Using Multithreading two threads return same value with different inputs?

Question

Hello I am ne to multithreading and trying to incorporate it into my program in order to get a particular calculation to converge faster. But when I create the two threads like below

        Thread firstThread = new Thread(() => { p0 = f(changePoint, result); });
        Thread secondThread = new Thread(() => { p1 = f(changePoint + 1, result); });
        firstThread.Start();
        secondThread.Start();
        firstThread.Join();
        secondThread.Join();

the values in p0 and p1 are the same(p0 has the same value as p1). Now if i take away the multithreading and just call them like this:

        p0 = f(changePoint, result);
        p1 = f(changePoint + 1, result);

different values are returned and everything works correctly.

What am i missing?

code of f(x,y)

public double f(double x,double result)
    {

        double temp = PCAcont.Future2Yrs(x).Last().FirstOrDefault().StatNetWorthToAssets.GetValueOrDefault();
        return temp - result;

    }

the main method

public double SecantMethod(double prec, int stepsCutoff, double changePoint, double result)
    {
        double p2, p1 = 0, p0 = 0;
        int i;

        Thread firstThread = new Thread(() => { p0 = f(changePoint, result); });
        Thread secondThread = new Thread(() => { p1 = f(changePoint + 1, result); });
        firstThread.Start();
        secondThread.Start();
        firstThread.Join();
        secondThread.Join();

        //p0 = f(changePoint, result);
        //p1 = f(changePoint + 1, result);
        p2 = p1 - f(p1, result) * (p1 - p0) / (f(p1, result) - f(p0, result));

        for (i = 0; System.Math.Abs(p2 - p1) > prec && i < stepsCutoff; i++)
        {
            p0 = p1;
            p1 = p2;
            p2 = p1 - f(p1, result) * (p1 - p0) / (f(p1, result) - f(p0,result));
        }
        if (i < stepsCutoff)
            return p2;
        else
        {
            System.Diagnostics.Debug.WriteLine("{0}.The method did not converge", p2);
            return double.NaN;
        }

    }

Can you show the code for `f(x,y)`? Or at least tell us what type `result` is, and whether it is modified inside `f()`? — hatchet - done with SOverflow, Dec 20 '12 at 22:05
Also, is that the exact code you're running? If your actual code is a for loop with each iteration setting up and starting a thread, the problem could have to do with how you're declaring and using p0 & p1. — hatchet - done with SOverflow, Dec 20 '12 at 22:12
Also Changepoint is getting passed into the method it is not a value from the loop iteration — bewilderedprogrammer, Dec 20 '12 at 22:19
If you have a closure (your `{ p1 = f(changePoint + 1, result); }`) inside a loop, there are some non-intuitive things that can go on. If a loop is involved you should show your actual code. — hatchet - done with SOverflow, Dec 20 '12 at 22:23
I have included the actual method in the main so you can see what is going on — bewilderedprogrammer, Dec 20 '12 at 22:30
Is the computation of `temp` in `f(x,y)` dependant of time? How long does it take to compute `f(x,y)`? — didierc, Dec 21 '12 at 02:18
blam it does receive different values for x but but looks like its only taking the value of the second(changePoint + 1) for both. didierc no it is not dependant on time, it does a about 3-40 seconds since I am using some foward recursion. — bewilderedprogrammer, Dec 21 '12 at 15:34

paparazzo · Accepted Answer · 2013-01-05T00:09:38.700

0

Based on you comment only taking the second.

Look at this

PCAcont.Future2Yrs(x).Last().FirstOrDefault().StatNetWorthToAssets.GetValueOrDefault();

PCAcont is not locked and you are getting the result of the second X.
You need to lock.
And lock past the return as possibly the linq is deferred.

edited Jan 05 '13 at 00:09

answered Dec 21 '12 at 17:32

paparazzo

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Using Multithreading two threads return same value with different inputs?

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