GNUplot

• The Random class in the java.util package provides pseudo-random numbers.

• We call them "pseudo-random" rather than random because the source of bits is an algorithm, and therefore cannot be truly random.

• You will need to use this class to generate the random numbers required by the sorting program of Homework 4.

• If you just quickly want a random number between 0.0 and (just less than) 1.0, then package Math has a method random, which is just a wrapper for instantiating Random one time, and then supplying values.

• You call it like this:

    double d = Math.random();   // 0.0 ... 1.0
  

• Some methods of the package Random are:

    public Random();            // no args constructor
    
    public Random(long seed);
    // using the same seed gives the same sequence of numbers
  
    public int nextInt();       // returns a random int
    public long nextLong();     // returns a random long
    public double nextDouble(); // returns a value in range 0.0 to 1.0
  
    public synchronized void setSeed(long seed);
    // resets the seed value
  
    public synchronized double nextGaussian();
    // a Gaussian distribution has mean 0.0, standard deviation 1.0
  

• nextGaussian provides numbers symmetrically distributed around zero, and successively unlikelier to be picked the further away from zero you get.

• If we collected a group of these numbers and plotted them, they would form the shape of a bell curve.

• To get a random value in a certain range (say, 1 to 6) to simulate the cast of a die, you would use:

    Random r = new Random();
    int myturn = 1 + Math.abs(r.nextInt()) % 6;
  

• To get a random double value in a certain range (say, 0.0 to 100.0) to simulate a percentage, you would scale it up with multiplication:

    double mypercent = r.nextDouble() * 100.0;
  

• You can instantiate Random with or without a seed.

• Seeding refers to seting (or reseting) the random number generator to a starting point.

• If you don't give the constructor a seed, it uses the current reading from the day/time clock (the millisecond part of this is going to be pretty close to random).

• If you choose to supply the seed, you will get the same series of random numbers every time you supply that same seed.

• This can be useful in testing and debugging code.

• The setSeed method can be used to reset the seed value at any time.

Executing a Program from a Java Application

• This section explains how to execute a program from Java and read the output of that program back into your application.

• In homework 4 you will have to use this class to call gnuplot from you program (although you will not need to read output back into your program) to generate the graph file.

• Executing a program from your Java program has four general steps:

  1. Get the object representing the current runtime enviornment. A call in class java.lang.Runtime does this.

  2. Call the exec method in the Runtime object with your command as an argument. This call creates a Process object. Give the full path name to the executable, and make sure the executable really exists on the system.

  3. Connect the output of the Process (which will be coming to you as an input stream) to an input stream reader in your program.

  4. You can either read individual characters from the input stream or layer a BufferedReader on it and read a line at a time, as in the code below.

• Here is an example of executing a pogram from Java:

  import java.io.*;
  
  public class exwho {
    
    public static void main(String args[]) {
      try {
        Runtime  rt = Runtime.getRuntime();   // step 1
  
        Process prcs = rt.exec("/bin/who");   // step 2
  
        InputStreamReader isr =                // step 3
           new InputStreamReader( prcs.getInputStream() );
  
        BufferedReader br = new BufferedReader( isr ); // step 4
  
        String line;
  
        while ((line = br.readLine()) != null)
           System.out.println(line);
      }
      catch (IOException ioe) { System.out.println(ioe); }
    }
  }
  

• This code uses a class inside System called Runtime which does a couple of runtime-related things.

• The main use is to provide a way to execute other programs.

• The code also uses the class Process that allows you to get in, out, err streams, and to destroy, or to wait for a process.

• The program /bin/who is a Unix program that lists the users on a system.

• Compiling and running the program gives this ouput (the output has been abbreviated because there were a lot of users on the system):

  $ javac exwho.java
  $ java exwho
  als84      pts/25       Apr  2 21:30	(98AAE98E.ipt.aol.com)
  avd4       pts/5        Apr  2 22:39	(cpw.math.columbia.edu)
  hh245      pts/6        Apr  2 21:41	(butler213a-06)
  vss8       pts/0        Apr  2 22:31	(handogod.schap.rhno.columbia.edu)
  

• Here you can see that I am logged on (and other users) at the stated time and location.

• For Homework 4, after having created the datafiles and the plot file for a set of sorts, you can create a graph within your program by executing gnuplot with the following code:

        Runtime rt = Runtime.getRuntime();
  
        Process prcs = rt.exec("gnuplot plot-random.gp");
  

• This code is assuming your gnuplot file is called plot-random.gp (also note that this code has to catch an IOException).