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## Solution for Programmming Exercise 8.6

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

### Exercise 8.6:

Exercise 3.2 asked you to find the integer in the range 1 to 10000 that has the largest number of divisors. Now write a program that uses multiple threads to solve the same problem. By using threads, your program will take less time to do the computation when it is run on a multiprocessor computer. At the end of the program, output the elapsed time, the integer that has the largest number of divisors, and the number of divisors that it has. The program can be modeled on the sample prime-counting program ThreadTest2.java from Subsection 8.5.3.

Discussion

Pseudocode for solving this problem without using threads is:

    maxDivisors = 0    // Maximum number of divisors seen so far
    for N from 1 to 10000:
       Let count be the number of divisors of N
       if (count > maxDivisors):
          maxDivisors = count  // New maximum number of divisors
          whichInt = N         // The integer that gave the maximum

To turn this into a multithreaded program, we have to divide the integers between 1 and 10000 into groups and assign each group of integers to a thread. In its run method, each thread finds the maximum number of divisors for integers in its assigned group. This is very similar to what was done in the prime-counting program ThreadTest2.java. The only problem is combining the final results of all the threads. In ThreadTest2.java, each thread counted some primes and recorded its results by calling a method

    synchronized private static void addToTotal(int x)

We can do something similar here. The thread actually has two pieces of data to report, the maximum number of divisors that it found and the integer that had that many divisors. The data from the thread has to be combined with the overall information about the maximum found by any thread so far. So, the reporting method becomes:

    /**
     * This method is called by a thread when it has completed its computation
     * to report the largest number of divisors that it found in its assigned
     * range of integers.  The information is used to update the variables
     * maxDivisorCount and intWithMaxDivisorCount.
     * @param maxCountFromThread largest divisor count in the thread's assigned
     *          range
     * @param intWithMaxFromThread the integer that had the maximum number
     *          of divisors.
     */
    synchronized private static void report(int maxCountFromThread,
          int intWithMaxFromThread) {
       if (maxCountFromThread > maxDivisorCount) {
          maxDivisorCount = maxCountFromThread;
          intWithMaxDivisorCount = intWithMaxFromThread;
       }
    }

The class that defines the threads, CountDivisorsThread, is very similar to the thread class from ThreadTest2.java. You can see it in the complete solution below.

In the program ThreadTest2.java, the number of integers to be processed was chosen to be evenly divisible by the number of threads. This made it easier to divide the integers among the threads, since every thread can be assigned exactly the same number of integers to process. For this program, I decided to drop the assumption that the number of integers is divisble by the number of threads. This makes the program more general, but it's a little harder to decide how to assign the integers to the threads.

If there are T threads and N integers, then each thread gets at least N/T integers (where N/T means the integer quotient, discarding any remainder). But after assigning N/T integers to each thread, there might still be a few integers left over. (The number of leftovers is actually N % T.) In my program, I just add these extra integers onto the group of integers assigned to the last thread, so the last thread might have more integers to process than the other threads. I do this by making sure that the last number assigned to the last thread is 10000.)

    int integersPerThread = 10000/numberOfThreads;
    int start = 1;  // Starting point of the range of ints for first thread.
    int end = start + integersPerThread;   // End point of the range of ints.
    for (int i = 0; i < numberOfThreads; i++) {
       if (i == numberOfThreads - 1) {
          end = 10000;  // Make sure that the last thread's range goes all
                        // the way up to 10000.  Because of rounding, this
                        // is not automatic.
       }
       worker[i] = new CountDivisorsThread( start, end );
       start = end+1;    // Determine the range of ints for the NEXT thread.
       end = start+integersPerThread;
    }

As one final change, instead of using the number 10000 as the nubmer of integers to be processed in my program, I use a constant named MAX to make it easier to change the range of integers that is considered.

The Solution

    /**
     * This program finds the number in the range 1 to some maximum that has the
     * largest number of divisors.  It prints that number and the number of divisors
     * that it has.  Note that there might be several numbers that have the maximum
     * number of divisors.  Only one of them is output.
     *
     * The program's work is divided among one to five threads.  The number
     * of threads is chosen by the user.
     */
    public class CountDivisorsUsingThreads {

       /**
        * The upper limit of the range of integers that is to be tested.
        */
       private final static int MAX = 10000;

       /**
        * The largest number of divisors found so far.  (Note: This is
        * volatile since it is referenced in unsynchrnized code in the
        * countDivisorsWithThreads() method.)
        */
       private volatile static int maxDivisorCount = 0;

       /**
        * An integer that has the maximum number of divisors seen so far.
        */
       private volatile static int intWithMaxDivisorCount;

