1. 1 Iteration

2. 2 Java looping  Options while do-while for  Allow programs to control how many times a statement list is executed

3. 3 Averaging  Problem Extract a list of positive numbers from standard input and produce their average  Numbers are one per line  A negative number acts as a sentinel to indicate that there are no more numbers to process  Observations Cannot supply sufficient code using just assignments and conditional constructs to solve the problem  Don’t how big of a list to process Need ability to repeat code as needed

4. 4 Averaging  Algorithm Prepare for processing Get first input While there is an input to process do {  Process current input  Get the next input } Perform final processing

5. 5 Averaging  Problem Extract a list of positive numbers from standard input and produce their average  Numbers are one per line  A negative number acts as a sentinel to indicate that there are no more numbers to process  Sample run Enter positive numbers one per line. Indicate end of list with a negative number. 4.5 0.5 1.3 Average 2.1

6. public class NumberAverage { // main(): application entry point public static void main(String[] args) { // set up the input // prompt user for values // get first value // process values one-by-one while (value >= 0) { // add value to running total // processed another value // prepare next iteration - get next value } // display result if (valuesProcessed > 0) // compute and display average else // indicate no average to display }

7. int valuesProcessed = 0; double valueSum = 0; // set up the input Scanner stdin = new Scanner (System.in); // prompt user for values System.out.println("Enter positive numbers 1 per line.\n" + "Indicate end of the list with a negative number."); // get first value double value = stdin.nextDouble(); // process values one-by-one while (value >= 0) { valueSum += value; ++valuesProcessed; value = stdin.nextDouble(); } // display result if (valuesProcessed > 0) { double average = valueSum / valuesProcessed; System.out.println("Average: " + average); } else { System.out.println("No list to average"); }

8. 8 While syntax and semantics Logical expression that determines whether Action is to be executed while ( Expression ) Action Action is either a single statement or a statement list within braces

9. 9 While semantics for averaging problem // process values one-by-one while ( value >= 0 ) { // add value to running total valueSum += value; // we processed another value ++valueProcessed; // prepare to iterate – get the next input value = stdin.nextDouble(); } Test expression is evaluated at the start of each iteration of the loop. If test expression is true, these statements are executed. Afterward, the test expression is reevaluated and the process repeats

10. 10 While Semantics Expression Action true false Expression is evaluated at the start of each iteration of the loop If Expression is true, Action is executed If Expression is false, program execution continues with next statement

11. 11 int valuesProcessed = 0; double valueSum = 0; double value = stdin.nextDouble(); while (value >= 0) { valueSum += value; ++valuesProcessed; value = stdin.nextDouble(); } if (valuesProcessed > 0) { double average = valueSum / valuesProcessed; System.out.println("Average: " + average); } else { System.out.println("No list to average"); } int valuesProcessed = 0; double valueSum = 0; double value = stdin.nextDouble(); while (value >= 0) { valueSum += value; ++valuesProcessed; value = stdin.nextDouble(); if (valuesProcessed > 0) { double average = valueSum / valuesProcessed; System.out.println("Average: " + average); Execution Trace Suppose input contains: 4.5 0.5 1.3 -1 0 valuesProcessed valueSum 0 value 4.5 Suppose input contains: 4.5 0.5 1.3 -1 4.5 1 Suppose input contains: 4.5 0.5 1.3 -1 0.5 5.0 2 1.3 6.3 Suppose input contains: 4.5 0.5 1.3 -1 3 Suppose input contains: 4.5 0.5 1.3 -1 average 2.1

12. 12 Converting text to strictly lowercase public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } System.out.println("\nConversion is:\n" + converted); }

13. 13 Sample run A Ctrl+z was entered. It is the Windows escape sequence for indicating end-of-file An empty line was entered

14. 14 Program trace public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } System.out.println("\nConversion is:\n" + converted); } public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } System.out.println("\nConversion is:\n" + converted); }

15. 15 Program trace Representation of lower case conversion of current input line converted += (currentConversion + "\n"); The append assignment operator updates the representation of converted to include the current input line Newline character is needed because method nextLine() "strips" them from the input

16. 16 Converting text to strictly lowercase public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } System.out.println("\nConversion is:\n" + converted); }

17. 17 Loop design  Questions to consider in loop design and analysis What initialization is necessary for the loop’s test expression? What initialization is necessary for the loop’s processing? What causes the loop to terminate? What actions should the loop perform? What actions are necessary to prepare for the next iteration of the loop? What conditions are true and what conditions are false when the loop is terminated? When the loop completes what actions are need to prepare for subsequent program processing?

