[Rd] fast version of split.data.frame or conversion from data.frame to list of its rows

From: Antonio Piccolboni <antonio_at_piccolboni.info>
Date: Mon, 30 Apr 2012 16:28:04 -0700

I was wondering if there is anything more efficient than split to do the kind of conversion in the subject. If I create a data frame as in

system.time({fd = data.frame(x=1:2000, y = rnorm(2000), id = paste("x&quot;, 1:2000, sep =""))})
  user system elapsed
  0.004 0.000 0.004

and then I try to split it

> system.time(split(fd, 1:nrow(fd)))

   user system elapsed
  0.333 0.031 0.415

You will be quick to notice the roughly two orders of magnitude difference in time between creation and conversion. Granted, it's not written anywhere that they should be similar but the latter seems interpreter-slow to me (split is implemented with a lapply in the data frame case) There is also a memory issue when I hit about 20000 elements (allocating 3GB when interrupted). So before I resort to Rcpp, despite the electrifying feeling of approaching the bare metal and for the sake of getting things done, I thought I would ask the experts. Thanks


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