Flip.java

Below is the syntax highlighted version of Flip.java from §2.2 Libraries.

/*************************************************************************
 *  Compilation:  javac Flip.java
 *  Execution:    java Flip N
 *  Dependencies: StdDraw.java
 *  
 *  Each experiment consists of flipping N fair coins. Plots (in real
 *  time) a histogram of the number of times i of the N coins are heads.
 *
 *  % java Flip 32
 *
 *  % java Flip 64
 *
 *  % java Flip 128
 *
 *************************************************************************/


public class Flip { 


    // number of heads when flipping N coins, each with prob p of heads
    public static int binomial(int N, double p)  { 
        int heads = 0; 
        for (int j = 0; j < N; j++) 
            if (StdRandom.bernoulli(p)) heads++; 
        return heads; 
    } 


    public static void main(String[] args) { 
        int N = Integer.parseInt(args[0]);   // number of coins to flip
        int[] freq = new int[N+1];           // freq[i] = # times you get i heads
 
        StdDraw.setCanvasSize(1000, 600);
        StdDraw.show(0);
        StdDraw.setXscale(0, N);
        StdDraw.setYscale(0, Math.sqrt(1.0 / N));



        while (true) {
            // flip N coins and tabulate the results
            for (int j = 0; j < N; j++) {
                int heads = binomial(N, 0.5);
                freq[heads]++;
            }

            StdDraw.clear();

            // plot histogram
            StdDraw.setPenRadius(1.0 / N);
            double total = StdStats.sum(freq);
            for (int i = 0; i <= N; i++) {
                StdDraw.line(i, 0, i, freq[i] / total);
            }

            // plot hypothesis
            StdDraw.setPenRadius();
            double mean = 0.5 * N;
            double var  = 0.25 * N;
            for (double i = 0.0; i < N; i += N/1000.0)
                StdDraw.point(i, Gaussian.phi(i, mean, Math.sqrt(var)));
            StdDraw.show(50);
        }
    }
}