18. 18 Reading a file  Background Same Scanner class! Scanner fileIn = new Scanner (new File (filename) ); The File class allows access to files It’s in the java.io package filename is a String

19. 19 Reading a file  Class File Allows access to files (etc.) on a hard drive  Constructor File (String s) Opens the file with name s so that values can be extracted Name can be either an absolute pathname or a pathname relative to the current working folder

20. 20 Reading a file Scanner stdin = new Scanner (System.in); System.out.print("Filename: "); String filename = stdin.nextLine(); Scanner fileIn = new Scanner (new File (filename)); String currentLine = fileIn.nextLine(); while (currentLine != null) { System.out.println(currentLine); currentLine = fileIn.nextLine(); } Scanner stdin = new Scanner (System.in); System.out.print("Filename: "); String filename = stdin.nextLine(); Scanner fileIn = new Scanner (new File (filename)); String currentLine = fileIn.nextLine(); while (currentLine != null) { System.out.println(currentLine); currentLine = fileIn.nextLine(); } Set up standard input streamDetermine file nameSet up file streamProcess lines one by oneGet first lineMake sure got a line to processDisplay current lineGet next lineMake sure got a line to process If not, loop is done Close the file stream

21. 21 Quick survey I feel I understand while loops… I feel I understand while loops… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

22. 22 Star Wars: Episode III trailer No, really! No, really!

23. 23 End of lecture on 16 March 2005  At this point, we moved back to the chapter 5 slides, and did slides 122- 145

24. 24 The For Statement currentTerm = 1; for ( int i = 0; i < 5; ++i ) { System.out.println(currentTerm); currentTerm *= 2; } After each iteration of the body of the loop, the update expression is reevaluated The body of the loop iterates while the test expression is true int Initialization step is performed only once -- just prior to the first evaluation of the test expression The body of the loop displays the current term in the number series. It then determines what is to be the new current number in the series

25. ForExpr Action truefalse ForInit PostExpr Evaluated once at the beginning of the for statements's execution The ForExpr is evaluated at the start of each iteration of the loop If ForExpr is true, Action is executed After the Action has completed, the PostExpression is evaluated If ForExpr is false, program execution continues with next statement After evaluating the PostExpression, the next iteration of the loop starts

26. 26 for statement syntax Logical test expression that determines whether the action and update step are executed for ( ForInit ; ForExpression ; ForUpdate ) Action Update step is performed after the execution of the loop body Initialization step prepares for the first evaluation of the test expression The body of the loop iterates whenever the test expression evaluates to true

27. 27 for vs. while  A for statement is almost like a while statement for ( ForInit; ForExpression; ForUpdate ) Action is ALMOST the same as: ForInit; while ( ForExpression ) { Action; ForUpdate; }  This is not an absolute equivalence! We’ll see when they are different below

28. 28 Variable declaration  You can declare a variable in any block: while ( true ) { int n = 0; n++; System.out.println (n); } System.out.println (n); Variable n gets created (and initialized) each time Thus, println() always prints out 1 Variable n is not defined once while loop ends As n is not defined here, this causes an error

29. 29 Variable declaration  You can declare a variable in any block: if ( true ) { int n = 0; n++; System.out.println (n); } System.out.println (n); Only difference from last slide

30. 30 System.out.println("i is " + i); } System.out.println("all done"); System.out.println("i is " + i); } System.out.println("all done"); i is 0 i is 1 i is 2 all done Execution Trace i 0 int i = 0;i < 3;++ifor () {int i = 0;i < 3;++i 123 Variable i has gone out of scope – it is local to the loop

