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Chapter 6

Chapter 6. Iteration. Chapter Goals. To be able to program loops with the while , for , and do statements To avoid infinite loops and off-by-one errors To understand nested loops To learn how to process input To implement simulations. Investment with Compound Interest.

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Chapter 6

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  1. Chapter 6 Iteration

  2. Chapter Goals To be able to program loops with the while , for , and do statements To avoid infinite loops and off-by-one errors To understand nested loops To learn how to process input To implement simulations

  3. Investment with Compound Interest Invest $10,000, 5% interest, compounded annually When will the balance be at least $20,000?

  4. while Statement while (condition)statement;repeats the statement while the condition is truewhile (balance < targetBalance){year++;double interest = balance * rate / 100;balance = balance + interest;}

  5. File Investment.java 1 /** 2 A class to monitor the growth of an investment that 3 accumulates interest at a fixed annual rate 4 */ 5 public class Investment 6 { 7 /** 8 Constructs an Investment object from a starting balance and

  6. 9 interest rate. 10 @param aBalance the starting balanece 11 @param aRate the interest rate in percent 12 */ 13 public Investment(double aBalance, double aRate) 14 { 15 balance = aBalance; 16 rate = aRate; 17 years = 0; 18 } 19

  7. 20 /** 21 Keeps accumulating interest until a target balance has 22 been reached. 23 @param targetBalance the desired balance 24 */ 25 public void waitForBalance(double targetBalance) 26 { 27 while (balance < targetBalance)

  8. 28 { 29 years++; 30 double interest = balance * rate / 100; 31 balance = balance + interest; 32 } 33 } 34 35 /** 36 Gets the current investment balance. 37 @return the current balance 38 */

  9. 39 public double getBalance() 40 { 41 return balance; 42 } 43 44 /** 45 Gets the number of years this investment has accumulated 46 interest.

  10. 47 @return the number of years since the start of the investment 48 */ 49 public int getYears() 50 { 51 return years; 52 } 53 54 private double balance; 55 private double rate; 56 private int years; 57 }

  11. File InvestmentTest.java 1/** 2 This program computes how long it takes for an investment 3 to double. 4 */ 5 public class InvestmentTest 6 { 7 public static void main(String[] args) 8 {

  12. 9 final double INITIAL_BALANCE = 10000; 10 final double RATE = 5; 11 Investment invest = new Investment(INITIAL_BALANCE, RATE); 12 invest.waitForBalance(2 * INITIAL_BALANCE); 13 int years = invest.getYears(); 14 System.out.println("The investment doubled after " 15 + years + " years"); 16 } 17 }

  13. Flowchart for while Loop

  14. Syntax 6.1. The while Statement • while (condition)statementExample: • while (balance < targetBalance) • { • year++; • double interest = balance * rate / 100; • balance = balance + interest; • } • Purpose: • To execute a statement while a condition is true

  15. Common Error: Infinite Loop while (year < 20) { balance = balance + balance * rate / 100; } while (year > 0) { year++; // oops, meant -- } Loops run forever--must kill program

  16. Common Error: Off-by-1 Error • year = 0;while (balance < targetBalance){year++;double interest = balance * rate / 100;balance = balance + interest;}System.out.println("Reached target after "+ year + " years."); • Should year start at 0 or 1? • Should the test be < or <=?

  17. Avoiding Off-by-1 Error • Run through a simple example:target balance = $20,000, interest rate 50%after one year: balance = $15,000after two years: balance = $22,500Therefore: year must start at 0 • interest rate 100%after one year: balance = $20,000loop should stopTherefore: must use < • Think, don't compile and try at random

  18. do Statement • Executes loop body at least once: • do statementwhile (condition); • Example: Validate input • double value;do{ String input = JOptionPane.showInputDialog( "Please enter a positive number"); value = Integer.parseInt(input);} while (input <= 0);

  19. Flowchart for do Loop

  20. for Statement • for (initialization; condition; update)statement • Example: for (int i = 1; i <= n; i++){double interest = balance * rate / 100;balance = balance + interest;} • Equivalent toinitialization;while (condition){ statement; update; }

  21. Flowchart for for Loop

  22. File Investment.java 1 /** 2 A class to monitor the growth of an investment that 3 accumulates interest at a fixed annual rate 4 */ 5 public class Investment 6 {

  23. 7 /** 8 Constructs an Investment object from a starting balance and 9 interest rate. 10 @param aBalance the starting balanece 11 @param aRate the interest rate in percent 12 */ 13 public Investment(double aBalance, double aRate) 14 { 15 balance = aBalance;

  24. 16 rate = aRate; 17 years = 0; 18 } 19 20 /** 21 Keeps accumulating interest until a target balance has 22 been reached. 23 @param targetBalance the desired balance 24 */

  25. 25 public void waitForBalance(double targetBalance) 26 { 27 while (balance < targetBalance) 28 { 29 years++; 30 double interest = balance * rate / 100; 31 balance = balance + interest; 32 } 33 }

