This thread program shows me different answers every time

Question

This is a Java Program to Find The Number with Largest Divisors from 1-500000.

public class Medium2 {  
  static int  count1 = 1;  
  static int  count2 = 1;  
  static int  big_count = 0;  
  static int  big = 0;  

Main method

  public static void main(String[] args) {  
    Runnable runnable1 = new Runnable() {  
      public void run() {  

The implementation goes here

    for (int num = 1; num <= 500000; num++) {  
      for (int i = 2; i <= num; i++) {  
        if (num % i == 0) {  //Actual Logic  
          count1++;  
        }  
      }  
      if (count1 > big_count) {  
        big_count = count1;  //Number of Divisors  
        big = num;  //Largest Number
      }  
      count1 = 1;  
    }  
  }  
};  

And the thread execution

Thread thread1 = new Thread(runnable1);  //Threads
Thread thread2 = new Thread(runnable1);  
thread1.start();  
thread2.start();  
try {  
  thread1.join();  
  thread2.join();  
} catch (InterruptedException ie) {  
  ;  
}  
System.out.println("Biggest: " + big + "\nNumber of Divisors for " + big + " = " + big_count); 
  }  
}  

But it gives different answers every time. The actual answer is : 498960 and 200 Divisors

Enclosure

"the actual answer is: " ... no, it isn't. the actual answer depends on the run, not on what you think. you get different results, because each time there are different results – True

Where does the "but" come from? Your program is knee-deep in data races. – Disentangle

thread will give different result on different runs – Spenserian

What's the question? It seems this code was written to demonstrate multi-threading race-conditions and giving different answers every time. – Knesset

You have gone out of your way to design a program which should give different results each time, so I don't understand what your question is. – Sherise

I just want to implement a program to Find the number with the largest Divisor in the range 1-500000 And I need it to implement it using Multi-threading But this program gives different answers every time. Seems like some threads get interrupted. How to avoid such errors ? – Enclosure

I guess that multi-threading is required here for a MapReduce approach, and not for doing exactly same calculation in parallel. – Obligate

Answer 1

Concerning your goal, your implementation should probably have problems. Since big_count and big is common for both threads and don't have any protection when threads are trying to modify those, your program should create errors.

Other than that, you are also not utilizing 2 threads, since both threads are doing calculation from 1 to 500000.

Since your calculation logic seems ok, you should get your desired output when you try with single thread.

If you want it to do by two threads, you can easily try this. (just to verify, not the nicest way)

• You should have big_count1, big1 and big_count2, big2. So that variables whose names end with '1' is only using by thread1 and variables whose names end with '2' is only using by thread2.

• Assign thread1 to check from 1 to 250000 and thread2 to from 250001 to 500000.

• After join() s, just compare big_count1 and big_count2, then you can deduce the final answer. :))

Scholl