如何：使用并行容器提高效率
本主题说明如何使用并行容器以并行方式高效地存储和访问数据。
代码示例以并行方式计算一组质数和 Carmichael 数。然后，对于每个 Carmichael 数，此代码将计算该数的质数因子。
示例
下面的示例显示了 is_prime 函数和 is_carmichael 函数，前者可确定输入值是否为质数，后者可确定输入值是否为 Carmichael 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;
}
下面的示例使用 is_prime 和 is_carmichael 函数计算多组质数和 Carmichael 数。该示例使用 concurrency::parallel_invoke 和 concurrency::parallel_for 算法来计算每个并行的设置。有关并行算法的更多信息，请参见并行算法。
本示例使用 concurrency::concurrent_queue 对象来保存 Carmichael 的一组数字，因为它以后将使用该对象为工作队列。它使用 concurrency::concurrent_vector 对象来保存质数的集，因为它以后将循环访问此设置为查找主要因素。
// 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);
}
});
});
下面的示例演示 prime_factors_of 函数，该函数使用试除法来查找给定值的所有质数因子。
此函数使用 concurrency::parallel_for_each 算法来循环访问集合的质数。concurrent_vector 对象使并行循环能够同时向结果中添加多个质数因子。
// 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;
}
此示例通过调用 prime_factors_of 函数计算 Carmichael 数的质数因子来处理 Carmichael 数队列中的每个元素。此示例使用任务组以并行方式执行此任务。有关任务组的更多信息，请参见任务并行（并发运行时）。
此示例将输出每个具有四个以上的质数因子的 Carmichael 数的质数因子。
// 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();
下面的代码演示完整示例，该示例使用并行容器计算 Carmichael 数的质数因子。
// 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();
}
此示例产生下面的示例输出。
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
将示例代码复制并将其粘贴在 Visual Studio 项目中，或将它粘贴到一个文件，名为 carmichael primes.cpp ，然后在 Visual Studio 命令提示符窗口中运行以下命令。
cl.exe /EHsc carmichael-primes.cpp