1. Object-Oriented Programming 95-712 MISM/MSIT Carnegie Mellon University Lecture 2: Program Control

2. Today We Look At Java operators Java operators Control structures Control structures More example programs More example programs

3. Java Operators An operator takes one or more “things” and produces a resultant “thing”. An operator takes one or more “things” and produces a resultant “thing”. “Things” are usually primitive types, but they are sometimes objects. “Things” are usually primitive types, but they are sometimes objects. The “things” operated upon are called operands. The “things” operated upon are called operands. An operator is just a function, but with a different syntax. An operator is just a function, but with a different syntax.

4. A Familiar Example The assignment operator and the addition operator are used (each exactly once!). The assignment operator and the addition operator are used (each exactly once!). This is a more familiar syntax than, e.g., This is a more familiar syntax than, e.g.,k.equals(i.add(j)); int i = 3, j = 4, k; k = i + j;

5. More Operator Facts All operators produce a value. All operators produce a value. Sometimes they produce side effects, i.e., they change the value of an operand. Sometimes they produce side effects, i.e., they change the value of an operand. Evaluation of a statement with several operators follows precedence rules. Use parentheses for readability. Evaluation of a statement with several operators follows precedence rules. Use parentheses for readability. (x + y) * z / 3 is different than x + y * z / 3 (x + y) * z / 3 is different than x + y * z / 3

6. Assignment Is Tricky, Part I public class Number { int i; } public class Assignment1 { public static void main(String[] args) { Number n1 = new Number(); n1.i = 2; n2.i = 5; n1.i = n2.i; n2.i = 10;// what is n1.i? }

7. Assignment Is Tricky, Part II public class Assignment2 { public static void main(String[] args) { Number n1 = new Number(); n1.i = 2; n2.i = 5; n1 = n2; n2.i = 10;// what is n1.i? n1.i = 20;// what is n2.i? }

8. A Picture Might Help n1n2 reference variables Number objects n2.in1.i Before assignment n1 = n2 n1n2 reference variables Number objects n2.in1.i After assignment n1 = n2 5 2 5 2

9. “Aliasing” In Function Calls public class PassObject { static void f(Number m) { m.i = 15; } public static void main(String[] args) { Number n = new Number(); f(n);// what is n.i now? }

10. Math Operators +, -, *, /, % +, -, *, /, % Integer division truncates, i.e., 16/3 = 5 Integer division truncates, i.e., 16/3 = 5 Modulus operator returns remainder on integer division, i.e., 16%3 = 1 Modulus operator returns remainder on integer division, i.e., 16%3 = 1 Shorthand: Shorthand: x += 4; is the same as x = x + 4; This works for the other math operators as well. This works for the other math operators as well.

11. Auto Increment and Decrement ++ increases by one, and -- decreases by one. ++ increases by one, and -- decreases by one. Two flavors of each: pre and post: Two flavors of each: pre and post: int i = 1, j; j = i++;// j = 1, i = 2 j = ++i;// j = 3, i = 3 j = i--;// j = 3, i = 2 j = --i;// j = 1, i = 1

12. Booleans and Relational Operators The boolean type has two possible values, true and false. The boolean type has two possible values, true and false. The relational operators >, >=,, >=, <, <=, == and != produce a boolean result. >, >=,, >=, <, <= are legal for all built-in types except booleans, == and != are legal for all.

13. Testing for (Non-)Equivalence The == and != operators need to be used with care with objects. The == and != operators need to be used with care with objects. public class Equivalence { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1 == n2);// prints false System.put.println(n1 != n2);// prints true }

14. The equals( ) Operator This exists for all objects (don’t need it for built-in types). This exists for all objects (don’t need it for built-in types). Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1.equals(n2);// prints true

15. The equals( ) Operator (cont.) But exists doesn’t necessarily mean properly defined! But exists doesn’t necessarily mean properly defined! class Number { int i; } : Number n1 = new Number(); Number n2 = new Number(); n1.i = 3; n2.i = 3; System.out.println(n1.equals(n2));// prints false

16. The equals( ) Operator (cont.) The equals( ) operator is properly defined for most Java library classes. The equals( ) operator is properly defined for most Java library classes. The default behavior is to compare references, so… The default behavior is to compare references, so… When you define a class, if you’re planning to use equals( ), you need to define it (i.e., override the default behavior). When you define a class, if you’re planning to use equals( ), you need to define it (i.e., override the default behavior).

