# [R] extraction from a list of vectors

Peter Dalgaard BSA p.dalgaard at biostat.ku.dk
Mon Jun 18 01:50:05 CEST 2001

```Jeff Miller <jmiller at xcaliber.com> writes:

> 	Hi ,
> 	I know this should be easy, but I haven't figured out a fast way to
> do it.
> 	I have a list of vectors: mylist <- list(vector1, vector2, ...,
> vectorN).
> 	(The vectors are made of integers. They vary in length from about 1
> to 10).
> 	I also have a vector of indices: myidx <- c( i1, i2, ..., iN).
> 	From these I'd like to make this: newvector <- c(vector1[i1],
> vector2[i2], ..., vectorN[iN]).
> 	It's easy to do this using a for loop, but I need something that
> will be fast for lists
> 	with lengths of about 100,000. I've thought about using lapply, but
> don't see how I can use it with "myidx".
> 		Jeff Miller

I don't think it *is* easy. Generally, one gets stalled on these
problems involving two or more parallel vectors. Occasionally the idea
of having a "multiapply" function pops up, but nothing has transpired
yet. The canonical way would be to use

sapply(1:N,function(i)mylist[i][myidx[i]])

but that is not likely to be much faster than a for loop. If all the
vectors were the same length, mylist could have been a matrix and
matrix indexing (myarray[cbind(1:N,myidx)]) would work, but there's
nothing similar for a list of vectors of unequal length. So if you
need the speed badly, I think you have to go to the C level. (A rather
nicely sized little exercise for learning the .External interface, I
would expect.)

--
O__  ---- Peter Dalgaard             Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics     2200 Cph. N
(*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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```