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Chapter 6

Chapter 6. Iteration. while loop. while ( condition ) statement ; repeats the statement while the condition is true while (balance < 2 * initial) { year++; balance = balance + balance * rate / 100; }. Program DoubleInv.java public class DoubleInv

erwin
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Chapter 6

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  1. Chapter 6 Iteration

  2. while loop • while (condition)statement; • repeats the statement while the condition is true • while (balance < 2 * initial){ year++; balance = balance + balance * rate / 100;}

  3. Program DoubleInv.java public class DoubleInv { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Interest rate:"); double rate = console.readDouble(); System.out.println("Initial balance:"); double initialBalance = console.readDouble(); int year = 0; double balance = initialBalance;

  4. // keep accumulating interest until balance doubles while (balance < 2 * initialBalance) { year++; double interest = balance * rate / 100; balance = balance + interest; } System.out.println("The investment doubled after " + year + ” years."); } }

  5. Figure 1 Flowchart of a while Loop

  6. Common Error: Infinite loops • while (year < 20){ balance = balance + balance * rate / 100; } • while (year > 0){ year++; // oops, meant -- . . .} • loop runs forever—must kill program

  7. for loop • for (init; condition; update)statement • Example: for (i = 1; i <= 10; i++) ... for (y = 20; y > 0; y--) ... • Equivalent toinit;while (condition){ statement; update; }

  8. Program Invest.java public class Invest { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Interest rate:"); double rate = console.readDouble(); final double INITIAL_BALANCE = 10000; final int NYEARS = 20; double balance = INITIAL_BALANCE;

  9. for (int year = 1; year <= NYEARS; year++) { double interest = balance * rate / 100; balance = balance + interest; System.out.println("year:” + year + ” balance: ” + balance); } } }

  10. Figure 2 Flowchart of a forLoop

  11. do loop • dostatement;while (condition); • Executes the statement at least once • Example:do{ System.out.println("Interest rate:"); rate = Console.readDouble();} while (rate <= 0);

  12. Figure 3 Flowchart of a doLoop

  13. Off-by-1 errors • year = 0;while (balance < 2 * initial){ year++; balance = balance + balance * rate / 100;}System.out.println("Doubled after " + year + " years."); • Should year start at 0 or 1? • Should the test be < or <=?

  14. Off-by-1 errors • Run through a simple example.initial balance = $100, interest rate 50%after one year: balance = $150after two years: balance = $225Þyear must start at 0 • interest rate 100%after one year: balance = $200loop should stop Þ must use < • Think, don't compile and try at random

  15. Semicolon errors • A semicolon that shouldn't be there • sum = 0;for (i = 1; i <= 10; i++); sum = sum + i;System.out.println(sum); • A missing semicolon • for (i = 1; i <= 10; sum = sum + i++)System.out.println(sum);

  16. Nested loops • Print table of powers xy 1 1 1 1 1 2 4 8 16 32 3 9 27 81 243 . . . • Print the rowsfor (int x = 0; x <= ROWS; x++){print row // uses another loop}

  17. Nested loops • Loop inside a loop:for (int x = 0; x <= ROWS; x++){ for (int y = 0; y <= COLS; y++) {compute valueprint value}System.out.println();} • Count iterations: ROWS * COLS

  18. Program Table.java public class Table { public static void main(String[] args) { final int COLUMN_WIDTH = 10; for (int x =1; x <= 10; x++) { // print table row for (int y = 1; y <= 8; y++) { int p = (int)Math.pow(x, y); // convert value to string

  19. String pstr = "” + p; // pad with spaces while (pstr.length() < COLUMN_WIDTH) pstr = ” ” + pstr; System.out.print(pstr); } System.out.println(); } } }

  20. Reading a Set of Numbers • boolean done = false;while (!done){ String line = console.readLine(); if (line == null) done = true; elseprocess data} • “Loop and a half”

  21. Console input • java Average3.52.6-1.2Ctrl+D (Unix) or Ctrl-Z (DOS) • Closes System.in • Control character is not passed to program

  22. Input/Output redirection • Put input in a file, say input.txt • java Average < input.txt • Now System.in reads from the file, not the keyboard • There is no Ctrl+D/Ctrl+Z character in the file • Can redirect output to a file:java MyProg > myfile.txt

