Chapter 6 Repetition Asserting Java © Rick Mercer 1

Algorithmic Pattern: The Determinate loop We often need to perform some action a specific number of times: Produce 89 paychecks. Count down to 0 (take 1 second of the clock). Compute grades for 81 students The determinate loop pattern repeats some action a specific number of times. 2

Determinate Loops with while The determinate loop pattern can be implemented with the Java while loop This template repeats a process n times: int n = /* how often we must repeat the process */ int counter = 1; while ( counter <= n ) { // the process to be repeated counter = counter + 1; } determinate loops must know the number of repetitions before they begin: know exactly how many employees, or students, or whatever that must be processed, for example 4

Example while loop produces an average Scanner keyboard = new Scanner(System.in); double sum = 0.0; double number; System.out.print("How many do you want to average? "); int n = keyboard.nextInt(); int counter = 1; // Do something n times while (counter <= n) { System.out.print("Enter number: "); // <- Repeat 3 number = keyboard.nextDouble(); // <- statements sum = sum + number; // <- n times counter = counter + 1; // make sure the loop stops } double average = sum / n; System.out.print("Average of "+ n + " numbers is "+ average);

Active Learning What is the output? : int j = 1; int n = 5; while(j <= n) { System.out.print(j + " "); j = j + 1; } j = 0; while(j <= 2 * n) { j = j + 2;

Indeterminate Loops Determinate loops have a limitation We must know n (the number of repetitions) in advance Many situations need us to repeat a set of statements an unspecified number of times: Processing report cards for every student in a school (or paychecks for all employees, or...) Allowing 1 to many ATM transactions Asking the user for specific input and allowing re-entry of input after invalid inputs 7

Some things that terminate indeterminate loops An indeterminate loop repeats a process until some stopping event terminates the repetition There are many such events, but we'll focus on these: User enters a special value indicating end of data A logical expression becomes false The Grid's mover hits the wall or an edge The end of a file is encountered Indeterminate loops do not need to know n in advance Indeterminate loops can actually determine n 8

Pattern Indeterminate loop Problem Some process must repeat an unknown number of times so some event is needed to terminate the loop. Algorithm while( the termination event has not occurred ) { execute several actions1 bring the loop closer to termination } Code while(dog.frontIsClear()) { dog.move(); Example }

While loop with a Scanner Sometimes a stream of input from the keyboard or a file needs to be read until there is no more data in the input stream Consider a Scanner object constructed with a String argument The string represents an input stream You will need Scanner in ControlFun

These assertions pass @Test public void showScanner() { Scanner scannerWithInts = new Scanner("1 2 3"); assertEquals(1, scannerWithInts.nextInt()); assertEquals(2, scannerWithInts.nextInt()); assertEquals(3, scannerWithInts.nextInt()); Scanner scanner = new Scanner("There are five words here."); assertEquals("There", scanner.next()); assertEquals("are", scanner.next()); assertEquals("five", scanner.next()); assertEquals("words", scanner.next()); assertEquals("here.", scanner.next()); }

Write method average to return the average of the doubles in a Scanner @Test public void testNum100s() { LoopFun cf = new LoopFun(); Scanner scanner1 = new Scanner("80.0 70.0 90.0"); Scanner scanner2 = new Scanner("1.0\t 2\n 3. 4.0"); assertEquals(80.0, cf.average(scanner). 0.01); assertEquals(2.5, cf.average(scanner). 0.01); }

Let sumOfNegatives return the sum of the negative integers in a Scanner @Test public void testSumOfNegs() { ControlFun cf = new ControlFun(); Scanner scanner0 = new Scanner("1 2 3"); Scanner scannerA = new Scanner("1 -2 3 -4"); Scanner scannerB = new Scanner("-1 -2 -3 -4"); assertEquals(0, cf.sumOfNegatives(scanner0)); assertEquals(-6, cf.sumOfNegatives(scannerA)); assertEquals(-10, cf.sumOfNegatives(scannerB)); }

What's wrong with this method? public int sumOfNegatives(Scanner scanner) { int result = 0; while (scanner.hasNextInt()) { if (scanner.nextInt() < 0) { result += scanner.nextInt(); } return result;