如何：使用并行容器提高效率

此主题展示如何使用并行容器高效地并行存储和访问数据。

示例代码并行计算质数和卡迈克尔数的集合。 然后，对于每个卡迈克尔数，代码计算该数的质因子。

示例：确定输入值是否为质数

下面的示例展示了 is_prime 函数，它确定输入值是否为质数，以及 is_carmichael 函数，它确定输入值是否为卡迈克尔数。

// 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;
}

// Determines whether the input value is a Carmichael number.
bool is_carmichael(const int n) 
{
   if (n < 2) 
      return false;

   int k = n;
   for (int i = 2; i <= k / i; ++i) 
   {
      if (k % i == 0) 
      {
         if ((k / i) % i == 0) 
            return false;
         if ((n - 1) % (i - 1) != 0)
            return false;
         k /= i;
         i = 1;
      }
   }
   return k != n && (n - 1) % (k - 1) == 0;
}

示例：计算质数和卡迈克尔数

下面的示例使用 is_prime 和 is_carmichael 函数来计算质数和卡迈克尔数的集合。 该示例使用 concurrency::parallel_invoke 和 concurrency::parallel_for 算法并行计算每个集。 有关并行算法的详细信息，请参阅并行算法。

此示例使用 concurrency::concurrent_queue 对象保存卡迈克尔数集，因为它稍后会将该对象用作工作队列。 它使用 concurrency::concurrent_vector 对象来保存质数集，因为它稍后将循环访问此集以查找质因子。

// The maximum number to test.
const int max = 10000000;

// Holds the Carmichael numbers that are in the range [0, max).
concurrent_queue<int> carmichaels;

// Holds the prime numbers that are in the range  [0, sqrt(max)).
concurrent_vector<int> primes;

// Generate the set of Carmichael numbers and the set of prime numbers
// in parallel.
parallel_invoke(
   [&] {
      parallel_for(0, max, [&](int i) {
         if (is_carmichael(i)) {
            carmichaels.push(i);
         }
      });
   },
   [&] {
      parallel_for(0, int(sqrt(static_cast<double>(max))), [&](int i) {
         if (is_prime(i)) {
            primes.push_back(i);
         }
      });
   });

示例：查找给定值的所有质因子

以下示例展示 prime_factors_of 函数，该函数使用试除法查找给定值的所有质因子。

此函数使用 concurrency::parallel_for_each 算法循环访问质数集合。 该 concurrent_vector 对象使并行循环能够并发地向结果中添加质数。

// Finds all prime factors of the given value.
concurrent_vector<int> prime_factors_of(int n, 
   const concurrent_vector<int>& primes)
{
   // Holds the prime factors of n.
   concurrent_vector<int> prime_factors;
   
   // Use trial division to find the prime factors of n.
   // Every prime number that divides evenly into n is a prime factor of n.
   const int max = sqrt(static_cast<double>(n));
   parallel_for_each(begin(primes), end(primes), [&](int prime)
   {
      if (prime <= max)
      {         
         if ((n % prime) == 0)
            prime_factors.push_back(prime);
      }
   });

   return prime_factors;
}

示例：处理卡迈克尔数队列中的每个元素

此示例通过调用 prime_factors_of 函数计算其质因子，来处理卡迈克尔数队列中的每个元素。 它使用任务组，以并行方式实现此操作。 有关任务组的详细信息，请参阅任务并行。

此示例为每个具有四个以上质因子的卡迈克尔数打印其质因子。

// Use a task group to compute the prime factors of each 
// Carmichael number in parallel.
task_group tasks;

int carmichael;
while (carmichaels.try_pop(carmichael))
{
   tasks.run([carmichael,&primes] 
   {
      // Compute the prime factors.
      auto prime_factors = prime_factors_of(carmichael, primes);

      // For brevity, print the prime factors for the current number only
      // if there are more than 4.
      if (prime_factors.size() > 4)
      {
         // Sort and then print the prime factors.
         sort(begin(prime_factors), end(prime_factors));

         wstringstream ss;
         ss << L"Prime factors of " << carmichael << L" are:";

         for_each (begin(prime_factors), end(prime_factors), 
            [&](int prime_factor) { ss << L' ' << prime_factor; });
         ss << L'.' << endl;

         wcout << ss.str();
      }
   });
}

// Wait for the task group to finish.
tasks.wait();