17. Logical Operators These are AND (&&), OR (||), and NOT (!). These are AND (&&), OR (||), and NOT (!). These work on booleans only; if you have old “C” habits, forget them! These work on booleans only; if you have old “C” habits, forget them! Use parentheses freely to group logical expressions. Use parentheses freely to group logical expressions. Logical expressions short-circuit; as soon as the result is known, evaluation stops. Logical expressions short-circuit; as soon as the result is known, evaluation stops.

18. Short-Circuiting Example public class ShortCircuit { static boolean test1(int val) {return val < 1;} static boolean test2(int val) {return val < 2;} static boolean test3(int val) {return val < 3;} public static void main(String[] args) { if (test1(0) && test2(2) && test3(2)) System.out.println(“Expression is true”); else System.out.println(“Expression is false”); }

19. Bitwise, Shift, Ternary Operators Bitwise & shift operators manipulate individual bits in integral primitive types. Bitwise & shift operators manipulate individual bits in integral primitive types. I almost never use them, except for masks, when memory is tight. YMMV I almost never use them, except for masks, when memory is tight. YMMV The ternary if-else operator looks like this: The ternary if-else operator looks like this: The result is either value0 or value1, depending on the truth of the boolean. The result is either value0 or value1, depending on the truth of the boolean. boolean-expression ? value0 : value1

20. The String + Operator The + operator is “overloaded” for String objects; it means concatenation. The + operator is “overloaded” for String objects; it means concatenation. It reminds me of good old C++… It reminds me of good old C++… If an expression begins with a String, then all the following operands of + will be converted into Strings: If an expression begins with a String, then all the following operands of + will be converted into Strings: int x = 0, y = 1; z = 2; String myString = “x, y, z ”; System.out.println(myString + x + y + z);

21. Casting A cast produces a temporary new value of a designated type. A cast produces a temporary new value of a designated type. Implicit and explicit casts: Implicit and explicit casts: int i = 2; float f = i;// OK, since f can hold all of i float g = 3.14159; //! int j = g;// not OK, loses information int k = (int) g;// OK, compiler is reassured

22. Execution Control: if-else if (boolean_expression) statement else if(boolean_expression) statement : else if(boolean_expression) statement else statement

23. if-else Example public int test(int testVal, int target) { int result = 0; if (testVal > target) result = +1; else if (testVal < target) result = -1; else { System.out.println(“They are equal”); result = 0; } return result; }

24. Execution Control: return Exit a method, returning an actual value or object, or not (if the return type is void). Exit a method, returning an actual value or object, or not (if the return type is void). public int test(int testVal, int target) { if (testVal > target) return = +1; else if (testVal < target) return = -1; else { System.out.println(“They are equal”); result = 0; }

25. Three Kinds of Iteration while (boolean_expression)// evaluate first statement_or_block do statement_or_block// evaluate last while (boolean_expression) for (initialization ; boolean_expression ; step) statement_or_block Example: for (int i = 0; i < myArray.size(); i++) { myArray[i] = 0; }

26. public class BreakAndContinue { public static void main(String[] args) { for (int i = 0; i < 100; i++) { if (i == 74) break;// out of for loop if (i % 9 != 0) continue;// next iteration System.out.println(i); } int i = 0;// this does work while (true) {// an infinite loop i++; int j = i * 27; if (j == 1269) break;// out of loop if (i % 10 != 0) continue;// top of loop }

27. Selection Via switch for (int i = 0; i < 100; i++) { char c = (char) (Math.random() * 26 + ‘a’); switch(c) { case ‘a’: case ‘e’: case ‘i’: case ‘o’: case ‘u’: System.out.println(“Vowel”); break; case ‘y’: case ‘w’: System.out.println(“Sometimes a vowel”); break; default: System.out.println(“Not a vowel”); }

28. Digression on Random Numbers Despite theory, most random number generators are more like “kids playing with matches”. Despite theory, most random number generators are more like “kids playing with matches”. –See “Random Number Generators: Good Ones Are Hard to Find” by S. Park and K. Miller, CACM Oct. 1988. Most random number generators use “multiplicative linear congruential” schemes: Most random number generators use “multiplicative linear congruential” schemes: – A modulus m, a large prime integer –A multiplier a, an integer in the range 2,3,…m-1 –These produce a sequence z 1, z 2, z 3 … using the iterative equation –z i+1 = f(z i ) = a*z % m

