R help - Aug 2010 - Index list by other list (w/ logical elements)?

I have two lists of the same shape, like this:

x <- list()
x[[1]] <- c("one","two")
x[[2]] <- c("three","four","five")

y <- list()
y[[1]] <- c(TRUE,FALSE)
y[[2]] <- c(FALSE,TRUE,TRUE)

I would like to index x "by" y, that is, the result in this case
should be:

 z
[[1]]
[1] "one"

[[2]]
[1] "four"  "five"


I was hoping 
sapply(x,"[",y) 
would work, but it doesn't.

I guess I need to sapply twice, like 
sapply(x, function() { sapply(y ... 
but I can't figure it out.

Many thanks for any pointers

Marianne


-- 
Marianne Promberger PhD, King's College London
http://promberger.info
R version 2.11.1 (2010-05-31)
Ubuntu 9.04

Marianne  -
    The function you're looking for is mapply:
> mapply(function(one,two)one[two],x,y)
[[1]]
[1] "one"

[[2]]
[1] "four" "five"

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu




On Tue, 24 Aug 2010, Marianne Promberger wrote:
> I have two lists of the same shape, like this:
>
> x <- list()
> x[[1]] <- c("one","two")
> x[[2]] <- c("three","four","five")
>
> y <- list()
> y[[1]] <- c(TRUE,FALSE)
> y[[2]] <- c(FALSE,TRUE,TRUE)
>
> I would like to index x "by" y, that is, the result in this case
> should be:
>
> z
> [[1]]
> [1] "one"
>
> [[2]]
> [1] "four"  "five"
>
>
> I was hoping
> sapply(x,"[",y)
> would work, but it doesn't.
>
> I guess I need to sapply twice, like
> sapply(x, function() { sapply(y ...
> but I can't figure it out.
>
> Many thanks for any pointers
>
> Marianne
>
>
> -- 
> Marianne Promberger PhD, King's College London
> http://promberger.info
> R version 2.11.1 (2010-05-31)
> Ubuntu 9.04
>
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