Chapter 6 Repetition Asserting Java Rick Mercer Algorithmic

Chapter 6 Repetition 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 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

Example for loop that 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. next. Int(); int counter = 1; // Do something n times while (counter <= n) { System. out. print("Enter number: "); // <- Repeat 3 number = keyboard. next. Double(); // <- 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) { System. out. print (j + " "); 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

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 Example while(my. Grid. front. Is. Clear()) { my. Grid. move(); }

Example Indeterminate Loop need Grid. java // Using random robot placement, instruct robot to get to // the wall in front, turn left, and move to the next wall public class Move. Around. The. Grid { public static void main(String[] args) { Grid g = new Grid(10, 15); while (g. front. Is. Clear()) { g. move(); } g. turn. Left(); while (g. front. Is. Clear()) { g. move(); } } }

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 2, methods 9 and 10: sum. In. Scanner and maximum. In. Scanner

These assertions pass @Test public void show. Scanner() { Scanner scanner. With. Ints = new Scanner("1 2 3"); assert. Equals(1, scanner. With. Ints. next. Int()); assert. Equals(2, scanner. With. Ints. next. Int()); assert. Equals(3, scanner. With. Ints. next. Int()); Scanner scanner = new Scanner("There are five words here. "); assert. Equals("There", scanner. next()); assert. Equals("are", scanner. next()); assert. Equals("five", scanner. next()); assert. Equals("words", scanner. next()); assert. Equals("here. ", scanner. next()); }

A test method to test num 100 s @Test public void test. Num 100 s() { Control. Fun cf = new Control. Fun(); Scanner scanner 0 = new Scanner("1 2 3"); Scanner scanner 1 = new Scanner("4 100 2 5"); Scanner scanner 3 = new Scanner("100 2 -3 5 3 2 -100 100"); assert. Equals(0, cf. num 100 s(scanner 0)); assert. Equals(1, cf. num 100 s(scanner 1)); assert. Equals(3, cf. num 100 s(scanner 3)); }

Answer public int num 100 s (Scanner scanner) { int result = 0; while (scanner. has. Next. Int()) { int next = scanner. next. Int(); if (next == 100) result++; } return result; }

Careful using next too often! These assertions should pass with the code that follows on the next slide @Test public void test. Sum. Of. Negs() { Control. Fun cf = new Control. Fun(); Scanner scanner 0 = new Scanner("1 2 3"); Scanner scanner. A = new Scanner("1 -2 3"); Scanner scanner. B = new Scanner("-4 1 -2 3"); assert. Equals(0, cf. sum. Of. Negatives(scanner 0)); assert. Equals(-2, cf. sum. Of. Negatives(scanner. A)); assert. Equals(-6, cf. sum. Of. Negatives(scanner. B)); }

What's wrong with this method? public int sum. Of. Negatives(Scanner scanner) { int result = 0; while (scanner. has. Next. Int()) { if (scanner. next. Int() < 0) { result += scanner. next. Int(); } } return result; }