29. Random Numbers (cont.) The sequence is initiated by choosing a seed. The sequence is initiated by choosing a seed. Example: f(z) = 6z %13. This produces the sequence...1, 6, 10, 8, 9, 2, 12, 7, 3, 5, 4, 11, 1,... Example: f(z) = 6z %13. This produces the sequence...1, 6, 10, 8, 9, 2, 12, 7, 3, 5, 4, 11, 1,... Example: f(z) = 7z % 13. This produces the sequence...1, 7, 10, 5, 9, 11, 12, 6, 3, 8, 4, 2, 1,... Example: f(z) = 7z % 13. This produces the sequence...1, 7, 10, 5, 9, 11, 12, 6, 3, 8, 4, 2, 1,... Is the latter somehow "less random"? Is the latter somehow "less random"? Example: f(z) = 5z %13. This produces the sequence...1, 5, 12, 8, 1... Example: f(z) = 5z %13. This produces the sequence...1, 5, 12, 8, 1...

30. Examples of Rotten RNGs IBM's RANDU for SYSTEM/360: IBM's RANDU for SYSTEM/360: –f(z) = 65539z % 2 31. Not full period, dependent on results of overflow operations, low-order bits cycle w/ period 2! Prime’s Scheffield Pascal, an even worse version Prime’s Scheffield Pascal, an even worse version SAS SAS Modula 2 Modula 2 Many LISPs and Prologs Many LISPs and Prologs Hundreds of CS textbooks Hundreds of CS textbooks Turbo Pascal Turbo Pascal One generator had a fixed point! One generator had a fixed point!

31. Using Java’s RNGs (cont.) java.lang.Math java.lang.Math –static double random()random in [0, 1.0) The sequence doesn’t seem to be repeatable The sequence doesn’t seem to be repeatable Bad for debugging Bad for debugging Good for experimental work Good for experimental work

32. What’s a Seed? Random number sequence -1321140701 1591074477 646222811 1633627825 -2039264851 -315534709 Here is a sequence I generated on Friday at 9:30, with the code: Random randomGenerator = new Random(); for (int i = 0; i < 10; i++) System.out.println(randomGenerator.nextInt()); i = 0 i = 1 i = 2 i = 3 i = 4 i = 5

33. What’s a Seed? Random number sequence -403585738 -2092042741 -333693675 -331823022 1651689643 133526058 I ran the same code again at 10:30, and got this: i = 0 i = 1 i = 2 i = 3 i = 4 i = 5

34. What’s a Seed? Random number sequence -1517918040 1115789266 -208917030 1019800440 -6116528751 1362132786 I ran this code at 10:35 Random randomGenerator = new Random(1234); for (int i = 0; i < 10; i++) System.out.println(randomGenerator.nextInt()); i = 0 i = 1 i = 2 i = 3 i = 4 i = 5

35. What’s a Seed? Random number sequence -1517918040 1115789266 -208917030 1019800440 -6116528751 1362132786 I ran this code at 10:45 and got the same sequence. Random randomGenerator = new Random(1234); for (int i = 0; i < 10; i++) System.out.println(randomGenerator.nextInt()); i = 0 i = 1 i = 2 i = 3 i = 4 i = 5

36. Using Java’s RNGs (cont.) java.util.Random java.util.Random –Constructors: »Random() »Random(long seed) –Methods: »nextInt()random in (-2 31, 2 31 -1) »nextInt(int n)random in [0, n) »nextFloat()random in [0, 1) »setSeed(long seed) java.lang.Math java.lang.Math –static double random()random in [0, 1.0)

37. Random Numbers In Action: The Monte Hall Paradox Monte Hall hosts a TV game Monte Hall hosts a TV game show, “Let’s Make a Deal”. One contestant is shown three One contestant is shown threedoors. Behind one of the doors is a Behind one of the doors is a “Fabulous Grand Prize”. Behind the other two doors are “worthless prizes”. Behind the other two doors are “worthless prizes”.

38. The Monte Hall Paradox The contestant selects a door. The contestant selects a door. Monte opens a door not selected by the contestant, and this always shows a worthless prize. Monte opens a door not selected by the contestant, and this always shows a worthless prize. The contestant can choose to change her selection to the other door not opened by Monte Hall. The contestant can choose to change her selection to the other door not opened by Monte Hall. Should the contestant change doors? Should the contestant change doors?

