The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » Software » comp.soft-sys.matlab

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: speeding up ACCUMARRAY
Replies: 12   Last Post: Nov 15, 2012 3:35 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Matt J

Posts: 4,997
Registered: 11/28/09
Re: speeding up ACCUMARRAY
Posted: Nov 13, 2012 6:36 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

"james bejon" wrote in message <k7u3e4$8ml$>...
> I must admit, I've always found accumarray to be remarkably fast, e.g.,
> % DATA
> subs = ceil(rand(1000000,1)*10);
> tic
> for i = 1:1000
> accumarray(subs, 1);
> end
> toc
> % Result = 1.40 seconds
> Is there a specific usage you have in mind?


I'm trying to find a way to implement a sparse matrix multiplication given I,J,S data while avoiding the overhead of actually building the sparse matrix and all of the sorting operations it requires. I have to do this for repeated sets of I,J,S data which are fairly easy/quick to generate, but if I have to load them into a sparse matrix each time, it will kill the speed.

I guess one of the reasons for my suspicions about accumarray performance is the speed differences I'm seeing between an accumarray operation and an equivalent sparse matrix multiplication in the example below. The accumarray approach is about twice as slow as the sparse matrix approach even though they both have the same data to work with. In fact, the sparse matrix multiplication is probably even handicapped relative to accumarray, since it has to search through x for the appropriate x(J), whereas in the accumarray approach, I exclude the construction of b=x(J) from the tic/toc.

%fake data setup

%pretend we build A from scratch
toc %Elapsed time is 0.062737 seconds.

%Apply A
toc %Elapsed time is 0.006868 seconds.

%Using accumarray
toc %Elapsed time is 0.012236 seconds.

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.