  23. Sentinels • End of input marker that isn't part of the data set • Example:java Sentinel13.52.61.20 • Or better, use a non-numerical sentinel like Q

  24. Sentinels • boolean done = false;while (!done){ String line = console.readLine(); if (line is the sentinel) done = true; elseprocess data}

  25. Program Average.java public class Average { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Enter data."); double sum = 0; int count = 0; // compute sum of all input values

  26. boolean done = false; while (!done) { String inputLine = console.readLine(); if (inputLine == null) done = true; else { double x = Double.parseDouble(inputLine); sum = sum + x; count++; } } // compute average if (count == 0) System.out.println("No data"); else System.out.println("Average = ” + sum / count); } }

  27. Program Sentinel1.java public class Sentinel1 { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Enter data (0 to finish):"); double sum = 0; int count = 0; // compute sum of all input values boolean done = false;

  28. while (!done) { String inputLine = console.readLine(); double x = Double.parseDouble(inputLine); if (x == 0) done = true; else { sum = sum + x; count++; } } // compute average if (count == 0) System.out.println("No data"); else System.out.println("Average = ” + sum / count); } }

  29. Program Sentinel2.java public class Sentinel2 { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Enter data (Q to finish):"); double sum = 0; int count = 0; // compute sum of all input values

  30. boolean done = false; while (!done) { String inputLine = console.readLine(); if (inputLine.equalsIgnoreCase("Q")) done = true; else { double x = Double.parseDouble(inputLine); sum = sum + x; count++; } } // compute average if (count == 0) System.out.println("No data"); else System.out.println("Average = ” + sum / count); } }

  31. String tokenization • Break up string into tokens (words delimited by white space) • StringTokenizer tokenizer = new StringTokenizer();while (tokenizer.hasMoreTokens()){ String token = tokenizer.nextToken();process token}

  32. Program Words.java import java.util.StringTokenizer; public class Words { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Enter Words:"); int count = 0; boolean done = false; while (!done) { String inputLine = console.readLine(); if (inputLine == null) done = true; else { // break up input line into words

  33. StringTokenizer tokenizer = new StringTokenizer(inputLine); while (tokenizer.hasMoreTokens()) { tokenizer.nextToken(); // read and discard count++; // count each word } } } System.out.println(count + "words"); } }

  34. Traversing characters in a string • char: character type—a single Unicode character • Character constants use single quotes: 'A', '\u00E9' • 'A'is not the same as "A" • String s = . . .;for (int i = 0; i < s.length(); i++){ char ch = s.charAt(i);process ch;}

  35. Program Reverse.java public class Reverse { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Please enter a string:"); String s = console.readLine(); String r = ""; for (int i = 0; i < s.length(); i++) { char ch = s.charAt(i); r = ch + r; // add ch in front } System.out.println(s + ” reversed is ” + r); } }

  36. Figure 6 The Buffon Needle Experiment

  37. Simulations • Random numbers:Random generator = new Random();int n = generator.nextInt(CHOICES);double x = generator.nextDouble(); • Throw die (random number between 1 and 6)int d = 1 + generator.nextInt(6); • Buffon needle: simulate needle throw double ylow = 2*generator.nextDouble();double angle = 180*generator.nextDouble();

  38. Program Dice.java import java.util.Random; public class Dice { public static void main(String[] args) { Random generator = new Random(); // roll dice ten times for (int i = 1; i <= 10; i++) { int d = 1 + generator.nextInt(6); System.out.print(d + ” "); } System.out.println(); } }

  39. Figure 7 Variables in a Trial of the Buffon Needle Experiment

  40. Program Buffon.java import java.util.Random; public class Buffon { public static void main(String[] args) { Random generator = new Random(); int hits = 0; final int NTRIES = 10000; for (int i = 1; i <= NTRIES; i++) { // simulate needle throw double ylow = 2 * generator.nextDouble(); double angle = 180 * generator.nextDouble();

  41. // compute high point of needle double yhigh = ylow + Math.sin(Math.toRadians(angle)); if (yhigh >= 2) hits++; } // print approximation of PI System.out.println("Tries / Hits = " + (NTRIES * 1.0) / hits); } }

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