Chapter 6A While Loops Asserting Java © Rick Mercer

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.

Determinate Loops Determinate Loops  The determinate loop pattern can be implemented with the Java while loop  This template executes some statements n times: — 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 int n = 0; /* how often we must repeat */ int counter = 1; while (counter <= n) { // TODO: Add the steps to be repeated counter = counter + 1; }

While loop output  What is the output? int j = 1; int n = 5; while (j <= n) { System.out.print(j + " "); j = j + 1; } j = 2 * n; while (j >= 0) { System.out.print(j + " "); j = j - 2; }

Problem Solving The Fibonacci numbers are a sequence of integers in which the first two elements are 1, and each following element is the sum of the two preceding elements. Write int fib(int n) to return the n th Fibonacci public void testFib() { LoopFun loops = new LoopFun(); assertEquals(1, loops.fib(1)); assertEquals(1, loops.fib(2)); assertEquals(2, loops.fib(3)); assertEquals(3, loops.fib(4)); assertEquals(5, loops.fib(5)); assertEquals(8, loops.fib(6)); assertEquals(13, loops.fib(7)); assertEquals(21, loops.fib(8)); }

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

Change DeterminateLoop.java to an indeterminate loop:  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 using a special value of the same type that is not meant to be processed — Perhaps a negative integer for tests that range from 0 through 100 only — Code Demo: Change DeterminateLoop to read until there is a sentinel (like things we will do in lab)

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

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 these actions bring the loop closer to termination } Code while(myGrid.frontIsClear()) { Example myGrid.move( ); }

// Using random robot placement, instruct the // robot to get to the wall in front. // // This program needs Grid.java in the same project // public class MoveAroundTheGrid { public static void main(String[] args) { // When using this 2 argument constructor, the // grid is surrounded by blocks with one "exit" // and the mover is facing a random direction // after being placed in a random location. Grid g = new Grid(10, 15); // Always get to a wall or the lone exit if lucky while (g.frontIsClear()) { g.move(); } }

While loop with a Scanner 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 project

These assertions 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()); }

A test method to test public void testNum100s() { LoopFun lf = new LoopFun(); Scanner scanner0 = new Scanner("1 2 3"); Scanner scanner1 = new Scanner(" "); Scanner scanner3 = new Scanner(" "); assertEquals(0, lf.num100s(scanner0)); assertEquals(1, lf.num100s(scanner1)); assertEquals(3, lf.num100s(scanner3)); }

Answer Answer public int num100s (Scanner scanner) { int result = 0; while (scanner.hasNextInt()) { int next = scanner.nextInt(); if (next == 100) result++; } return result; }

Careful using next too often!  These assertions should pass with the code that follows on the next public void testSumOfNegs() { ControlFun lf = new ControlFun(); Scanner scanner0 = new Scanner("1 2 3"); Scanner scannerA = new Scanner("1 -2 3"); Scanner scannerB = new Scanner(" "); assertEquals(0, lf.sumOfNegatives(scanner0)); assertEquals(-2, lf.sumOfNegatives(scannerA)); assertEquals(-6, lf.sumOfNegatives(scannerB)); }

What's wrong with this method? 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; }