[R] Extracting multiple elements from a list

[Prof Brian Ripley]

[] works on a list and extracts multiple elements.  However, that is not 
it seems what you want, rather, unions of elements of elements of lists.
That is a pretty unusual need, and perhaps you could explain you you need 
it.

Andy's solution still need something like

do.call("c",  lapply(alist, function(x) x$vec))

but then it *is* unions of elements of elements of lists.


On Thu, 15 Jan 2004, Liaw, Andy wrote:

> Aren't sapply()/lapply() sufficient for this?
> 
> > sapply(alist, function(x) x$vec)
>      [,1] [,2]
> [1,]    1    5
> [2,]    2    6
> [3,]    3    7
> [4,]    4    8
> > lapply(alist, function(x) x$vec)
> [[1]]
> [1] 1 2 3 4
> 
> [[2]]
> [1] 5 6 7 8
> 
> HTH,
> Andy
> 
> > From: Waichler, Scott R
> > 
> > For a long time I've wanted a way to conveniently extract 
> > multiple elements
> > from a list, which [[ doesn't allow.  Can anyone suggest an efficient
> > function to do this?  Wouldn't it be a sensible addition to R?
> > 
> > For example,
> > 
> > alist <- list()
> > alist[[1]] <- list()
> > alist[[1]]$name <- "first"
> > alist[[1]]$vec <- 1:4
> > alist[[2]] <- list()
> > alist[[2]]$name <- "second"
> > alist[[2]]$vec <- 5:8
> > both.vec <- c(alist[[1]]$vec, alist[[2]]$vec)
> > 
> > Can I get both.vec without c() or an explicit loop?
> > 
> > and
> > 
> > new.names <- c("one", "two")
> > alist[[1]]$name <- new.names[1]
> > alist[[2]]$name <- new.names[2]
> > 
> > Could I assign the new values in a quasi-vectorized way?
> > 
> > Thanks,
> > Scott Waichler
> > Pacific Northwest National Laboratory
> > Richland, WA   USA
> > 
> 
> 
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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