[R] multicore by(), like mclapply?

[Joshua Wiley]

I could be waay off base here, but my concern about presplitting the data is that you will have your data, and a second copy of our data that is something like a list where each element contains the portion of the data for that split.  Good speed wise, bad memory wise.  My hope with the technique I showed (again I may not have accomplished it) was to only have at anyone time, the original data and a copy of the particular elements being worked with.  Of course  this is not an issue if you have plenty of memory.

On Oct 10, 2011, at 12:19, Thomas Lumley <tlumley at uw.edu> wrote:

> On Tue, Oct 11, 2011 at 7:54 AM, ivo welch <ivo.welch at gmail.com> wrote:
>> hi josh---thx.  I had a different version of this, and discarded it
>> because I think it was very slow.  the reason is that on each
>> application, your version has to scan my (very long) data vector.  (I
>> have many thousand different cases, too.)  I presume that by() has one
>> scan through the vector that makes all splits.
> 
> by.data.frame() is basically a wrapper for tapply(), and the key line
> in tapply() is
>   ans <- lapply(split(X, group), FUN, ...)
> which should be easy to adapt for mclapply.
> 
> -- 
> Thomas Lumley
> Professor of Biostatistics
> University of Auckland