The answer is not found same for the factorial (Parallel and Normal)

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Why is not The answer same following? Why is not The answer same following?

Normal coding :

C#

for (long i = 1; i <= 5; i++)
 {
   sum = sum * i;
 }

Parallel Coding :

C#

Parallel.For(1, 5, delegate(int i){ sum = sum * i; });

result of parallel=24
result of Normal=120
why ?

Posted 3-Dec-11 10:08am

jackrasha

Updated 3-Dec-11 10:21am

Monjurul Habib

v2

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Monjurul Habib 3-Dec-11 16:22pm

Edited: Code block.

3 solutions

Solution 2

Hi,

You have tried to parallize a calculation where the result of one iteration is dependent on the previous. If the sum was not used in the multiplication the value would not be transferred to the next iteration.
You can only parallize stuff where iterations have no relation to each other.

Your result may vary on the machine you run it on when you do it like this, the sum value needs closure th make sure it's not effected outside the loop.

Hope this helps.

Regards, AT

Posted 3-Dec-11 10:55am

Addy Tas

Solution 3

The reason is,,,, the value of sum was changed from the 1st for loop and used again in the second block of codes. Try using 2 different variables like: sum1 and sum2

Regards,
Eduard

Posted 3-Dec-11 13:02pm

Eduard Lu

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Wonde Tadesse · Accepted Answer · 2011-12-03T10:31:00

Solution 1

In the parallel for loop the loop size is not inclusive to the loop where as in normal for loop the loop size included. Take look at for detail for Parallel Programming[^] MSDN article.

Posted 3-Dec-11 10:31am

Wonde Tadesse