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POParallel.for causes different results
 

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    <p>I am currently trying to improve a C# project I am working on. Specifically, my goal is to parallelize some operations to reduce processing time. I am starting with small snippets just to get the hang of it. The following code (not parallel) works correctly (as expected)</p> <pre><code>for (int i = 0; i &lt; M; i++) { double d; try { d = Double.Parse(lData[i]); } catch (Exception) { throw new Exception("Wrong formatting on data number " + (i + 1) + " on line " + (lCount + 1)); } sg[lCount % N][i] = d; } </code></pre> <p>By using the following (parallel) code I would expect to obtain the exact same results, but that is not the case.</p> <pre><code>Parallel.For(0, M, i =&gt; { double d; try { d = Double.Parse(lData[i]); } catch (Exception) { throw new Exception("Wrong formatting on data number " + (i + 1) + " on line " + (lCount + 1)); } sg[lCount % N][i] = d; }); </code></pre> <p>The part of the program these snippets are from reads data from a file, one line at a time. Each line is a sequence of comma-separated double precision numbers, that I put in the vector lData[] using String.Split(). Every M lines, the data sequence starts over with a new data frame (hence the <code>% M</code> in the element index when i assign the values).</p> <p>It is my understanding (clearly wrong) that by putting the code from the (serial) for-loop in the third parameter of <code>Parallel.For</code> I parallelize its execution. This shouldn't change the results. Is the problem in the fact that the threads are all accessing to lCount and M? Should I make thread-local copies?</p> <p>Thanks.</p> <p>(since I'm new I am not allowed to create the <code>Parallel.For</code> tag)</p> <p>EDIT: I ran some more tests. Basically I looked at an output earlier in the code than what I did before. It would appear that the parallel version of my code does not fill the <code>sg[][]</code> array entirely. Rather, some values are left to their defaults (0, in my case).</p> <p>EDIT 2 (to answer some of the comments): <code>lData[]</code> is a <code>string[]</code> obtained by using <code>string.Split()</code>. The original string I am splitting is read from my data files. I wrote the code that generates them, so they are generally well-formatted (I still used the <code>try-catch</code> construct out of habit). Just before the for-loop (wither parallel or serial) I check to verify that <code>lData[]</code> has the correct number of values (M). If it doesn't, I throw an exception that prevents the program from reaching the for-loop in question. <code>sg[][]</code> is a N by M array of type <code>double</code> (there was a typo in the snippets, now corrected; In my original code this error was not present). After I read N lines from the file the array <code>sg[][]</code> contains a whole data set. After the for-loop (either parallel or serial) there is a portion of come that looks like this: lCount++; //counting the lines I have already read if((lCount % N) == 0) { //do things with sg[][] //reset sg[][] } So, I am on purpose overwriting all lines of <code>sg[][]</code>. The for-loop's whole purpose is to update the values in <code>sg[][]</code>.</p>
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    1. COAside: you can increase performance by using `Double.TryParse` instead of exception handling.
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    2. COThanks @James, I didn't know that.
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    3. COFor the missing values, did you receive the exception? Also if your processing depends on order, Parallel.For does not execute in specific order.
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