       /**
        * This method is called by a thread when it has completed its computation
        * to report the largest number of divisors that it found in its assigned
        * range of integers.  The information is used to update the variables
        * maxDivisorCount and intWithMaxDivisorCount.
        * @param maxCountFromThread largest divisor count in the thread's assigned
        *          range
        * @param intWithMaxFromThread the integer that had the maximum number
        *          of divisors.
        */
       synchronized private static void report(int maxCountFromThread,
             int intWithMaxFromThread) {
          if (maxCountFromThread > maxDivisorCount) {
             maxDivisorCount = maxCountFromThread;
             intWithMaxDivisorCount = intWithMaxFromThread;
          }
       }

       /**
        * A thread belonging to this class counts the number of divisors for all
        * the integers in an assigned range of integers.  The range is specified
        * in the constructor.  The thread finds the integer in the range that
        * has the largest number of divisors, and a number that has that many
        * divisors.  At the end of its computation, the thread reports its answer
        * by calling the report() method.
        */
       private static class CountDivisorsThread extends Thread {
          int count = 0;
          int min, max;
          public CountDivisorsThread(int min, int max) {
             this.min = min;
             this.max = max;
          }
          public void run() {
    //         System.out.println("Thread " + this + " testing range " +
    //               min + " to " + max);  // For testing.
    //         long startTime = System.currentTimeMillis();
             int maxDivisors = 0;
             int whichInt = 0;
             for (int i = min; i < max; i++) {
                int divisors = countDivisors(i);
                if (divisors > maxDivisors) {
                   maxDivisors = divisors;
                   whichInt = i;
                }
             }
    //         long elapsedTime = System.currentTimeMillis() - startTime;
    //         System.out.println("Thread " + this + " used " +
    //               (elapsedTime/1000.0) + " seconds.");   // or testing.
             report(maxDivisors,whichInt);
          }
       }

       /**
        * Finds the number in the range 1 to 10000 that has the largest number of
        * divisors, dividing the work among a specified number of threads.
        */
       private static void countDivisorsWithThreads(int numberOfThreads) {
          System.out.println("\nCounting divisors using " +
                numberOfThreads + " threads...");
          long startTime = System.currentTimeMillis();
          CountDivisorsThread[] worker = new CountDivisorsThread[numberOfThreads];
          int integersPerThread = MAX/numberOfThreads;
          int start = 1;  // Starting point of the range of ints for first thread.
          int end = start + integersPerThread;   // End point of the range of ints.
          for (int i = 0; i < numberOfThreads; i++) {
             if (i == numberOfThreads - 1) {
                end = MAX;  // Make sure that the last thread's range goes all
                             // the way up to MAX.  Because of rounding, this
                             // is not automatic.
             }
             worker[i] = new CountDivisorsThread( start, end );
             start = end+1;    // Determine the range of ints for the NEXT thread.
             end = start+integersPerThread;
          }
          maxDivisorCount = 0;
          for (int i = 0; i < numberOfThreads; i++)
             worker[i].start();
          for (int i = 0; i < numberOfThreads; i++) {
                 // Wait for each worker thread to die, because the results
                 // are not complete until all threads have completed and
                 // reported their results.
             while (worker[i].isAlive()) {
                try {
                   worker[i].join();
                }
                catch (InterruptedException e) {
                }
             }
          }
          long elapsedTime = System.currentTimeMillis() - startTime;
          System.out.println("\nThe largest number of divisors " +
                "for numbers between 1 and " + MAX + " is " + maxDivisorCount);
          System.out.println("An integer with that many divisors is " +
                intWithMaxDivisorCount);
          System.out.println("Total elapsed time:  " +
                (elapsedTime/1000.0) + " seconds.\n");
       }

       /**
        * Gets the number of threads from the user and calls countDivisorsWithThreads()
        * to do the actual work.
        */
       public static void main(String[] args) {
          int numberOfThreads = 0;
          while (numberOfThreads < 1 || numberOfThreads > 10) {
             System.out.print("How many threads do you want to use  (1 to 10) ?  ");
             numberOfThreads = TextIO.getlnInt();
             if (numberOfThreads < 1 || numberOfThreads > 10)
                System.out.println("Please enter a number from 1 to 10 !");
          }
          countDivisorsWithThreads(numberOfThreads);
       }

        /**
         * Finds the number of divisors of the integer N.  Note that this method does
         * the counting in a stupid way, since it tests every integer in the range
         * 1 to N to see whether it evenly divides N.
         */
       public static int countDivisors(int N) {
          int count = 0;
          for (int i = 1; i <= N ; i++) {
             if ( N % i == 0 )
                count ++;
          }
          return count;
       }

    }

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