31. 31 for vs. while  An example when a for loop can be directly translated into a while loop: int count; for ( count = 0; count < 10; count++ ) { System.out.println (count); }  Translates to: int count; count = 0; while (count < 10) { System.out.println (count); count++; }

32. 32 for vs. while  An example when a for loop CANNOT be directly translated into a while loop: for ( int count = 0; count < 10; count++ ) { System.out.println (count); }  Would (mostly) translate as: int count = 0; while (count < 10) { System.out.println (count); count++; } count IS defined here count is NOT defined here only difference

33. 33 for loop indexing  Java (and C and C++) indexes everything from zero  Thus, a for loop like this: for ( int i = 0; i < 10; i++ ) {... }  Will perform the action with i being value 0 through 9, but not 10  To do a for loop from 1 to 10, it would look like this: for ( int i = 1; i <= 10; i++ ) {... }

34. 34 Quick survey I feel I understand for loops… I feel I understand for loops… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

35. 35 Fractals

36. 36 End of lecture on 21 March 2005  Also did last few slides of the set for chapter 5, and the test 2 review slides

37. 37 Nested loops int m = 2; int n = 3; for (int i = 0; i < n; ++i) { System.out.println("i is " + i); for (int j = 0; j < m; ++j) { System.out.println(" j is " + j); } i is 0 j is 0 j is 1 i is 1 j is 0 j is 1 i is 2 j is 0 j is 1

38. 38 Nested loops int m = 2; int n = 4; for (int i = 0; i < n; ++i) { System.out.println("i is " + i); for (int j = 0; j < i; ++j) { System.out.println(" j is " + j); } i is 0 i is 1 j is 0 i is 2 j is 0 j is 1 i is 3 j is 0 j is 1 j is 2

39. 39 do-while loops  We are going to skip these Thus, they won’t be on the exams You can look at them in the slides and/or the book

40. 40 The do-while statement  Syntax do Action while (Expression)  Semantics Execute Action If Expression is true then execute Action again Repeat this process until Expression evaluates to false  Action is either a single statement or a group of statements within braces Action true false Expression

41. 41 Picking off digits  Consider System.out.print("Enter a positive number: "); int number = stdin.nextInt(); do { int digit = number % 10; System.out.println(digit); number = number / 10; } while (number != 0);  Sample behavior Enter a positive number: 1129 9 2 1 1

42. 42 while vs. do-while  If the condition is false: while will not execute the action do-while will execute it once while ( false ) { System.out.println (“foo”); } do { System.out.println (“foo”); } while ( false ); never executed executed once

43. 43 while vs. do-while  A do-while statement can be translated into a while statement as follows: do { Action; } while ( WhileExpression );  can be translated into: boolean flag = true; while ( flag || WhileExpression ) { flag = false; Action; }

44. 44 A digression: Perl again  Consider the statement: if ( !flag ) {... } else {... }  Perl has a command unless : unless ( flag ) {... } else {... }  An unless command is a if statement with a negated condition  It can get a bit confusing, though The else of an unless…

45. 45 A digression: Perl again  Consider the statement: while ( !flag ) {... }  Perl has a command until : until ( flag ) {... }  An until command is a while loop with a negated condition  As most people are quite used to if-else and while, unless and until are rarely used

46. 46 The continue keyword  The continue keyword will immediately start the next iteration of the loop The rest of the current loop is not executed for ( int a = 0; a <= 10; a++ ) { if ( a % 2 == 0 ) { continue; } System.out.println (a + " is odd"); }  Output:1 is odd 3 is odd 5 is odd 7 is odd 9 is odd

47. 47 The break keyword  The break keyword will immediately stop the execution of the loop Execution resumes after the end of the loop for ( int a = 0; a <= 10; a++ ) { if ( a == 5 ) { break; } System.out.println (a + " is less than five"); }  Output:0 is less than five 1 is less than five 2 is less than five 3 is less than five 4 is less than five

