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POParallelize a loop using std::thread and good practices
 

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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/10792157/c-2011-stdthread-simple-example-to-parallelize-a-loop">C++ 2011 : std::thread : simple example to parallelize a loop?</a> </p> </blockquote> <p>Consider the following program that distribute a computation over the elements of a vector (I never used std::thread before):</p> <pre><code>// vectorop.cpp // compilation: g++ -O3 -std=c++0x vectorop.cpp -o vectorop -lpthread // execution: time ./vectorop 100 50000000 // (100: number of threads, 50000000: vector size) #include &lt;iostream&gt; #include &lt;iomanip&gt; #include &lt;cstdio&gt; #include &lt;vector&gt; #include &lt;thread&gt; #include &lt;cmath&gt; #include &lt;algorithm&gt; #include &lt;numeric&gt; // Some calculation that takes some time template&lt;typename T&gt; void f(std::vector&lt;T&gt;&amp; v, unsigned int first, unsigned int last) { for (unsigned int i = first; i &lt; last; ++i) { v[i] = std::sin(v[i])+std::exp(std::cos(v[i]))/std::exp(std::sin(v[i])); } } // Main int main(int argc, char* argv[]) { // Variables const int nthreads = (argc &gt; 1) ? std::atol(argv[1]) : (1); const int n = (argc &gt; 2) ? std::atol(argv[2]) : (100000000); double x = 0; std::vector&lt;std::thread&gt; t; std::vector&lt;double&gt; v(n); // Initialization std::iota(v.begin(), v.end(), 0); // Start threads for (unsigned int i = 0; i &lt; n; i += std::max(1, n/nthreads)) { // question 1: // how to compute the first/last indexes attributed to each thread // with a more "elegant" formula ? std::cout&lt;&lt;i&lt;&lt;" "&lt;&lt;std::min(i+std::max(1, n/nthreads), v.size())&lt;&lt;std::endl; t.push_back(std::thread(f&lt;double&gt;, std::ref(v), i, std::min(i+std::max(1, n/nthreads), v.size()))); } // Finish threads for (unsigned int i = 0; i &lt; t.size(); ++i) { t[i].join(); } // question 2: // how to be sure that all threads are finished here ? // how to "wait" for the end of all threads ? // Finalization for (unsigned int i = 0; i &lt; n; ++i) { x += v[i]; } std::cout&lt;&lt;std::setprecision(15)&lt;&lt;x&lt;&lt;std::endl; return 0; } </code></pre> <p>There is already two questions embedded in the code. </p> <p>A third one would be: is this code is completely ok or could it be written in a more elegant way using std::threads ? I do not know the "good practices" using std::thread...</p>
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    1. CO"question 2" is answered by the comment immediately preceding it.
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    2. COAlso for question 2 you can use a barrier.
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    3. COthe comment is from me, so I do not know if a join loop finish all threads before going to the next instruction.
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