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

Explore how to implement parallel versions of std::count_if and std::copy_if in C++. Understand the challenges of concurrent writes, learn to manage chunk sizes based on cores, and apply synchronization strategies to handle data races for efficient parallel algorithms.

We'll cover the following...

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:

C++
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 ...