如何：在已完成的任务之间选择

项目
06/09/2015

本示例演示如何使用 concurrency::choice 和 concurrency::join 类选择用于完成搜索算法的第一个任务。

示例

下面的示例以并行方式执行两个搜索算法，并选择第一个算法来完成。该示例定义了保存员工的数字标识符和薪水的 employee 类型。 find_employee 函数会查找第一个具有提供的标识符或薪水的员工。 find_employee 函数还处理没有员工具有提供的标识符或薪水的情况。 wmain 函数创建一个 employee 对象数组，并搜索若干标识符和薪水值。

该示例使用 choice 对象在以下情况之间进行选择：

存在一个具有提供的标识符的员工。
存在一个具有提供的薪水的员工。
没有员工具有提供的标识符或薪水。

对于前两种情况，该示例使用 concurrency::single_assignment 对象保存标识符，使用另一个 single_assignment 对象保存薪水。该示例对第三种情况使用 join 对象。 join 对象由两个附加 single_assignment 对象组成，一个用于没有员工具有提供的标识符的情况，一个用于没有员工具有提供的薪水的情况。 join 对象在其每个成员收到一条消息时发送一条消息。在该示例中，join 对象在没有员工具有提供的标识符或薪水时发送一条消息。

该示例使用 concurrency::structured_task_group 对象并行运行这两个搜索算法。每个搜索任务写入一个 single_assignment 对象，以指示是否存在给定的员工。该示例使用 concurrency::receive 函数获取第一个缓冲区的索引，该缓冲区包含用于输出结果的消息和 switch 块。

// find-employee.cpp 
// compile with: /EHsc
#include <agents.h>
#include <ppl.h>
#include <array>
#include <iostream>
#include <random>

using namespace concurrency;
using namespace std;

// Contains information about an employee. 
struct employee
{
   int id;
   float salary;
};

// Finds the first employee that has the provided id or salary. 
template <typename T>
void find_employee(const T& employees, int id, float salary)
{
   // Holds the salary for the employee with the provided id.
   single_assignment<float> find_id_result;

   // Holds the id for the employee with the provided salary.
   single_assignment<int> find_salary_result;


   // Holds a message if no employee with the provided id exists.
   single_assignment<bool> id_not_found;

   // Holds a message if no employee with the provided salary exists.
   single_assignment<bool> salary_not_found;

   // Create a join object for the "not found" buffers.
   // This join object sends a message when both its members holds a message  
   // (in other words, no employee with the provided id or salary exists).
   auto not_found = make_join(&id_not_found, &salary_not_found);


   // Create a choice object to select among the following cases: 
   // 1. An employee with the provided id exists. 
   // 2. An employee with the provided salary exists. 
   // 3. No employee with the provided id or salary exists.
   auto selector = make_choice(&find_id_result, &find_salary_result, &not_found);


   // Create a task that searches for the employee with the provided id.
   auto search_id_task = make_task([&]{
      auto result = find_if(begin(employees), end(employees), 
         [&](const employee& e) { return e.id == id; });
      if (result != end(employees))
      {
         // The id was found, send the salary to the result buffer.
         send(find_id_result, result->salary);
      }
      else
      {
         // The id was not found.
         send(id_not_found, true);
      }
   });

   // Create a task that searches for the employee with the provided salary.
   auto search_salary_task = make_task([&]{
      auto result = find_if(begin(employees), end(employees), 
         [&](const employee& e) { return e.salary == salary; });
      if (result != end(employees))
      {
         // The salary was found, send the id to the result buffer.
         send(find_salary_result, result->id);
      }
      else
      {
         // The salary was not found.
         send(salary_not_found, true);
      }
   });

   // Use a structured_task_group object to run both tasks.
   structured_task_group tasks;
   tasks.run(search_id_task);
   tasks.run(search_salary_task);

   wcout.setf(ios::fixed, ios::fixed);
   wcout.precision(2);

   // Receive the first object that holds a message and print a message. 
   int index = receive(selector);
   switch (index)
   {
   case 0:
      wcout << L"Employee with id " << id << L" has salary " 
            << receive(find_id_result);
      break;
   case 1:
      wcout << L"Employee with salary " << salary << L" has id " 
            << receive(find_salary_result);
      break;
   case 2:
      wcout << L"No employee has id " << id << L" or salary " << salary;
      break;
   default:
      __assume(0);
   }
   wcout << L'.' << endl;

   // Cancel any active tasks and wait for the task group to finish.
   tasks.cancel();
   tasks.wait();
}

int wmain()
{
   // Create an array of employees and assign each one a  
   // random id and salary. 

   array<employee, 10000> employees;

   mt19937 gen(15);
   const float base_salary = 25000.0f;
   for (int i = 0; i < employees.size(); ++i)
   {
      employees[i].id = gen()%100000;

      float bonus = static_cast<float>(gen()%5000);
      employees[i].salary = base_salary + bonus;
   }

   // Search for several id and salary values.

   find_employee(employees, 14758, 30210.00);
   find_employee(employees, 340, 29150.00);
   find_employee(employees, 61935, 29255.90);
   find_employee(employees, 899, 31223.00);
}

该示例产生下面的输出。

该示例使用 concurrency::make_choice helper 函数创建 choice 对象，使用 concurrency::make_join helper 函数创建 join 对象。