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POC++ OpenMP for-loop global variable problems
 

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  1. POC++ OpenMP for-loop global variable problems
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    <p>I'm new to OpenMP and from what I have read about OpenMP 2.0, which comes standard with Microsoft Visual Studio 2010, global variables are considered troublesome and error prone when used in parallel programming. I have also been adopting this feeling since I have found very little on how to deal with global variables and static global variables efficiently, or at all for that matter.</p> <p>I have this snippet of code which runs but because of the local variable created in the parallel block I don't get the answer I'm looking for. I get 8 different print outs (because that how many threads I have on my PC) instead of 1 answer. I know that it's because of the local variables "list" created in the parallel block but this code will not run if I move the "list" variable and make it a global variable. Actually the code does run but it never gives me an answer back. This is the sample code that I would like to modify to use a global "list" variable :</p> <pre><code>#pragma omp parallel { vector&lt;int&gt; list; #pragma omp for for(int i = 0; i &lt; 50000; i++) { list.push_back(i); } cout &lt;&lt; list.size() &lt;&lt; endl; } </code></pre> <p>Output:</p> <pre><code>6250 6250 6250 6250 6250 6250 6250 6250 </code></pre> <p>They add up to 50000 but I did not get the one answer with 50000, instead it's divided up.</p> <p>Solution:</p> <pre><code> vector&lt;int&gt; list; #pragma omp parallel { #pragma omp for for(int i = 0; i &lt; 50000; i++) { cout &lt;&lt; i &lt;&lt; endl; #pragma omp critical { list.push_back(i); } } } cout &lt;&lt; list.size() &lt;&lt; endl; </code></pre>
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