[R] fast subsetting of lists in lists
Alexander Senger
senger at physik.hu-berlin.de
Tue Dec 7 15:47:52 CET 2010
Hello,
my data is contained in nested lists (which seems not necessarily to be
the best approach). What I need is a fast way to get subsets from the data.
An example:
test <- list(list(a = 1, b = 2, c = 3), list(a = 4, b = 5, c = 6),
list(a = 7, b = 8, c = 9))
Now I would like to have all values in the named variables "a", that is
the vector c(1, 4, 7). The best I could come up with is:
val <- sapply(1:3, function (i) {test[[i]]$a})
which is unfortunately not very fast. According to R-inferno this is due
to the fact that apply and its derivates do looping in R rather than
rely on C-subroutines as the common [-operator.
Does someone now a trick to do the same as above with the faster
built-in subsetting? Something like:
test[<somesubsettingmagic>]
Thank you for your advice
Alex
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