示例：完成的并行容器代码示例

以下代码展示了完整示例，该示例使用并行容器来计算卡迈克尔数的质因子。

// carmichael-primes.cpp
// compile with: /EHsc
#include <ppl.h>
#include <concurrent_queue.h>
#include <concurrent_vector.h>
#include <iostream>
#include <sstream>

using namespace concurrency;
using namespace std;

// 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;
}

// Determines whether the input value is a Carmichael number.
bool is_carmichael(const int n) 
{
   if (n < 2) 
      return false;

   int k = n;
   for (int i = 2; i <= k / i; ++i) 
   {
      if (k % i == 0) 
      {
         if ((k / i) % i == 0) 
            return false;
         if ((n - 1) % (i - 1) != 0)
            return false;
         k /= i;
         i = 1;
      }
   }
   return k != n && (n - 1) % (k - 1) == 0;
}

// Finds all prime factors of the given value.
concurrent_vector<int> prime_factors_of(int n, 
   const concurrent_vector<int>& primes)
{
   // Holds the prime factors of n.
   concurrent_vector<int> prime_factors;
   
   // Use trial division to find the prime factors of n.
   // Every prime number that divides evenly into n is a prime factor of n.
   const int max = sqrt(static_cast<double>(n));
   parallel_for_each(begin(primes), end(primes), [&](int prime)
   {
      if (prime <= max)
      {         
         if ((n % prime) == 0)
            prime_factors.push_back(prime);
      }
   });

   return prime_factors;
}

int wmain()
{
   // The maximum number to test.
   const int max = 10000000;
   
   // Holds the Carmichael numbers that are in the range [0, max).
   concurrent_queue<int> carmichaels;

   // Holds the prime numbers that are in the range  [0, sqrt(max)).
   concurrent_vector<int> primes;
   
   // Generate the set of Carmichael numbers and the set of prime numbers
   // in parallel.
   parallel_invoke(
      [&] {
         parallel_for(0, max, [&](int i) {
            if (is_carmichael(i)) {
               carmichaels.push(i);
            }
         });
      },
      [&] {
         parallel_for(0, int(sqrt(static_cast<double>(max))), [&](int i) {
            if (is_prime(i)) {
               primes.push_back(i);
            }
         });
      });

   // Use a task group to compute the prime factors of each 
   // Carmichael number in parallel.
   task_group tasks;

   int carmichael;
   while (carmichaels.try_pop(carmichael))
   {
      tasks.run([carmichael,&primes] 
      {
         // Compute the prime factors.
         auto prime_factors = prime_factors_of(carmichael, primes);

         // For brevity, print the prime factors for the current number only
         // if there are more than 4.
         if (prime_factors.size() > 4)
         {
            // Sort and then print the prime factors.
            sort(begin(prime_factors), end(prime_factors));

            wstringstream ss;
            ss << L"Prime factors of " << carmichael << L" are:";

            for_each (begin(prime_factors), end(prime_factors), 
               [&](int prime_factor) { ss << L' ' << prime_factor; });
            ss << L'.' << endl;

            wcout << ss.str();
         }
      });
   }

   // Wait for the task group to finish.
   tasks.wait();
}

此示例生成以下示例输出。

Prime factors of 9890881 are: 7 11 13 41 241.
Prime factors of 825265 are: 5 7 17 19 73.
Prime factors of 1050985 are: 5 13 19 23 37.

编译代码

复制示例代码，并将它粘贴到 Visual Studio 项目中，或粘贴到名为 carmichael-primes.cpp 的文件中，再在 Visual Studio 命令提示符窗口中运行以下命令。

cl.exe /EHsc carmichael-primes.cpp

另请参阅

并行容器和对象
任务并行
concurrent_vector 类
concurrent_queue 类
parallel_invoke 函数
parallel_for 函数
task_group 类