1. Determinate Loop Pattern, Java's for Statement, and Scanner objects
2nd part of Chapter 6: Repetition

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).
Simulate playing a game of "Let's Make a Deal" 10,000 times
The determinate loop pattern repeats some action a specific number of times

4. Determinate Loops This template repeats a process n times
replace comments with appropriate statements
int n = /* how often we must repeat the process */
for( int j = 1; j <= n; j = j + 1 ) {
// the process to be repeated }
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

5. General Form The Java for loop
for ( initial statement ; loop-test ; update-step) {
repeated-part }
When a for loop is encountered, the initial-statement is executed (used here quite often as int j = 1). The loop-test evaluates. If loop-test is false, the for loop terminates. If loop-test is true, the repeated-part executes followed by the update-step.

6. Flow chart view of a for loop
Initial statement
True
False
Loop test
Iterative part
update-step

7. Example for loop that produces an average
Scanner keyboard = new Scanner(System.in);
double sum = 0.0;
System.out.print("How many do you want to average? ");
int n = keyboard.nextInt();
// Do something n times
for (int j = 1; j <= n; j = j + 1) {
System.out.print("Enter number: "); // <- Repeat 3
int number = keyboard.nextInt(); // <- statements
sum = sum + number; // <- n times }
double average = sum / n;
System.out.print("Average = " + average);

8. Code Demo: Use the debugger to trace this code
int n = 5;
for (int j = 1; j <= n; j = j + 1) {
System.out.println(j); }
for (int k = 10; k >= 0; k = k - 2) {
System.out.println(k);

9. Other Incrementing Operators
It is common to see determinate loops of this form where n is the number of repetitions
for( int j = 1; j <= n; j++ ) }
// }
The unary ++ and -- operators add 1 and subtract 1 from their operands, respectively.
int n = 0;
n++; // n is now 1 equivalent to n=n+1; or n+=1;
n++; // n is now 2
n--; // n is now 1 again
The expression count++ is equivalent to the more verbose count = count + 1;

10. Other Assignment Operators
Java has several assignment operators in addition to = (-= and +=)
j -= 2; is the equivalent of j = j - 2;
sum += x; is the equivalent of sum = sum + x;
What is sum when a user enters 7 and 8?
int sum = 0;
int x = 0;
System.out.print("Enter a number: ");
x = keyboard.nextInt(); // user enters 7
sum += x;
x = keyboard.nextInt(); // user enters 8

11. Indeterminate 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 2, methods 9 and 10: sumInScanner and maximumInScanner

12. 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()); }

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

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

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

16. 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;