Created
August 25, 2019 12:00
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Demonstrates parallel sum of array elements.
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#include <iostream> | |
#include <chrono> | |
#include <numeric> | |
#include <random> | |
#include <thread> | |
#include <future> | |
#include <exception> | |
double* randomArray(size_t size, int from, int to) { | |
std::random_device rd; | |
std::mt19937 gen(rd()); | |
std::uniform_real_distribution<double> dis(from, to); | |
double* arr = new double[size]; | |
for (size_t i = 0; i < size; i++) | |
{ | |
arr[i] = dis(gen); | |
} | |
return arr; | |
} | |
void print(double* arr, size_t size) { | |
std::cout << "["; | |
for (size_t i = 0; i < size; i++) | |
{ | |
std::cout << arr[i]; | |
if (i < size - 1) { | |
std::cout << ", "; | |
} | |
} | |
std::cout << "]" << std::endl; | |
} | |
template <typename Iterator> | |
typename std::iterator_traits<Iterator>::value_type | |
adder(Iterator begin, Iterator end) | |
{ | |
using T = typename std::iterator_traits<Iterator>::value_type; | |
return std::accumulate(begin, end, T()); | |
} | |
template <typename Iterator> | |
typename std::iterator_traits<Iterator>::value_type parallel_sum(Iterator begin, Iterator end, size_t n) | |
{ | |
using T = typename std::iterator_traits<Iterator>::value_type; | |
int size = std::distance(begin, end); | |
if (size < n) { | |
throw std::invalid_argument("size shouldn't be less than n"); | |
} | |
// Calculate number of futures required... | |
int numberOfFutures = size / n; | |
// See if leftovers are present (at the end)... | |
int leftovers = size % n; | |
int numberOfFuturesWithLeftovers = numberOfFutures + ((leftovers > 0) ? 1 : 0); | |
std::future<T>* fs = new std::future<T>[numberOfFuturesWithLeftovers]; | |
Iterator it = begin; | |
for (size_t i = 0; it != end - leftovers; it += n, ++i) { | |
fs[i] = std::async(std::launch::async, adder<Iterator>, it, it + n); | |
} | |
if (leftovers > 0) { | |
fs[numberOfFutures] = std::async(std::launch::async, adder<Iterator>, it, end); | |
} | |
double sum = 0.; | |
for (size_t i = 0; i < numberOfFuturesWithLeftovers; ++i) | |
{ | |
sum += fs[i].get(); | |
} | |
return sum; | |
} | |
// https://codereview.stackexchange.com/questions/45557/summing-values-in-a-vector-using-threads | |
int main() | |
{ | |
size_t size = 2000; | |
double* arr = randomArray(size, -100, 100); | |
//print(arr, size); | |
// https://en.cppreference.com/w/cpp/algorithm/accumulate | |
std::cout << "Nor Sum=" << std::accumulate(arr, arr + size, 0.) << std::endl; | |
std::chrono::time_point<std::chrono::high_resolution_clock> start, finish; | |
// N = 10 | |
start = std::chrono::high_resolution_clock::now(); | |
double r10 = parallel_sum(arr, arr + size, 10); | |
finish = std::chrono::high_resolution_clock::now(); | |
std::cout << "Par Sum10=" << r10 << ", time=" << std::chrono::duration_cast<std::chrono::nanoseconds>(finish - start).count() / (double)1000000 << std::endl; | |
// N = 100 | |
start = std::chrono::high_resolution_clock::now(); | |
double r100 = parallel_sum(arr, arr + size, 100); | |
finish = std::chrono::high_resolution_clock::now(); | |
std::cout << "Par Sum100=" << r100 << ", time=" << std::chrono::duration_cast<std::chrono::nanoseconds>(finish - start).count() / (double)1000000 << std::endl; | |
// N = 1000 | |
start = std::chrono::high_resolution_clock::now(); | |
double r1000 = parallel_sum(arr, arr + size, 1000); | |
finish = std::chrono::high_resolution_clock::now(); | |
std::cout << "Par Sum1000=" << r1000 << ", time=" << std::chrono::duration_cast<std::chrono::nanoseconds>(finish - start).count() / (double)1000000 << std::endl; | |
std::cout << "Finished." << std::endl; | |
std::cin.get(); | |
} |
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