[R] fast subsetting of lists in lists

[Gerrit Eichner]

Hello, Alexander,

does

utest <- unlist(test)
utest[ names( utest) == "a"]

come close to what you need?

Hth,

Gerrit


On Tue, 7 Dec 2010, Alexander Senger wrote:

> 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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