for counter <= N Remember that after the loop, the counter (if still in scope) will be beyond the limit of your condition if we started from 0, counter would be N, if we started from 1, counter would be N+1

25. 6/28/2004 Copyright 2004, by the authors of these slides, and Ateneo de Manila University. All rights reserved L9: Loops Slide 25 Some Exercises Using Loops List all even numbers less than a given limit Approach 1: Use an if-statement inside a for-loop Approach 2: Arrange it so that the for-loop does skips through the odd numbers Print out all 16 pairs of numbers from the set {0,1,2,3} What if the numbers have to be distinct? What if the the order does not matter (i.e., the pair 1,2 is the same as the pair 2,1) Print out a block of asterisks of a given size