Mehran Sahami Handout #16 CS 106A October 10, 2007

Mehran Sahami Handout #16
CS 106A October 10, 2007
Examples of Programs Using Random Numbers
Portions of this handout by Eric Roberts
/*
* File: RollDice.java
* -------------------
* This program simulates rolling some number of dice until
* the maximal value on the all the dice is rolled.
*/
import acm.program.*;
import acm.util.*;
public class RollDice extends ConsoleProgram {
/* Number of sides on each die */
private static final int NUM_SIDES = 6;
public void run() {
int numDice = readInt("Number of dice: ");
int maxRoll = numDice * NUM_SIDES;
int numRolls = 0;
while (true) {
int roll = rollDice(numDice);
numRolls++;
if (roll == maxRoll) break;
println("Rolled " + roll);
}
println("Rolled " + maxRoll + " after " + numRolls + " rolls");
}
/* Returns the total of rolling numDice dice */
private int rollDice(int numDice) {
int total = 0;
for (int i = 0; i < numDice; i++) {
total += rgen.nextInt(1, NUM_SIDES);
}
return total;
}
/* Private instance variables */
private RandomGenerator rgen = RandomGenerator.getInstance();
}
– 2 –
/* * File: ColorChangingSquare.java * ------------------------------ * This program puts up a square in the center of the window * and randomly changes its color every second. */ import acm.graphics.*; import acm.program.*; import acm.util.*; public class ColorChangingSquare extends GraphicsProgram { /* Size of the square in pixels */ private static final int SQUARE_SIZE = 100; /* Pause time in milliseconds */ private static final int PAUSE_TIME = 1000; public void run() { GRect square = new GRect(SQUARE_SIZE, SQUARE_SIZE); square.setFilled(true); add(square, (getWidth() - SQUARE_SIZE) / 2, (getHeight() - SQUARE_SIZE) / 2); /* Note: we meant to have this infinite loop */ while (true) { square.setColor(rgen.nextColor()); pause(PAUSE_TIME); } } /* Private instance variables */ private RandomGenerator rgen = RandomGenerator.getInstance(); }
– 3 –
/* * File: PiApproximation.java * -------------------------- * This program computes an approximation to pi by simulating * a dart board, as described in Chapter 6, Programming Exercise 3 * of "The Art and Science of Java". The general technique * is called Monte Carlo integration. */ import acm.program.*; import acm.util.*; public class PiApproximation extends ConsoleProgram { /* Number of darts to throw. */ private static final int NDARTS = 10000; public void run() { int inside = 0; for (int i = 0; i < NDARTS; i++) { double x = rgen.nextDouble(-1.0, +1.0); double y = rgen.nextDouble(-1.0, +1.0); /* Consider circle of radius = 1, centered at (0, 0) */ if (((x * x) + (y * y)) < 1.0) { inside++; } } /* * Note: area of circle = PI * r * r = PI * 1 * 1 = PI * area of square = side * side = 2 * 2 = 4 * So, PI/4 is the fraction of darts landing in circle: * darts in circle = NDARTS * PI/4 * PI = (4 * darts in circle)/NDARTS */ double pi = (4.0 * inside) / NDARTS; println("Pi is approximately " + pi); } /* Private instance variables */ private RandomGenerator rgen = RandomGenerator.getInstance(); }