how-to-write-a-parallel-for-loop.md

description	title	ms.date	helpviewer_keywords	ms.assetid
Learn more about: How to: Write a parallel_for Loop	How to: Write a parallel_for Loop	11/04/2016	writing a parallel_for loop [Concurrency Runtime] parallel_for function, example	adb4d64e-5514-4b70-8dcb-b9210e6b5a1c

How to: Write a parallel_for Loop

This example demonstrates how to use concurrency::parallel_for to compute the product of two matrices.

Example: Compute the product of two matrices

The following example shows the matrix_multiply function, which computes the product of two square matrices.

[!code-cppconcrt-parallel-matrix-multiply#1]

Example: Compute a matrix multiply in parallel

The following example shows the parallel_matrix_multiply function, which uses the parallel_for algorithm to perform the outer loop in parallel.

[!code-cppconcrt-parallel-matrix-multiply#2]

This example parallelizes the outer loop only because it performs enough work to benefit from the overhead for parallel processing. If you parallelize the inner loop, you will not receive a gain in performance because the small amount of work that the inner loop performs does not overcome the overhead for parallel processing. Therefore, parallelizing the outer loop only is the best way to maximize the benefits of concurrency on most systems.

Example: Finished parallel_for loop code sample

The following more complete example compares the performance of the matrix_multiply function versus the parallel_matrix_multiply function.

[!code-cppconcrt-parallel-matrix-multiply#3]

The following sample output is for a computer that has four processors.

serial: 3853
parallel: 1311

Compiling the Code

To compile the code, copy it and then paste it in a Visual Studio project, or paste it in a file that is named parallel-matrix-multiply.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe /EHsc parallel-matrix-multiply.cpp

See also

Parallel Algorithms
parallel_for Function