  26. 34 35 /** 36 Keeps accumulating interest for a given number of years. 37 @param n the number of years 38 */ 39 public void waitYears(int n) 40 {

  27. 41 for (int i = 1; i <= n; i++) 42 { 43 double interest = balance * rate / 100; 44 balance = balance + interest; 45 } 46 years = years + n; 47 } 48

  28. 49 /** 50 Gets the current investment balance. 51 @return the current balance 52 */ 53 public double getBalance() 54 { 55 return balance; 56 } 57

  29. 58 /** 59 Gets the number of years this investment has accumulated 60 interest. 61 @return the number of years since the start of the investment 62 */ 63 public int getYears() 64 { 65 return years; 66 }

  30. 67 68 private double balance; 69 private double rate; 70 private int years; 71 }

  31. File InvestmentTest.java 1 /** 2 This program computes how much an investment grows in 3 a given number of years. 4 */ 5 public class InvestmentTest 6 { 7 public static void main(String[] args) 8 { 9 final double INITIAL_BALANCE = 10000;

  32. 10 final double RATE = 5; 11 final int YEARS = 20; 12 Investment invest = new Investment(INITIAL_BALANCE, RATE); 13 invest.waitYears(YEARS); 14 double balance = invest.getBalance(); 15 System.out.println("The balance after " + YEARS + 16 " years is " + balance); 17 } 18 }

  33. Syntax 6.2. The for Statement • for (initialization; condition; update )statement • Example: • for (int i = 1; i <= n; i++){double interest = balance * rate / 100;balance = balance + interest;} • Purpose: • To execute an initialization, then keep executing a statement and updating an expression while a condition is true

  34. Common Errors: Semicolons • A semicolon that shouldn't be theresum = 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);

  35. Nested Loops • Create triangle pattern[][][][][][][][][][] • Loop through rowsfor (int i = 1; i <= n; i++){// make triangle row}

  36. Make triangle row is another loopfor (int j = 1; j <= i; j++)r = r + "[]";r = r + "\n"; • Put loops together => Nested loops

  37. File Triangle.java 1 /** 2 This class describes triangle objects that can be displayed 3 as shapes like this: 4 [] 5 [][] 6 [][][] 7 */

  38. 8 public class Triangle 9 { 10 /** 11 Constructs a triangle. 12 @param aWidth the number of [] in the last row of the triangle. 13 */ 14 public Triangle(int aWidth) 15 { 16 width = aWidth; 17 }

  39. 18 19 /** 20 Computes a string representing the triangle. 21 @return a string consisting of [] and newline characters 22 */ 23 public String toString() 24 { 25 String r = "";

  40. 26 for (int i = 1; i <= width; i++) 27 { 28 // make triangle row 29 for (int j = 1; j <= i; j++) 30 r = r + "[]"; 31 r = r + "\n"; 32 } 33 return r; 34 } 35 36 private int width; 37 }

  41. File TriangleTest.java 1 /** 2 This program tests the Triangle class. 3 */ 4 public class TriangleTest 5 { 6 public static void main(String[] args) 7 { 8 Triangle small = new Triangle(3); 9 System.out.println(small.toString());

  42. 10 11 Triangle large = new Triangle(15); 12 System.out.println(large.toString()); 13 } 14 }

  43. Reading Input Values • General pattern: • boolean done = false;while (!done){ String input = read input; if (end of input indicated) done = true; else {process input }} • "Loop and a half"

  44. File DataSet.java 1 /** 2 Computes the average of a set of data values. 3 */ 4 public class DataSet 5 { 6 /** 7 Constructs an empty data set. 8 */

  45. 9 public DataSet() 10 { 11 sum = 0; 12 count = 0; 13 maximum = 0; 14 } 15 16 /** 17 Adds a data value to the data set 18 @param x a data value 19 */

  46. 20 public void add(double x) 21 { 22 sum = sum + x; 23 if (count == 0 || maximum < x) maximum = x; 24 count++; 25 } 26 27 /** 28 Gets the average of the added data. 29 @return the average or 0 if no data has been added 30 */

  47. 31 public double getAverage() 32 { 33 if (count == 0) return 0; 34 else return sum / count; 35 } 36 37 /** 38 Gets the largest of the added data. 39 @return the maximum or 0 if no data has been added 40 */

  48. 41 public double getMaximum() 42 { 43 return maximum; 44 } 45 46 private double sum; 47 private double maximum; 48 private int count; 49 }

  49. File InputTest.java 1 import javax.swing.JOptionPane; 2 3 /** 4 This program computes the average and maximum of a set 5 of input values. 6 */ 7 public class InputTest 8 {

  50. 9 public static void main(String[] args) 10 { 11 DataSet data = new DataSet(); 12 13 boolean done = false; 14 while (!done) 15 { 16 String input = JOptionPane.showInputDialog("Enter value, Cancel to quit"); 17 if (input == null) 18 done = true;

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