[R] a question on list manipulation

[Greg Snow]

Here is one approach, whether it is better than the basic loop or not is up to you:

> x <- list(A=c("d", "e", "f"), B=c("d", "e"), C=c("d"))
> 
> tmp <- unlist(x)
> tmp2 <- sub( '[0-9]+$', '', names(tmp) )
> 
> x.new <- split( tmp2, tmp )
> x.new
$d
[1] "A" "B" "C"

$e
[1] "A" "B"

$f
[1] "A"


Of course this version will have some problems if the names of your list elements end with digits that you don't want stripped off (but you can work around that by preprocessing the list names).

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of zhenjiang xu
> Sent: Friday, August 05, 2011 11:04 AM
> To: Duncan Murdoch
> Cc: r-help
> Subject: Re: [R] a question on list manipulation
> 
> Exactly! Sorry I get others misunderstood. The uppercase/lowercase is
> only a toy example (and a bad one; yours is better than mine). My
> question is a more general one: a list is basically a one-to-many
> matching, from the names of a list to the elements belonging to each
> name. I'd like to reverse the matching, from all the elements to the
> names of the list.
> 
> On Fri, Aug 5, 2011 at 12:53 PM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
> > On 05/08/2011 12:05 PM, zhenjiang xu wrote:
> >>
> >> Hi R users,
> >>
> >> I have a list:
> >> >  x
> >> $A
> >> [1] "a"  "b"  "c"
> >> $B
> >> [1] "b"  "c"
> >> $C
> >> [1] "c"
> >>
> >> I want to convert it to a lowercase-to-uppercase list like this:
> >> >  y
> >> $a
> >> [1] "A"
> >> $b
> >> [1] "A"  "B"
> >> $c
> >> [1] "A"  "B"  "C"
> >>
> >> In a word, I want to reverse the list names and the elements under
> >> each list name. Is there any quick way to do that? Thanks
> >
> > I interpreted this question differently from the others, and your
> example is
> > ambiguous as to which is the right interpretation.  I thought you
> wanted to
> > swap names and elements,  so
> >
> >> x <- list(A=c("d", "e", "f"), B=c("d", "e"), C=c("d"))
> >> x
> > $A
> > [1] "d" "e" "f"
> >
> > $B
> > [1] "d" "e"
> >
> > $C
> > [1] "d"
> >
> > would become
> >
> >> list(d=c("A", "B", "C"), e=c("A", "B"), f="A")
> > $d
> > [1] "A" "B" "C"
> >
> > $e
> > [1] "A" "B"
> >
> > $f
> > [1] "A"
> >
> > I don't know a slick way to do this; I'd just do it by brute force,
> looping
> > over the names of x.
> >
> > Duncan Murdoch
> >
> 
> 
> 
> --
> Best,
> Zhenjiang
> 
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