PPLParallelLoopPattern.cpp

// Parallel For Loop Pattern
// Performs the same independent operation for each element in a collection or a fixed number of iterations.
//
// Complied: Visual Studio 2013. 
// Uses Microsoft's Parallel Patterns Library

#include <iostream>
#include <ppl.h>
#include <vector>
#include <numeric>
#include <chrono>

using namespace ::std;

// Simulate a large workload. Consumes a lot of stack.
long Work(const long &i) {
 if (i == 0 || i == 1)
 return i;
 return Work(i - 1) + Work(i - 2);;
}

void Display(const vector<long>& v) {
 cout << endl;
 for (auto const &i : v) {
 cout << i << ",";
 }
 cout << endl;
}

void Time(std::function<void()> f) {
 auto t1 = chrono::high_resolution_clock::now();
 f();
 auto t2 = chrono::high_resolution_clock::now();
 auto tt = (t2 - t1);
 cout << "Time: " << tt.count();
}

vector<long> SequentialForLoop(const vector<long> &input) {
 vector<long> results(input.size());
 for (auto i = 0; i < input.size(); i++) {
 results[i] = Work(input[i]);
 }
 return results;
}

vector<long> ParallelForLoop(const vector<long> &input) {
 vector<long> results(input.size());
 Concurrency::parallel_for(0u, input.size(), [&results, &input](size_t i) {
 results[i] = Work(input[i]); // Each write to result[i] will be independent
 });
 return results;
}

vector<long> SequentialForEachLoop(const vector<long> &input) {
 vector<long> results;
 for_each(input.cbegin(), input.cend(), [&results](size_t i) {
 results.push_back(Work(i));
 });

 // Equivalent to
 //for (auto &i : input) {
 // results.push_back(Work(i));
 //}

 return results;
}

vector<long> ParallelForEachLoop(const vector<long> &input) {
 Concurrency::combinable<vector<long>>results;
 Concurrency::parallel_for_each(input.cbegin(), input.cend(), [&results](long i) {
 results.local().push_back(Work(i));
 });
 // Return combined results
 return results.combine([](vector<long> v1, const vector<long> v2) { 
 v1.insert(v1.end(), v2.cbegin(), v2.cend());
 return v1;
 });
}

int main()
{
 vector<long> input(20);
 std::iota(input.begin(), input.end(), 17);

 Time([&input]() { Display(SequentialForLoop(input)); });
 Time([&input]() { Display(ParallelForLoop(input)); });
 Time([&input]() { Display(SequentialForEachLoop(input)); });
 Time([&input]() { Display(ParallelForEachLoop(input)); });

 cout << endl << "Press enter to exit." << endl;
 cin.ignore();
 return 0;
}