48. 48 Quick survey I feel I understand loops, break, continue, etc… I feel I understand loops, break, continue, etc… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

49. 49 Human stupidity

50. 50 Four Hobos  An example of a program that uses nested for loops  Credited to Will Shortz, crossword puzzle editor of the New York Times And NPR’s Sunday Morning Edition puzzle person  This problem is in section 6.10 of the text

51. 51 Problem  Four hobos want to split up 200 hours of work  The smart hobo suggests that they draw straws with numbers on it  If a straw has the number 3, then they work for 3 hours on 3 days (a total of 9 hours)  The smart hobo manages to draw the shortest straw  How many ways are there to split up such work?  Which one did the smart hobo choose?

52. 52 Analysis  We are looking for integer solutions to the formula: a 2 +b 2 +c 2 +d 2 = 200 Where a is the number of hours & days the first hobo worked, b for the second hobo, etc.  We know the following: Each number must be at least 1 No number can be greater than 200 = 14 That order doesn’t matter  The combination (1,2,1,2) is the same as (2,1,2,1) Both combinations have two short and two long straws  We will implement this with nested for loops

53. 53 Implementation public class FourHobos { public static void main (String[] args) { for ( int a = 1; a <= 14; a++ ) { for ( int b = 1; b <= 14; b++ ) { for ( int c = 1; c <= 14; c++ ) { for ( int d = 1; d <= 14; d++ ) { if ( (a <= b) && (b <= c) && (c <= d) ) { if ( a*a+b*b+c*c+d*d == 200 ) { System.out.println ("(" + a + ", " + b + ", " + c + ", " + d + ")"); }

54. 54 Results  The output: (2, 4, 6, 12) (6, 6, 8, 8)  Not surprisingly, the smart hobo picks the short straw of the first combination

55. 55 Alternate implementation  We are going to rewrite the old code in the inner most for loop: if ( (a <= b) && (b <= c) && (c <= d) ) { if ( a*a+b*b+c*c+d*d == 200 ) { System.out.println ("(" + a + ", " + b + ", " + c + ", " + d + ")"); } }  First, consider the negation of ( (a <= b) && (b <= c) && (c <= d) ) It’s ( !(a <= b) || !(b <= c) || !(c <= d) ) Or ( (a > b) || (b > c) || (c > d) )

56. 56 Alternate implementation  This is the new code for the inner-most for loop: if ( (a > b) || (b > c) || (c > d) ) { continue; } if ( a*a+b*b+c*c+d*d != 200 ) { continue; } System.out.println ("(" + a + ", " + b + ", " + c + ", " + d + ")");

57. 57 Revised implementation public class FourHobos { public static void main (String[] args) { for ( int a = 1; a <= 14; a++ ) { for ( int b = 1; b <= 14; b++ ) { for ( int c = 1; c <= 14; c++ ) { for ( int d = 1; d <= 14; d++ ) { if ( (a > b) || (b > c) || (c > d) ) { continue; } if ( a*a+b*b+c*c+d*d != 200 ) { continue; } System.out.println ("(" + a + ", " + b + ", " + c + ", " + d + ")"); }

58. 58 Quick survey I feel I understand that example… I feel I understand that example… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

59. 59 Today’s demotivators

60. 60 Data set manipulation  Another example to develop code for  Develops a class  Uses loops and if-elses  Booyea!

61. 61 Data set manipulation  Often five values of particular interest Minimum Maximum Mean (average) Standard deviation Size of data set  Let’s design a data set representation The data set represents a series of numbers Note that the numbers themselves are not remembered by the DataSet  Only properties of the set (average, minimum, etc.)  We’re going to ignore the standard deviation

62. 62 Data set properties (instance variables)  private int n Number of values in the data set being represented  private double minimumValue Minimum value in the data set being represented  private double maximumValue Maximum value in the data set being represented  private double xSum The sum of values in the data set being represented

63. 63 End of lecture on 28 March 2005  I want to start 2 slides back  Also had Anita’s talk, and went over the second midterm, and did the mid-semester survey today

