histogram.java

用java编的代码

/* 6.2 Histogram.java
 * Generate 100 random numbers ~Unif[0,1], calc mean, sd and plot a histogram of distribution.
 * i)    Why is the calculation of the standard deviation inefficient? How 
 *          could we improve on it?
 * ii)   Can we have a different number of decimal places in the names of 
 *          columns in the table we print out? What happens if we don't use a 
 *          formatted number?
 */

import java.text.*;
class Histogram {
   /* Main Method handle top level tasks */
   public static void main(String[] args) {
      double[] array = MyUtil.generateRandomArray(100);

      System.out.println("Mean = " + mean(array));
      System.out.println("Standard Deviation = " + sd(array));

      double[] counts = countInterval(10, array);
      drawHistogram(counts);
   }

   public static double mean(double[] array) {
      double sum = 0;
      int n = array.length;

      for(int i=0; i<n; i++) {
         sum += array[i];
      }

      return sum / n;
   }

   public static double sd(double[] array) {
      double sum = 0;
      int n = array.length;

      for(int i=0; i<n; i++) {
         sum += Math.pow(mean(array) - array[i], 2);
      }

      return Math.sqrt(sum / n);
   }

   /* Splits [0,1] into n intervals and counts how many elements of array 
    * fall into these intervals */
   public static double[] countInterval(int n, double[] array) {
      double [] counts = new double[n];
      double width = 1.0 / n;

      for(int i=0; i<n; i++) counts[i] = 0;

      for(int i =0; i<array.length; i++) {
         int idx = (int) (array[i] / width);
         counts[idx]++;
      }

      return counts;
   }

   public static void drawHistogram(double[] counts) {
      // This object formats numbers nicely
      NumberFormat formatter = new DecimalFormat("0.00"); 

      double width = 1.0 / counts.length;
      for(int i=0; i<counts.length; i++) {
         String dots = "";
         for(int j=0; j<counts[i]; j++) dots += "*";
         String name = formatter.format(i*width);
         name += "-";
         name += formatter.format((i+1)*width);
         System.out.println(name + " : " + dots);
      }
   }
}