39. The Monte Hall Paradox Very intelligent people are confused! Very intelligent people are confused! –Any initial door choice has a 1/3 chance of winning. –Why would changing the door choice increase or decrease the odds of winning the “Fabulous Grand Prize”? –Can’t we settle this with a computer simulation?

40. The Door Class public class Door { boolean open; boolean hasGrandPrize; boolean chosenByContestant; }

41. The Game Class import java.util.*; public class Game { Door door1, door2, door3; void setUpGame() { door1 = new Door();door2 = new Door();door3 = new Door(); // initialize all the Door variables to false Random r = new Random(); int grandPrizeDoor = r.nextInt(3); switch(grandPrizeDoor) { case 0: door1.hasGrandPrize = true; break; case 1: // etc. }

42. Reality Check For Game Class void printStateOfDoors() { System.out.println("Door 1 is " + (door1.open ? " open, " : "not open, ") + (door1.hasGrandPrize ? "is the grand prize, and " : "is not the grand prize, and ") + (door1.chosenByContestant ? "is chosen." : "is not chosen.") ); System.out.println("Door 2 is " + (door2.open ? " open, " : "not open, ") + (door2.hasGrandPrize ? "is the grand prize, and " : "is not the grand prize, and ") + (door2.chosenByContestant ? "is chosen." : "is not chosen.") ); System.out.println("Door 3 is " + (door3.open ? " open, " : "not open, ") + (door3.hasGrandPrize ? "is the grand prize, and " : "is not the grand prize, and ") + (door3.chosenByContestant ? "is chosen." : "is not chosen.") ); }

43. The Game Class public class Game { Door door1, door2, door3; void setUpGame() { // shown earlier} void contestantChooseDoor() { Random r = new Random(); int contestantDoor = r.nextInt(3); switch(contestantDoor) { case 0: door1.chosenByContestant = true; break; case 1: door2.chosenByContestant = true; break; case 2: door3.chosenByContestant = true; break; }

44. The PlayManyGames Class public class PlayManyGames { public static void main(String[] args) { Game theGame = new Game(); theGame.setUpGame(); theGame.printStateOfDoors(); theGame.contestantChooseDoor(); theGame.printStateOfDoors(); }

45. A Problem: First Run Door 1 is not open, is not the grand prize, and is not chosen. Door 2 is not open, is not the grand prize, and is not chosen. Door 3 is not open, is the grand prize, and is not chosen. Door 1 is not open, is not the grand prize, and is not chosen. Door 2 is not open, is not the grand prize, and is not chosen. Door 3 is not open, is the grand prize, and is chosen.

46. A Problem: Second Run Door 1 is not open, is not the grand prize, and is not chosen. Door 2 is not open, is the grand prize, and is not chosen. Door 3 is not open, is not the grand prize, and is not chosen. Door 1 is not open, is not the grand prize, and is not chosen. Door 2 is not open, is the grand prize, and is chosen. Door 3 is not open, is not the grand prize, and is not chosen.

47. A Problem: Third Run Door 1 is not open, is the grand prize, and is not chosen. Door 2 is not open, is not the grand prize, and is not chosen. Door 3 is not open, is not the grand prize, and is not chosen. Door 1 is not open, is the grand prize, and is chosen. Door 2 is not open, is not the grand prize, and is not chosen. Door 3 is not open, is not the grand prize, and is not chosen.

48. The Solution The RNGs in both methods of the Game class are starting from the same seed. The RNGs in both methods of the Game class are starting from the same seed. Solution: Take the RNG out of the methods, and into the Game class itself. Solution: Take the RNG out of the methods, and into the Game class itself. But each instance of the Game class will still start at the same place. But each instance of the Game class will still start at the same place. Solution: Make the RNG a static variable. Solution: Make the RNG a static variable.

49. The New Game Class public class Game { Door door1, door2, door3; static Random r = new Random(); void setUpGame() { int grandPrizeDoor = r.nextInt(3); switch(grandPrizeDoor) { // etc. } void contestantChooseDoor() { int contestantDoor = r.nextInt(3); switch(contestantDoor) { // etc. }

50. Completing the Simulation Add Game methods for Add Game methods for –Monte choosing a door to open –The player switching or not –Deciding if the player has won Add PlayManyGames code to Add PlayManyGames code to –Call the new Game methods –Put everything in a loop –Prompt the user for # of trials, switching policy –Display the results