Chapter 6. Repetition. Asserting Java. Rick Mercer

Transcription

1 Chapter 6 Repetition Asserting Java Rick Mercer

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.

3 Pattern: Determinate Loop Problem: Do something exactly n times, where n is known in advance Algorithm determine n repeat the following n times perform these actions Code Example: public int charsin(char ch, String str) { int found = 0; int index = 0; while(index < str.length()) { if(str.charat(index) == ch) found = found + 1; index = index + 1; return found;

4 While Loop flowchart Boolean Expression true false { statement_1; statement_2; statement_n;

5 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

6 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);

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

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

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

10 Pattern Problem Indeterminate loop 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

11 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

12 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());

13 Write method average to return the average of the doubles in a public void testnum100s() { LoopFun cf = new LoopFun(); Scanner scanner1 = new Scanner(" "); Scanner scanner2 = new Scanner("1.0\t 2\n "); assertequals(80.0, cf.average(scanner). 0.01); assertequals(2.5, cf.average(scanner). 0.01);

14 Let sumofnegatives return the sum of the negative integers in a public void testsumofnegs() { ControlFun cf = new ControlFun(); Scanner scanner0 = new Scanner("1 2 3"); Scanner scannera = new Scanner(" "); Scanner scannerb = new Scanner(" "); assertequals(0, cf.sumofnegatives(scanner0)); assertequals(-6, cf.sumofnegatives(scannera)); assertequals(-10, cf.sumofnegatives(scannerb));

15 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;