1. Lesson 7 Iteration Structures

2. Iteration is the third control structure we will explore. Iteration simply means to do something repeatedly. All iteration control structures test a condition each time before the loop is executed.

3. Types of Loops There are three types of loops that Java employs. Pretest Loop- test a condition each time before the loop is executed. Posttest Loop - test a condition after each loop execution. Fixed repetition - cause a loop to be executed a predetermined number of times.

4. Three Iteration Control Structures The three iteration control structures are the while, do/while, and for. The difference between them is the means by which they control the exiting of the loop. The while is a pretest loop. The do/while is a posttest loop. The for is a fixed repetition loop.

5. The WHILE loop If the test expression is true the loop statements are executed. If the test expression is false the loop statements are bypassed. To get out of the loop something must change the expression to false, otherwise the the result will be an infinite loop.

6. Syntax for the While Loop while (test expression) { statement1; statement2; statementn; }

7. While Loop Example (counter) //Compute 1 + 2 +... + 100 class counter { public static void main (String Args[]) { int sum = 0, cntr = 1; while (cntr <=100) { sum += cntr; cntr++; System.out.println (sum); }

8. While Loop Example (Square Roots) //Display the square roots of 25, 20, 15, and 10 class SquareRoots { public static void main (String args[]) { int number = 25; while (number >=10) { System.out.println ("The square root of " + number + " is “ + Math.sqrt(number)); number -=5; }

9. The DO/WHILE loop The do/while loop is tested at the end of the loop compared to the while which is tested at the beginning. The loop statements will always be executed once. To break the loop the test expression must become false.

10. Syntax for DO/WHILE do { statement1; statement2; statementn; } while (test expression);

11. class dowhile { public static void main(String args[]) { int myNum; //generate random numbers until we get 5 do { myNum = (int)(Math.random() * 10); System.out.print(myNum); } while (myNum !=5); }

12. Using Counters and Accumulators Numeric variables used within a repetition structure to calculate subtotals, totals, and averages –Counter Used for counting something –Accumulator Used for accumulating (adding together) something

13. Sentinel Values Values used to end loops Should be easily distinguishable from the valid data used by the program Also called trip values or trailer values

14. Sentinel values Our program can read numbers repeatedly until it encounters a special value called a sentinel that marks the end of the list. For example, if all the numbers in the list are positive, then the sentinel could be -1.

15. import TerminalIO.KeyboardReader; class sentinel { public static void main(String args[]) {KeyboardReader reader = new KeyboardReader(); double number, sum = 0; int count = 0; while (true) { number = reader.readDouble("Enter a positive number or -1 to quit: "); if (number == -1) break; sum += number; count++; } System.out.println("The average is " + sum/count); }

16. The FOR loop The for loop runs a fixed number of times The first thing that is done is the initialization of the counter. If the results of the test expression is true, then the statements are executed. Each time the loop is executed, the loop counter must be incremented or decremented.

17. Syntax for FOR loop for (initialize counter; test counter; update counter) { statement1; statement2; statementn; }

18. class Racer { public static void main (String args[]) { System.out.println("GO!"); for (int lap=1; lap <=10; lap++) { System.out.println("Completed " + lap + " laps."); } System.out.println("Finish!"); }

19. Nested Loops Many times it is desirable to have looping operations within loops. This is called nested looping. Each inner loop will as many times as desired X the number of times the outer loop runs.

20. Each iteration of the outer loop causes multiple iterations of the inner loop. class NestedForLoop { public static void main (String args[]) { for (int i = 0; i <3; i++) { for (int j = 0; j < 3; j++) { System.out.println("i = " + i + ", j = " + j); }

21. Break and Continue Option If you want a loop to terminate if some desired value is found, it is possible to use the break statement to force the loop to quit. If you want one iteration of the loop to be terminated, it is possible to use a continue statement to force the loop to quit one iteration and continue on with another.

22. Syntax for BREAK while (test expression) { statement1; statement2; if (test expression) break; statementn; }

23. class breakExample { public static void main (String args[]) { while (true) { System.out.print ("true"); break; } System.out.println(" out of loop"); } The loop will iterate once because the condition is true, but the break statement takes the program right out of the loop.

24. Syntax for CONTINUE for (initialize counter; test counter; update counter) { statement1; statement2; if (test expression) continue; statementn; }

25. The continue statement causes the loop to stop where it is and loop again from the beginning, causing an infinite loop. class ContinueExample { public static void main (String args[]) { boolean b = true; while (b) { if (b) continue; System.out.println("never gets here"); b = false; }

26. Bottoms Up On The Fifth Cup