Introduction to Computers and
Programming
Lecture 12: Math.random()
Professor: Evan Korth
New York University
Road Map
• Math.random
• Reading:
– Liang 5: Chapter 4: Section 4.8.5
– Liang 6: Chapter 5: Section 5.9.5
– Liang 7: Chapter 5: Section 5.9.5
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?
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
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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?
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Random-Number Generation
– ( int ) ( Math.random() * 6 )
• Produces integers from 0 - 5
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// Fig. 6.7: RandomIntegers.java
// Shifted, scaled random integers.
import javax.swing.JOptionPane;
Outline
RandomIntegers.
java
public class RandomIntegers {
public static void main( String args[] )
{
int value;
String output = "";
Produce integers in range 1-6
// loop 20 times
for ( int counter = 1; counter <= 20; counter++ ) {
// pick random integer between 1 and 6
value = 1 + ( int ) ( Math.random() * 6 );
output += value + "
";
// append value to output
Line 16
Produce integers in
range 1-6
Line 16
Math.random
returns doubles. We
cast the double as an
int
// if counter divisible by 5, append newline toMath.random
String output returns doubles.
if ( counter % 5 == 0 )
We cast the double as an int
output += "\n";
} // end for
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All rights reserved.
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JOptionPane.showMessageDialog( null, output,
"20 Random Numbers from 1 to 6",
JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
// terminate application
Outline
RandomIntegers.
java
} // end main
} // end class RandomIntegers
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All rights reserved.
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Outline
// Fig. 6.8: RollDie.java
// Roll a six-sided die 6000 times.
import javax.swing.*;
RollDie.java
public class RollDie {
public static void main( String args[] )
{
Produce integers
int frequency1 = 0, frequency2 = 0, frequency3 = 0,
frequency4 = 0, frequency5 = 0, frequency6 = 0, face;
Line 14
in range 1-6Produce integers in
range 1-6
// summarize results
for ( int roll = 1; roll <= 6000; roll++ ) {
face = 1 + ( int ) ( Math.random() * 6 );
// determine roll value and increment appropriate counter
switch ( face ) {
case 1:
++frequency1;
break;
Lines 17-43
Increment appropriate
frequency counter,
depending on
randomly generated
number
Increment appropriate frequency counter,
depending on randomly generated number
case 2:
++frequency2;
break;
case 3:
++frequency3;
break;
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All rights reserved.
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case 4:
++frequency4;
break;
case 5:
++frequency5;
break;
case 6:
++frequency6;
break;
} // end switch
} // end for
JTextArea outputArea = new JTextArea();
outputArea.setText( "Face\tFrequency" + "\n1\t" + frequency1 +
"\n2\t" + frequency2 + "\n3\t" + frequency3 +
"\n4\t" + frequency4 + "\n5\t" + frequency5 +
"\n6\t" + frequency6 );
Outline
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!
JOptionPane.showMessageDialog( null, outputArea,
"Rolling a Die 6000 Times", JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
// terminate application
} // end main
} // end class RollDie
 2003 Prentice Hall, Inc.
All rights reserved.
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 2003 Prentice Hall, Inc. All rights reserved.