1. Chapter 6, Part 1
While Loops
Asserting Java
© Rick Mercer 1

2. 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

4. 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; } 4

6. While loop output What is the output? int j = 1; int n = 5;
while (j <= n) {
System.out.print(j + " ");
j = j + 1; }
j = 10;
while (j >= 0) {
j = j - 3;

7. 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 nth Fibonacci number.
@Test
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)); } 7

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 8

9. 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 9

10. 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: Find the average of an unknown number of inputs: use a sentinel such as -1

11. 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( );

12. // 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(); } }

13. 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

14. 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()); }

15. A test method to test num100s
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)); }

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

17. Careful using next too often!
These assertions should pass with the code that follows on the next slide
@Test
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)); }

18. 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;