How to: Use Cancellation to Break from a Parallel Loop

This example shows how to use cancellation to implement a parallel search algorithm.

The following example uses cancellation to search for an element in an array. The parallel_find_any function uses the Concurrency::parallel_for algorithm and a Concurrency::structured_task_group object to search for the position that contains the given value. When a work function finds the value, it calls the Concurrency::structured_task_group::cancel method to cancel future work. The runtime cancels any active tasks and does not start new ones.


// parallel-array-search.cpp
// compile with: /EHsc
#include <ppl.h>
#include <iostream>
#include <random>

using namespace Concurrency;
using namespace std;

// Returns the position in the provided array that contains the given value, 
// or -1 if the value is not in the array.
template<typename T>
int parallel_find_any(const T a[], size_t count, const T& what)
{
   // The position of the element in the array. 
   // The default value, -1, indicates that the element is not in the array.
   int position = -1;

   // Use parallel_for to search for the element. 
   // The task group enables a work function to cancel the overall 
   // operation when it finds the result.

   structured_task_group tasks;
   tasks.run_and_wait([&]
   {
      parallel_for(std::size_t(0), count, [&](int n) {
         if (a[n] == what)
         {
            // Set the return value and cancel the remaining tasks. 
            position = n;            
            tasks.cancel();
         }
      });
   });

   return position;
}

int wmain()
{
   const size_t count = 10000;
   int values[count];

   // Fill the array with random values.
   mt19937 gen(34);
   for (size_t i = 0; i < count; ++i)
   {
      values[i] = gen()%10000;
   }

   // Search for any position in the array that contains value 3123.
   const int what = 3123;
   int position = parallel_find_any(values, count, what);
   if (position >= 0)
   {
      wcout << what << L" is at position " << position << L'.' << endl;
   }
   else
   {
      wcout << what << L" is not in the array." << endl;
   }  
}


The following shows sample output for this example.

3123 is at position 4739.

The Concurrency::parallel_for algorithm acts concurrently. Therefore, it does not perform the operations in a pre-determined order. If the array contains multiple instances of the value, the result can be any one of its positions.

Copy the example code and paste it in a Visual Studio project, or paste it in a file that is named parallel-array-search.cpp and then run the following command in a Visual Studio 2010 Command Prompt window.

cl.exe /EHsc parallel-array-search.cpp

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