C++ 2011 : std::thread : simple example to parallelize a loop?

Question

C++ 2011 includes very cool new features, but I can\'t find a lot of example to parallelize a for-loop. So my very naive question is : how do you parallelize a simple for lo

Answer 1

AFAIK the simplest way to parallelize a loop, if you are sure that there are no concurrent access possible, is by using OpenMP.

It is supported by all major compilers except LLVM (as of August 2013).

Example :

for(int i = 0; i < n; ++i)
{
   tab[i] *= 2;
   tab2[i] /= 2;
   tab3[i] += tab[i] - tab2[i];
}

This would be parallelized very easily like this :

#pragma omp parallel for
for(int i = 0; i < n; ++i)
{
   tab[i] *= 2;
   tab2[i] /= 2;
   tab3[i] += tab[i] - tab2[i];
}

However, be aware that this is only efficient with a big number of values.

If you use g++, another very C++11-ish way of doing would be using a lambda and a for_each, and use gnu parallel extensions (which can use OpenMP behind the scene) :

__gnu_parallel::for_each(std::begin(tab), std::end(tab), [&] () 
{
    stuff_of_your_loop();
});

However, for_each is mainly thought for arrays, vectors, etc... But you can "cheat" it if you only want to iterate through a range by creating a Range class with begin and end method which will mostly increment an int.

Note that for simple loops that do mathematical stuff, the algorithms in #include and #include can all be parallelized with G++.