HOW TO：使用可組合的類別結合集合

本主題將說明如何使用 concurrency::combinable 來計算質數的集合類別。

範例

下列範例會計算一組質數兩次。每次計算都會將結果儲存在 std::bitset 物件中。此範例會先循序計算這個集合，然後再以平行方式計算這個集合。範例也會將執行這兩個計算的所需時間列印至主控台。

這個範例會使用 concurrency::parallel_for 演算法和combinable來產生執行緒區域集合的物件。然後會使用 concurrency::combinable::combine_each 方法來組合成最終組的執行緒區域集合。

// parallel-combine-primes.cpp
// compile with: /EHsc
#include <windows.h>
#include <ppl.h>
#include <bitset>
#include <iostream>

using namespace concurrency;
using namespace std;

// Calls the provided work function and returns the number of milliseconds 
// that it takes to call that function.
template <class Function>
__int64 time_call(Function&& f)
{
   __int64 begin = GetTickCount();
   f();
   return GetTickCount() - begin;
}

// Determines whether the input value is prime.
bool is_prime(int n)
{
   if (n < 2)
      return false;
   for (int i = 2; i < n; ++i)
   {
      if ((n % i) == 0)
         return false;
   }
   return true;
}

const int limit = 40000;

int wmain()
{
   // A set of prime numbers that is computed serially.
   bitset<limit> primes1;

   // A set of prime numbers that is computed in parallel.
   bitset<limit> primes2;

   __int64 elapsed;

   // Compute the set of prime numbers in a serial loop.
   elapsed = time_call([&] 
   {
      for(int i = 0; i < limit; ++i) {
         if (is_prime(i))
            primes1.set(i);
      }
   });
   wcout << L"serial time: " << elapsed << L" ms" << endl << endl;

   // Compute the same set of numbers in parallel.
   elapsed = time_call([&] 
   {
      // Use a parallel_for loop and a combinable object to compute 
      // the set in parallel. 
      // You do not need to synchronize access to the set because the 
      // combinable object provides a separate bitset object to each thread.
      combinable<bitset<limit>> working;
      parallel_for(0, limit, [&](int i) {
         if (is_prime(i))
            working.local().set(i);
      });

      // Merge each thread-local computation into the final result.
      working.combine_each([&](bitset<limit>& local) {
         primes2 |= local;
      });
   });
   wcout << L"parallel time: " << elapsed << L" ms" << endl << endl;
}

下列是針對配備四個處理器之電腦的範例輸出。

serial time: 312 ms

parallel time: 78 ms

編譯程式碼

將範例程式碼複製並貼上它在 Visual Studio 專案中，或將它貼在檔名為 primes.cpp-合併平行- ，然後執行下列命令，Visual Studio 的命令提示字元] 視窗中。

cl.exe /EHsc parallel-combine-primes.cpp

請參閱

參考

combinable 類別

combinable::combine_each 方法

概念

平行容器和物件