C++ High Performance/

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Implementing Parallel std::count_if() and std::copy_if()

Implementing Parallel std::count_if() and std::copy_if()

Learn how to implement the std::count_if() function and parallelize the std::copy_if() operation.

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Implementation of std::count_if()

A nice thing about divide and conquer is that it can be applied to many problems. We can easily use the same technique to implement a parallel version of std::count_if(), with the difference being that we need to accumulate the returned value, like this:

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template <typename It, typename Pred>
auto par_count_if(It first, It last, Pred pred, size_t chunk_sz) {
  auto n = static_cast<size_t>(std::distance(first, last));
  if (n <= chunk_sz)
    return std::count_if(first, last, pred);
  auto middle = std::next(first, n / 2);
  auto fut = std::async(std::launch::async, [=, &pred] {
    return par_count_if(first, middle, pred, chunk_sz);
  });
  auto num = par_count_if(middle, last, pred, chunk_sz);
  return num + fut.get();
}

As you can see, the only difference here is that we need to sum ...