64. 64 Constructors  public DataSet() Initializes a representation of an empty data set  public DataSet(String s) Initializes the data set using the values from the file with name s  public DataSet(File filep) Initializes the data set using the values from the file represented by filep  We aren’t going to develop that here…

65. 65 Methods  public double getMinimum() Returns the minimum value in the data set If the data set is empty, then Double.NaN is returned  Double.NaN is the Java double value representing the status not-a-number  public double getMaximum() Returns the maximum value in the data set If the data set is empty, then Double.NaN is returned

66. 66 More methods  public double getAverage() Returns the average value in the data set If the data set is empty, then Double.NaN is returned  public int getSize() Returns the number of values in the data set being represented

67. 67 More more methods  public void addValue(double x) Adds the value x to the data set being represented  public void clear() Sets the representation to that of an empty data set  public void load(String s) Adds the vales from the file with name s to the data set being represented  public void load(File filep) Adds the vales from the file represented by filep to the data set being represented  Left to interested student

68. 68 Example usage DataSet dataset = new DataSet("age.txt"); System.out.println(); System.out.println("Minimum: " + dataset.getMinimum()); System.out.println("Maximum: " + dataset.getMaximum()); System.out.println("Mean: " + dataset.getAverage()); System.out.println("Size: " + dataset.getSize()); System.out.println(); dataset.clear(); dataset.load("stature.txt"); System.out.println("Minimum: " + dataset.getMinimum()); System.out.println("Maximum: " + dataset.getMaximum()); System.out.println("Mean: " + dataset.getAverage()); System.out.println("Size: " + dataset.getSize()); System.out.println(); dataset.clear();

69. 69 Example usage dataset.load("foot-length.txt"); System.out.println("Minimum: " + dataset.getMinimum()); System.out.println("Maximum: " + dataset.getMaximum()); System.out.println("Mean: " + dataset.getAverage()); System.out.println("Size: " + dataset.getSize()); System.out.println(); dataset.clear(); System.out.println("Minimum: " + dataset.getMinimum()); System.out.println("Maximum: " + dataset.getMaximum()); System.out.println("Mean: " + dataset.getAverage()); System.out.println("Size: " + dataset.getSize()); System.out.println();

70. 70 Example usage

71. 71 Methods getMinimum() and getMaximum()  Straightforward implementations given correct setting of instance variables public double getMinimum() { return minimumValue; } public double getMaximum() { return maximumValue; }

72. 72 Method getSize()  Straightforward implementations given correct setting of instance variables public int getSize() { return n; }

73. 73 Method getAverage()  Need to take into account that data set might be empty public double getAverage() { if (n == 0) { return Double.NaN; } else { return xSum / n; } }

74. 74 DataSet constructors  Straightforward using clear() and load() public DataSet() { clear(); } public DataSet(String s) { load(s); }

75. 75 Facilitator clear() public void clear() { n = 0; xSum = 0; minimumValue = Double.NaN; maximumValue = Double.NaN; }

76. 76 Facilitator addValue() public void addValue(double x) { xSum += x; ++n; if (n == 1) { minimumValue = x; maximumValue = x; } else if (x < minimumValue) { minimumValue = x; } else if (x > maximumValue) { maximumValue = x; } }

77. 77 Facilitator load() public void load(String s) { // get a reader for the file Scanner fileIn = new Scanner (new File(s)); // add values one by one String currentLine = fileIn.nextLine(); while (currentLine != null) { double x = Double.parseDouble(currentLine); addValue(x); currentLine = fileIn.nextLine(); } // close up file }

78. 78 Quick survey I felt I understood the material in this slide set… I felt I understood the material in this slide set… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

79. 79 Quick survey The pace of the lecture for this slide set was… The pace of the lecture for this slide set was… a) Fast b) About right c) A little slow d) Too slow

80. 80 Quick survey How interesting was the material in this slide set? Be honest! How interesting was the material in this slide set? Be honest! a) Wow! That was SOOOOOOO cool! b) Somewhat interesting c) Rather boring d) Zzzzzzzzzzz

81. 81 Sand Castles

82. 82 Survey time!