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‎docs/parallel/concrt/codesnippet/CPP/how-to-write-a-parallel-for-each-loop_1.cpp

Lines changed: 59 additions & 47 deletions
Original file line numberDiff line numberDiff line change
@@ -9,66 +9,78 @@
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using namespace concurrency;
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using namespace std;
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// Calls the provided work function and returns the number of milliseconds
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// that it takes to call that function.
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// Returns the number of milliseconds that it takes to call the passed in function.
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template <class Function>
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__int64 time_call(Function&& f)
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{
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__int64 begin = GetTickCount();
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f();
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return GetTickCount() - begin;
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__int64 begin = GetTickCount();
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f();
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return GetTickCount() - begin;
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}
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// Determines whether the input value is prime.
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// Determines whether the input is a prime.
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bool is_prime(int n)
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{
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if (n < 2)
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return false;
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for (int i = 2; i < n; ++i)
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{
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if ((n % i) == 0)
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return false;
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}
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return true;
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if (n < 2)
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{
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return false;
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}
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for (int i = 2; i < int(std::sqrt(n)) + 1; ++i)
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{
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if (n % i == 0)
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{
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return false;
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}
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}
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return true;
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}
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int wmain()
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{
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// Create an array object that contains 200000 integers.
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array<int, 200000> a;
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// Create an array object that contains 200000 integers.
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array<int, 200000> a;
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// Initialize the array such that a[i] == i.
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int n = 0;
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generate(begin(a), end(a), [&] {
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return n++;
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});
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// Initialize the array such that a[i] == i.
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int n = 0;
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generate(begin(a), end(a), [&]
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{
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return n++;
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});
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LONG prime_count;
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__int64 elapsed;
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// Use the for_each algorithm to count, serially, the number
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// of prime numbers in the array.
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LONG prime_count = 0L;
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__int64 elapsed = time_call([&]
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{
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for_each(begin(a), end(a), [&](int n)
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{
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if (is_prime(n))
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{
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++prime_count;
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}
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});
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});
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wcout << L"serial version: " << endl
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<< L"found " << prime_count << L" prime numbers" << endl
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<< L"took " << elapsed << L" ms" << endl << endl;
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// Use the for_each algorithm to count the number of prime numbers
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// in the array serially.
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prime_count = 0L;
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elapsed = time_call([&] {
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for_each (begin(a), end(a), [&](int n ) {
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if (is_prime(n))
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++prime_count;
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});
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});
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wcout << L"serial version: " << endl
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<< L"found " << prime_count << L" prime numbers" << endl
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<< L"took " << elapsed << L" ms" << endl << endl;
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// Use the parallel_for_each algorithm to count, in parallel, the number
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// of prime numbers in the array.
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prime_count = 0L;
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elapsed = time_call([&]
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{
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parallel_for_each(begin(a), end(a), [&](int n)
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{
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if (is_prime(n))
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{
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InterlockedIncrement(&prime_count);
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}
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});
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});
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// Use the parallel_for_each algorithm to count the number of prime numbers
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// in the array in parallel.
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prime_count = 0L;
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elapsed = time_call([&] {
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parallel_for_each (begin(a), end(a), [&](int n ) {
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if (is_prime(n))
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InterlockedIncrement(&prime_count);
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});
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});
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wcout << L"parallel version: " << endl
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<< L"found " << prime_count << L" prime numbers" << endl
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<< L"took " << elapsed << L" ms" << endl << endl;
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wcout << L"parallel version: " << endl
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<< L"found " << prime_count << L" prime numbers" << endl
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<< L"took " << elapsed << L" ms" << endl << endl;
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}

‎docs/parallel/concrt/how-to-write-a-parallel-for-each-loop.md

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---
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description: "Learn more about: How to: Write a parallel_for_each Loop"
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title: "How to: Write a parallel_for_each Loop"
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ms.date: "11/04/2016"
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description: "Learn more about how to write a parallel_for_each Loop"
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title: "How to: write a parallel_for_each Loop"
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ms.date: 09/12/2022
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helpviewer_keywords: ["writing a parallel_for_each loop [Concurrency Runtime]", "parallel_for_each function, example"]
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ms.assetid: fa9c0ba6-ace0-4f88-8681-c7c1f52aff20
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---
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# How to: Write a parallel_for_each Loop
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# How to: Write a `parallel_for_each` Loop
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This example shows how to use the [concurrency::parallel_for_each](reference/concurrency-namespace-functions.md#parallel_for_each) algorithm to compute the count of prime numbers in a [std::array](../../standard-library/array-class-stl.md) object in parallel.
9+
This example shows how to use the [`concurrency::parallel_for_each`](reference/concurrency-namespace-functions.md#parallel_for_each) algorithm to compute the count of prime numbers in a [`std::array`](../../standard-library/array-class-stl.md) object in parallel.
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## Example
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The following example computes the count of prime numbers in an array two times. The example first uses the [std::for_each](../../standard-library/algorithm-functions.md#for_each) algorithm to compute the count serially. The example then uses the `parallel_for_each` algorithm to perform the same task in parallel. The example also prints to the console the time that is required to perform both computations.
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The following example computes the count of prime numbers in an array two times. The example first uses the [`std::for_each`](../../standard-library/algorithm-functions.md#for_each) algorithm to compute the count serially. The example then uses the `parallel_for_each` algorithm to perform the same task in parallel. The example also prints to the console the time that is required to perform both computations.
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[!code-cpp[concrt-parallel-count-primes#1](../../parallel/concrt/codesnippet/cpp/how-to-write-a-parallel-for-each-loop_1.cpp)]
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The following sample output is for a computer that has four processors.
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The following sample output is for a computer that has four cores.
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```Output
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serial version:
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found 17984 prime numbers
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took 6115 ms
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took 125 ms
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parallel version:
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found 17984 prime numbers
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took 1653 ms
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took 63 ms
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```
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## Compiling the Code
@@ -35,9 +34,9 @@ To compile the code, copy it and then paste it in a Visual Studio project, or pa
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## Robust Programming
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The lambda expression that the example passes to the `parallel_for_each` algorithm uses the `InterlockedIncrement` function to enable parallel iterations of the loop to increment the counter simultaneously. If you use functions such as `InterlockedIncrement` to synchronize access to shared resources, you can present performance bottlenecks in your code. You can use a lock-free synchronization mechanism, for example, the [concurrency::combinable](../../parallel/concrt/reference/combinable-class.md) class, to eliminate simultaneous access to shared resources. For an example that uses the `combinable` class in this manner, see [How to: Use combinable to Improve Performance](../../parallel/concrt/how-to-use-combinable-to-improve-performance.md).
37+
The lambda expression that the example passes to the `parallel_for_each` algorithm uses the `InterlockedIncrement` function to enable parallel iterations of the loop to increment the counter simultaneously. If you use functions such as `InterlockedIncrement` to synchronize access to shared resources, you can present performance bottlenecks in your code. You can use a lock-free synchronization mechanism, for example, the [`concurrency::combinable`](../../parallel/concrt/reference/combinable-class.md) class, to eliminate simultaneous access to shared resources. For an example that uses the `combinable` class in this manner, see [How to: Use combinable to improve performance](../../parallel/concrt/how-to-use-combinable-to-improve-performance.md).
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## See also
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[Parallel Algorithms](../../parallel/concrt/parallel-algorithms.md)<br/>
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[parallel_for_each Function](reference/concurrency-namespace-functions.md#parallel_for_each)
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[Parallel algorithms](../../parallel/concrt/parallel-algorithms.md)\
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[`parallel_for_each` Function](reference/concurrency-namespace-functions.md#parallel_for_each)

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