1. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #1 More Programming Constructs -- Introduction In this lecture: –internal data representation –conversions between one data type and another –more operators –more selection statements –more repetition statements –array declaration and use –arrays of objects –parameters and arrays –multidimensional arrays

2. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #2 Internal Data Representation We discussed earlier that every piece of information stored on a computer is represented as binary values What is represented by the following binary string? 01100001001010 You can't tell just from the bit string itself. We take specific binary values and apply an interpretation to them

3. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #3 Representing Integers There are four types of integers in Java, each providing a different bits to store the value Each has a sign bit. If it is 1, the number is negative; if it is 0, the number is positive byte short int long s 7 bits s 15 bits s 31 bits s63 bits

4. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #4 Two's Complement Integers are stored in signed two's complement format A positive value is a straightforward binary number A negative value is represented by inverting all of the bits of the corresponding positive value, then adding 1 To "decode" a negative value, invert all of the bits and add 1 Using two's complement makes internal arithmetic processing easier

5. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #5 Two's Complement The number 25 is represented in 8 bits ( byte ) as 00011001 To represent -25, first invert all of the bits 11100110 then add 1 11100111 Note that the sign bit reversed, indicating the number is negative

6. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #6 Overflow and Underflow Storing numeric values in a fixed storage size can lead to overflow and underflow problems Overflow occurs when a number grows too large to fit in its allocated space Underflow occurs when a number shrinks too small to fit in its allocated space Example: int num = Integer.MAX_VALUE; // 2,147,483,647 num = num + 1; // -2,147,483,648 // which is Integer.MIN_VALUE

7. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #7 Representing Floating Point Values A decimal (base 10) floating point value can be defined by the following equation sign * mantissa * 10 exponent where –sign is either 1 or -1 –mantissa is a positive value that represents the significant digits of the number –exponent is a value that indicates how the decimal point is shifted relative to the mantissa

8. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #8 Representing Floating Point Values For example, the number -843.977 can be represented by -1 * 843977 * 10 -3 Floating point numbers can be represented in binary the same way, except that the mantissa is a binary number and the base is 2 instead of 10 sign * mantissa * 2 exponent Floating point values are stored by storing each of these components in the space allotted

9. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #9 Representing Characters As described earlier, characters are represented according to the Unicode Character Set The character set matches a unique number to each character to be represented Storing the character is therefore as simple as storing the binary version of the number that represents it For example, the character 'z' has the Unicode value 122, which is represented in 16 bits as 0000000001111010

10. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #10 Representing Characters Because they are stored as numbers, Java lets you perform some arithmetic processing on characters For example, because 'A' is stored as Unicode value 65, the statement char ch = 'A' + 5; will store the character 'F' in ch (Unicode value 70) This relationship is occasionally helpful

11. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #11 Conversions Each data value and variable is associated with a particular data type It is sometimes necessary to convert a value of one data type to another Not all conversions are possible. For example, boolean values cannot be converted to any other type and vice versa Even if a conversion is possible, we need to be careful that information is not lost in the process

12. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #12 Widening Conversions Widening conversions are generally safe because they go from a smaller data space to a larger one The widening conversions are: From byte short char int long float To short, int, long, float, or double int, long, float, or double long, float, or double float or double double

13. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #13 Narrowing Conversions Narrowing conversions are more dangerous because they usually go from a smaller data space to a larger one The narrowing conversions are: From byte short char int long float double To char byte or char byte or short byte, short, or char byte, short, char, or int byte, short, char, int or long byte, short, char, int, long, or float

14. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #14 Performing Conversions In Java, conversion between one data type and another can occur three ways Assignment conversion - when a value of one type is assigned to a variable of another type Arithmetic promotion - occurs automatically when operators modify the types of their operands Casting - an operator that forces a value to another type

15. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #15 Assignment Conversion For example, if money is a float variable and dollars is an int variable (storing 82), then money = dollars; converts the value 82 to 82.0 when it is stored The value in dollars is not actually changed Only widening conversions are permitted through assignment Assignment conversion can also take place when passing parameters (which is a form of assignment)

16. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #16 Arithmetic Promotion Certain operators require consistent types for their operands For example, if sum is a float variable and count is an int variable, then the statement result = sum / count; internally converts the value in count to a float then performs the division, producing a floating point result The value in count is not changed

17. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #17 Casting A cast is an operator that is specified by a type name in parentheses It is placed in front of the value to be converted The following example truncates the fractional part of the floating point value in money and stores the integer portion in dollars dollars = (int) money; The value in money is not changed If a conversion is possible, it can be done through a cast

18. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #18 Arithmetic Operators The Java Virtual Machine can only perform integer, long, float and double operations Therefore byte and short operations are performed as integer for example: byte c = 4, d = 5; byte e = (byte) c + d;

19. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #19 More Operators We've seen several operators of various types: arithmetic, equality, relational There are many more in Java to make use of: –increment and decrement operators –logical operators –assignement operators –the conditional operator

20. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #20 The Increment and Decrement Operators The increment operator (++) adds one to its integer or floating point operand The decrement operator (--) subtracts one The statement count++; is essentially equivalent to count = count + 1;

21. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #21 The Increment and Decrement Operators The increment and decrement operators can be applied in prefix (before the variable) or postfix (after the variable) form When used alone in a statement, the prefix and postfix forms are basically equivalent. That is, count++; is equivalent to ++count;

22. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #22 The Increment and Decrement Operators When used in a larger expression, the prefix and postfix forms have a different effect In both cases the variable is incremented (decremented) But the value used in the larger expression depends on the form Expression count++ ++count count-- --count Operation add 1 subtract 1 Value of Expression old value new value old value new value

23. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #23 The Increment and Decrement Operators If count currently contains 45, then total = count++; assigns 45 to total and 46 to count If count currently contains 45, then total = ++count; assigns the value 46 to both total and count

24. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #24 The Increment and Decrement Operators If sum contains 25, then the statement System.out.println (sum++ + " " + ++sum + " " + sum + " " + sum--); prints the following result: 25 27 27 27 and sum contains 26 after the line is complete

25. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #25 Logical Operators There are three logical operators in Java: They all take boolean operands and produce boolean results Logical NOT is unary (one operand), but logical AND and OR are binary (two operands) Operator ! && || Operation Logical NOT Logical AND Logical OR

26. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #26 Logical NOT The logical NOT is also called logical negation or logical complement If a is true, !a is false; if a is false, then !a is true Logical expressions can be shown using truth tables a false true !a true false

27. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #27 Logical AND The expression a && b is true if both a and b are true, and false otherwise Truth tables show all possible combinations of all terms a false true b false true false true a && b false true

28. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #28 Logical OR The expression a || b is true if a or b or both are true, and false otherwise a false true b false true false true a || b false true

29. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #29 Assignment Operators Often we perform an operation on a variable, then store the result back into that variable Java provides assignment operators that simplify that process For example, the statement num += count; is equivalent to num = num + count;

30. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #30 Assignment Operators There are many such assignment operators, always written as op=, such as: Operator += -= *= /= %= Example x += y x -= y x *= y x /= y x %= y Equivalent To x = x + y x = x - y x = x * y x = x / y x = x % y

31. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #31 Assignment Operators The right hand side of an assignment operator can be a complete expression The entire right-hand expression is evaluated first, then combined with the additional operation Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num);

32. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #32 The Conditional Operator Java has a conditional operator that evaluates a boolean condition that determines which of two expressions is evaluated The result of the chosen expression is the result of the entire conditional operator Its syntax is: condition ? expression1 : expression2 If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

33. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #33 The Conditional Operator It is similar to an if-else statement, except that it is an expression that returns a value For example: larger = (num1 > num2) ? num1 : num2; If num1 is greater that num2, then num1 is assigned to larger ; otherwise, num2 is assigned to larger The conditional operator is ternary, meaning it requires three operands

34. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #34 The Conditional Operator Another example: System.out.println ("Your change is " + count + (count == 1) ? "Dime" : "Dimes"); If count equals 1, "Dime" is printed, otherwise "Dimes" is printed

35. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #35 Another Selection Statement The if and the if-else statements are selection statements, allowing us to select which statement to perform next based on some boolean condition Another selection construct, called the switch statement, provides another way to choose the next action The switch statement evaluates an expression, then attempts to match the result to one of a series of values Execution transfers to statement list associated with the first value that matches

36. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #36 The switch Statement The syntax of the switch statement is: switch (expression) { case value1: statement-list1 case value2: statement-list2 case … }

37. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #37 The switch Statement The expression must evaluate to an integral value, such as an integer or character The break statement is usually used to terminate the statement list of each case, which causes control to jump to the end of the switch statement and continue A default case can be added to the end of the list of cases, and will execute if no other case matches

38. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #38 int acount = 0, ecount = 0, icount = 0, ocount = 0; int ucount = 0, other = 0, index = 0; String quote = "We are the Borg. Resistance is futile."; while (index < quote.length()) { switch (quote.charAt (index)) { case 'a': acount++; break; case 'e': ecount++; break; // other cases come here... default: other++; } index++; }

39. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #39 More Repetition Constructs In addition to while loops, Java has two other constructs used to perform repetition: the do statement the for statement Each loop type has its own unique characteristics You must choose which loop type to use in each situation

40. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #40 The do Statement The do statement has the following syntax: do statement while (condition); The statement is executed until the condition becomes false It is similar to a while statement, except that its termination condition is evaluated after the loop body

41. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #41 The do Statement statement condition false true

42. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #42 The do Statement The key difference between a do loop and a while loop is that the body of the do loop will execute at least once If the condition of a while loop is false initially, the body of the loop is never executed Another way to put this is that a while loop will execute zero or more times and a do loop will execute one or more times

43. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #43 The for Statement The syntax of the for loop is for (intialization; condition; increment) statement; which is equivalent to initialization; while (condition) { statement; increment; }

44. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #44 The for Statement Like a while loop, the condition of a for statement is tested prior to executing the loop body Therefore, a for loop will execute zero or more times It is well suited for executing a specific number of times, known in advance Note that the initialization portion is only performed once, but the increment portion is executed after each iteration

45. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #45 The for Statement statement condition false true initialization increment

46. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #46 The for Statement Examples: for (int count=1; count < 75; count++) System.out.println (count); for (int num=5; num <= total; num *= 2) { sum += num; System.out.println (sum); }

47. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #47 The for Statement Each expression in the header of a for loop is optional –If the initialization is left out, no initialization is performed –If the condition is left out, it is always considered to be true, and therefore makes an infinite loop –If the increment is left out, no increment operation is performed Both semi-colons are always required

48. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #48 Arrays An array is an ordered list of values Each value has a numeric index An array of size N is indexed from zero to N-1 The following array of integers has a size of 10 and is indexed from 0 to 9 0 1 2 3 4 5 6 7 8 9 scores 79 87 94 82 67 98 87 81 74 91

49. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #49 Arrays A particular value in an array is referenced using the array name followed by the index in brackets For example, the expression scores[4] refers to the value 67 (which is the 5th value in the array) That expression represents a place to store a single integer, can can be used wherever an integer variable can For example, it can be assigned a value, printed, used in a calculation

50. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #50 Arrays An array stores multiple values of the same type That type can be primitive types or objects Therefore, we can create an array of integers, or an array of characters, or an array of String objects, etc. In Java, the array itself is an object Therefore the name of the array is an object reference variable, and the array itself is instantiated separately

51. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #51 Declaring Arrays The scores array could be declared as follows: int[] scores = new int[10]; Note that the type of the array does not specify its size, but each object of that type has a specific size The type of the variable scores is int[] (an array of integers) It is set to a newly instantiated array of 10 integers

52. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #52 Declaring Arrays Some examples of array declarations: float[] prices = new float[500]; boolean[] flags; flags = new boolean[20]; char[] codes = new char[1750];

53. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #53 Bounds Checking Once an array is created, it has a fixed size An index used in an array reference must specify a valid element That is, they must be in bounds (0 to N-1) The Java interpreter will throw an exception if an array index is out of bounds This is called automatic bounds checking Its common to inadvertently introduce off-by-one errors when using arrays

54. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #54 Bounds Checking Each array object has a public constant called length that stores the size of the array It is referenced through the array name (just like any other object): scores.length Note that length holds the number of elements, not the largest index

55. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #55 final int STUDENTS = 100; InputReader in = new InputReader(); float[] test = new float[STUDENTS], shifted = new float[STUDENTS]; float sum = 0, bonus; float average; for (int i = 0; i < STUDENTS; i++) { test[I] = in.readFloat(“Enter grade “ + i); sum += test[i]; } average = sum / STUDENTS; for (int i = 0; i < STUDENTS; i++) shifted[i] = (float) Math.sqrt((double) (10 * test[i])); // print shifted[] and test[]...

56. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #56 Array Declarations Revisited The brackets of the array type can be associated with the element type or with the name of the array Therefore float[] prices; and float prices[]; are essentially equivalent The first format is usually more readable

57. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #57 Initializer Lists An initializer list can be used to instantiate and initialize an array in one step The values are delimited by braces and separated by commas Examples: int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; char[] letter_grades = {'A', 'B', 'C', 'D', 'F'};

58. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #58 Initializer Lists Note that when an initializer list is used: –the new operator is not used –no size value is specified The size of the array is determined by the number of items in the initializer list An initializer list can only be used in the declaration of an array

59. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #59 Arrays of Objects The elements of an array can be object references The declaration String[] words = new String[25]; reserves space to store 25 references to String objects It does NOT create the String objects themselves Each object stored in an array must be instantiated separately

60. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #60 class Children { public static void main (String[] args) { String[] name_list = {"Joshua", "Bethany", "Megan", "Eric"}; for (int name = 0; name < name_list.length; name++) System.out.println (name_list[name]); } // method main } // class Children

61. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #61 class Reverse { public static void main (String[] args) { InputReader in = new InputReader(); String[] names = new String[5]; for (int name = 0; name < names.length; name++) names[name] = in.readString(“Enter string “ + i); System.out.println ("The strings you entered, in reverse:"); for (int name = names.length-1; name >= 0; name--) System.out.println (names[name]); } // method main } // class Reverse

62. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #62 Arrays as Parameters An entire array can be passed to a method as a parameter Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other Changing an array element in the method changes the original An array element can be passed to a method as well, and follow the parameter passing rules of that element's type

63. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #63 class ArrayExample { public static void copy(int[] a1, int[] a2) { for (int i = 0; i < a1.length; i++) a1[i] = a2[i]; } public static void referenceCopy(int[] a1, int[] a2) { a1 = a2; } public static void multiply(int[] a, int c) { for (int i = 0; i < a.length; i++) a[i] *= c; } public static int minElement(int[] a) { int min = Integer.MAX_VALUE; for (int i = 0; i < a.length; i++) if (a[i] < min) min = a[i]; return min; }

64. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #64 class ArrayExample { // … continued from previous slide public static int[] clone(int[] a) { int[] clone = new int[a.length]; for (int i = 0; i < a.length; i++) clone[i] = a[i]; return clone; } int[] a1 = {11, 22, 11, 33, 44, 55}; int[] a2 = {99, 99, 99, 105, -3}; ArrayExample.copy(a2,a1); ArrayExample.copy(a1,a2); // what happens here? ArrayTools.referenceCopy(a1,a2); int[] a3 = ArrayExample.clone(a1);

65. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #65 Multidimensional Arrays A one-dimensional array stores a simple list of values A two-dimensional array can be thought of as a table of values, with rows and columns A two-dimensional array element is referenced using two index values To be precise, a two-dimensional array in Java is an array of arrays, therefore each row can have a different length

66. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #66 Multidimensional Arrays An initializer list can be used to create and set up a multidimensional array Each element in the list is itself an initializer list Note that each array dimension has its own length constant

67. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #67 class Table2D { private int[][] table = { {28, 84, 47, 72}, {69, 26}, {91, 40, 28}, {42, 34, 37}, {13, 26, 57, 35} }; public void print() { for (int row=0; row < table.length; row++) { for (int col=0; column < table[row].length; col++) System.out.print (table[row][column] + " "); System.out.println(); } } // method print // … method sumColumn in next slide … } // class Table2D

68. The Interdisciplinary Center, Herzelia Lecture 5, Introduction to CS - Information Technologies Slide #68 public int sumColumn (int column) { int sum = 0; for (int row=0; row < table.length; row++) if (column < table[row].length) sum += table[row][column]; return sum; }