Road Map

1. Introduction to Computers and ProgrammingLecture 12: Math.random()Professor: Evan KorthNew York University

2. Road Map • Math.random • Reading: • Liang 5: Chapter 4: Section 4.8.5 • Liang 6: Chapter 5: Section 5.9.5

3. review • What is a method? • What information can you learn about a method from its header? • What does it mean to invoke a method? • What is the difference between a formal parameter and an actual parameter? • Why don’t we have to import the Math class? • What is abstraction in computer science?

4. random numbers • Often we want our programs to generate random numbers. • games of chance • testing without user interaction • java.lang.Math provides a method which can be used to generate random numbers

5. Random-Number Generation • Java random-number generators • Math.random() • Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0. • What if we want to generate random integers?

6. Random-Number Generation • ( int ) ( Math.random() * 6 ) • Produces integers from 0 - 5

7. Produce integers in range 1-6 Math.random returns doubles. We cast the double as an int 1 // Fig. 6.7: RandomIntegers.java 2 // Shifted, scaled random integers. 3 import javax.swing.JOptionPane; 4 5 public class RandomIntegers { 6 7 public static void main( String args[] ) 8 { 9 int value; 10 String output = ""; 11 12 // loop 20 times 13 for ( int counter = 1; counter <= 20; counter++ ) { 14 15 // pick random integer between 1 and 6 16 value = 1 + ( int ) ( Math.random() * 6 ); 17 18 output += value + " "; // append value to output 19 20 // if counter divisible by 5, append newline to String output 21 if ( counter % 5 == 0 ) 22 output += "\n"; 23 24 } // end for 25 RandomIntegers.javaLine 16Produce integers in range 1-6Line 16Math.random returns doubles. We cast the double as an int

8. 26 JOptionPane.showMessageDialog( null, output, 27 "20 Random Numbers from 1 to 6", 28 JOptionPane.INFORMATION_MESSAGE ); 29 30 System.exit( 0 ); // terminate application 31 32 } // end main 33 34 } // end class RandomIntegers RandomIntegers.java

9. Produce integers in range 1-6 Increment appropriate frequency counter, depending on randomly generated number 1 // Fig. 6.8: RollDie.java 2 // Roll a six-sided die 6000 times. 3 import javax.swing.*; 4 5 public class RollDie { 6 7 public static void main( String args[] ) 8 { 9 int frequency1 = 0, frequency2 = 0, frequency3 = 0, 10 frequency4 = 0, frequency5 = 0, frequency6 = 0, face; 11 12 // summarize results 13 for ( int roll = 1; roll <= 6000; roll++ ) { 14 face = 1 + ( int ) ( Math.random() * 6 ); 15 16 // determine roll value and increment appropriate counter 17 switch ( face ) { 18 19 case1: 20 ++frequency1; 21 break; 22 23 case2: 24 ++frequency2; 25 break; 26 27 case3: 28 ++frequency3; 29 break; 30 RollDie.javaLine 14Produce integers in range 1-6Lines 17-43Increment appropriate frequency counter, depending on randomly generated number

10. 31 case4: 32 ++frequency4; 33 break; 34 35 case5: 36 ++frequency5; 37 break; 38 39 case6: 40 ++frequency6; 41 break; 42 43 } // end switch 44 45 } // end for 46 47 JTextArea outputArea = new JTextArea(); 48 49 outputArea.setText( "Face\tFrequency" + "\n1\t" + frequency1 + 50 "\n2\t" + frequency2 + "\n3\t" + frequency3 + 51 "\n4\t" + frequency4 + "\n5\t" + frequency5 + 52 "\n6\t" + frequency6 ); 53 54 JOptionPane.showMessageDialog( null, outputArea, 55 "Rolling a Die 6000 Times", JOptionPane.INFORMATION_MESSAGE ); 56 57 System.exit( 0 ); // terminate application 58 59 } // end main 60 61 } // end class RollDie RollDie.java This is different from the example I showed in the compiler. I left it this way to show you that showMessageDialog() can take other objects besides String for the message. Remember the API. You do not need to know JTextArea for this class!