# Mathematica Q&A: Sow, Reap, and Parallel Programming

April 20, 2011 — Andrew Moylan, Technical Communication & Strategy

Got questions about Mathematica? The Wolfram Blog has answers! We’ll regularly answer selected questions from users around the web. You can submit your question directly to the Q&A Team using this form.

This week’s question comes from Patrick, a student:

How can I use Sow & Reap across parallel kernels?

Before we answer this question, a review of the useful functions Sow and Reap is in order.

Sow and Reap are used together to build up a list of results during a computation. Sow[expr] puts expr aside to be collected later. Reap collects these and returns a list:

The first part of the list is the regular result of the computation. The second part is everything that was “sown”.

Sow and Reap are ideally suited to situations in which you don’t know in advance how many results you will get. For example, suppose that you want to find simple initial conditions that lead to “interesting” results in Conway’s game of life, the famous two-dimensional cellular automaton:

We’ll call a result “interesting” if it has at least n cells alive (live cells have value 1):

Now we’ll check 1,000 random 6X6 initial conditions for the “interesting” property of having at least 200 live cells after 100 steps. Whenever the result is InterestingQ, we create and Sow a simple visualization:

One interesting result was found and sown. If we want to conduct a larger search, we ought to take advantage of Mathematica‘s automatic parallelization.

This brings us to our main question: What’s the right way to Sow results in the parallel subkernels, but Reap them in the master kernel?

In this case the answer is particularly simple: define a version of Sow that always runs on the master kernel.

SetSharedFunction specifies that a function should always run on the master kernel.

Now we can apply Parallelize to our search. Here’s a search of 10,000 random initial conditions:

Note that Reap and Sow remain unaffected, and you can use them just as usual in algorithms running on parallel subkernels. Only our new function ParallelSow runs on the master kernel.

If you have a question you’d like answered in this blog, you can submit it to the Q&A Team using this form.

 Love this post! Game of life a of great interest to many. To search through the computation universe is a bit easier using ParallelSow! Thanks a bunch! Posted by Samuel Chen    April 20, 2011 at 6:39 pm
 I think that to match your description to your code “at least 250 live cells after 100 steps” should read “at least 200 live cells after 100 steps”. Other than that a really nice example of a parallelizable problem! Posted by Simon    April 20, 2011 at 7:43 pm
 Hi Simon, Thank you for bringing this to our attention. The post has been changed. Regards, The Wolfram Blog Team Posted by Wolfram Blog    April 21, 2011 at 8:53 am
 Excellent! I finally understood Sow&Reap after many years. Keep on doing this type of posts. Posted by Erik Mahieu    April 21, 2011 at 1:57 am
 The post is great! However, somehow 1+2+3 was calculated as 7 instead of 6… Posted by rszabolcs    April 22, 2011 at 4:53 am
 @rszabolcs, the sum was 1 + 2*3, not 1+2+3. Thanks very much for posting this reply! Very useful! Posted by Patrick    April 22, 2011 at 12:34 pm
 Hi rszabolcs, The result was 7 because the calculation was 1 + 2 * 3 rather than 1 + 2 + 3. Posted by Andrew Moylan    April 22, 2011 at 7:00 pm
 Nice post! I particularly like the definition of the “Game of Life” in terms of CellularAutomaton. This seems like an appropriate place to ask: will you consider renaming SetSharedFunction back to SharedDownValues? A motivating example: Function is usually used as an OwnValue, not a DownValue, so SetSharedFunction is caught in a quandary here. Posted by Ben    April 22, 2011 at 8:38 pm
 @rszabolcs It says 1 + 2 * 3 (multiplication of two and three) ;) Otherwise you would’ve found a very interesting bug! Posted by Sander Huisman    April 24, 2011 at 3:17 am
 I’m sorry, I’m blind, indeed! :-) Posted by rszabolcs    April 25, 2011 at 12:28 pm
 If the ParallelSow and Reap constructs are used with a ParallelDo for iterating across a set of parameters, would the ordering of the ParallelDo be enforced so that a given set of parameters were associated with a reaped list, OR would this correspondence depend on how quickly each kernel finished the computation associated with a given set of parameters? Posted by Brian    November 19, 2013 at 4:56 pm