Chapter 7 Exercise 21, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

***7.21 (Game: bean machine) The bean machine, also known as a quincunx or the Galton box, is a device for statistics experiments named after English scientist Sir Francis Galton. It consists of an upright board with evenly spaced nails (or pegs) in a triangular form, as shown in Figure 7.13. 

Balls are dropped from the opening of the board. Every time a ball hits a nail, it has a 50% chance of falling to the left or to the right. The piles of balls are accumulated in the slots at the bottom of the board. Write a program that simulates the bean machine. Your program should prompt the user to enter the number of the balls and the number of the slots in the machine. Simulate the falling of each ball by printing its path. For example, the path for the ball in Figure 7.13b is LLRRLLR and the path for the ball in Figure 7.13c is RLRRLRR. Display the final buildup of the balls in the slots in a histogram. Here is a sample run of the program:

Enter the number of balls to drop: 5
 Enter the number of slots in the bean machine: 8
 LRLRLRR
 RRLLLRR
 LLRLLRR
 RRLLLLL
 LRLRRLR

           O
           O
   O O O


 (Hint: Create an array named slots. Each element in slots stores the number of balls in a slot. Each ball falls into a slot via a path. The number of Rs in a path is the position of the slot where the ball falls. For example, for the path LRLRLRR, the ball falls into slots[4], and for the path is RRLLLLL, the ball falls into slots[2].) 

import java.util.Scanner;
public class    Exercise_21 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.print("Enter the number of balls to drop: ");
        int numberOfBalls = input.nextInt();
        String[] ballPaths = new String[numberOfBalls];

        System.out.print("Enter the number of slots in the bean machine: ");
        int numberOfSlots = input.nextInt();
        int[] slots = new int[numberOfSlots];

        // display ball paths
        for (int i = 0; i < numberOfBalls; i++) {
            ballPaths[i] = dropBall(slots);
            System.out.printf("%10s\n", ballPaths[i]);
        }

        // display game
        System.out.println("");
        printArray(slots, numberOfBalls);
    }

    public static String dropBall(int[] slot) {

        StringBuilder ballPath = new StringBuilder();


        for (int i = 0; i < slot.length - 1; i++) {
            int random = (int)(Math.random() * 10) % 2;
            if (random > 0) ballPath.append("R");
            else ballPath.append("L");
        }
        int position = getBallPosition(ballPath.toString(), 'R');
        slot[position]++;
        return ballPath.toString();

    }

    public static int getBallPosition(String str, char a) {

        int count = 0;
        for (int i = 0; i < str.length(); i++) {

            if (str.charAt(i) == a) count++;

        }

        return count;
    }


    public static void printArray(int[] slots, int numberOfBalls) {

            while (!isEmpty(slots)) {

                if (isRowEmpty(slots, numberOfBalls)) {
                    numberOfBalls--;
                    continue;
                }
                for (int i = 0; i < slots.length; i++) {

                    if (slots[i] >= numberOfBalls) {
                        System.out.printf("%2c", 'O');
                        slots[i]--;
                    }
                    else System.out.printf("%2c", ' ');

                }
                numberOfBalls--;
                System.out.println("");

            }

    }

    public static boolean isEmpty(int[] slots) {

        for (int slot : slots) {
            if (slot != 0) {
                return false;
            }
        }

        return true;
    }

    public static boolean isRowEmpty(int[] slots, int rowNum) {

        for (int slot : slots) {
            if (slot == rowNum) {
                return false;
            }
        }

        return true;
    }
}