From: William Dunlap <wdunlap_at_tibco.com>

Date: Sat, 18 Aug 2012 21:56:13 +0000

a d e

3 9 27

> df$Matrix <- matrix(1001:1006, ncol=2, nrow=3) > f4(df, "Matrix")

[,1] [,2]

a 1001 1004

d 1002 1005

e 1003 1006

R-devel_at_r-project.org mailing list

https://stat.ethz.ch/mailman/listinfo/r-devel Received on Sat 18 Aug 2012 - 22:12:26 GMT

Date: Sat, 18 Aug 2012 21:56:13 +0000

That would have been essentially my suggestion as well. I prefer its clarity (and speed). I didn't know if you wanted your solution to also apply to matrices embedded in data.frames. In S+ rownames<-() works on vectors (because it calls the generic rowId<-()) so the following works:

> f4 <- function(df, column) { tmp <- df[[column]] ; rownames(tmp) <- rownames(df) ; tmp} > nv <- c(a=1,d=17,e=101) > df <- data.frame(VAR=nv, Two=3^(1:3)) > f4(df, 2)

a d e

3 9 27

> df$Matrix <- matrix(1001:1006, ncol=2, nrow=3) > f4(df, "Matrix")

[,1] [,2]

a 1001 1004

d 1002 1005

e 1003 1006

I forget if R has something like rowIds() (it is to names and rownames as NROW is to length and nrow).

Bill Dunlap

Spotfire, TIBCO Software

wdunlap tibco.com

> -----Original Message-----

*> From: r-devel-bounces_at_r-project.org [mailto:r-devel-bounces_at_r-project.org] On Behalf
**> Of Winston Chang
**> Sent: Saturday, August 18, 2012 11:54 AM
**> To: Martin Maechler
**> Cc: R. Devel List
**> Subject: Re: [Rd] Quiz: How to get a "named column" from a data frame
**>
**> This isn't super-concise, but has the virtue of being clear:
**>
**> nv <- c(a=1, d=17, e=101)
**> df <- as.data.frame(cbind(VAR = nv))
**>
**> identical(nv, setNames(df$VAR, rownames(df)))
**> # TRUE
**>
**>
**> It seems to be more efficient than the other methods as well:
**>
**> f1 <- function() setNames(df$VAR, rownames(df))
**> f2 <- function() t(df)[1,]
**> f3 <- function() as.matrix(df)[,1]
**>
**> r <- microbenchmark(f1(), f2(), f3(), times=1000)
**> r
**> # Unit: microseconds
**> # expr min lq median uq max
**> # 1 f1() 14.589 17.0315 18.608 19.3220 89.388
**> # 2 f2() 68.057 70.8735 72.240 75.8065 3707.012
**> # 3 f3() 58.153 61.2600 62.521 65.0380 238.483
**>
**> -Winston
**>
**>
**>
**> On Sat, Aug 18, 2012 at 10:03 AM, Martin Maechler
**> <maechler_at_stat.math.ethz.ch> wrote:
**> > Today, I was looking for an elegant (and efficient) way
**> > to get a named (atomic) vector by selecting one column of a data frame.
**> > Of course, the vector names must be the rownames of the data frame.
**> >
**> > Ok, here is the quiz, I know one quite "cute"/"slick" answer, but was
**> > wondering if there are obvious better ones, and
**> > also if this should not become more idiomatic (hence "R-devel"):
**> >
**> > Consider this toy example, where the dataframe already has only
**> > one column :
**> >
**> >> nv <- c(a=1, d=17, e=101); nv
**> > a d e
**> > 1 17 101
**> >
**> >> df <- as.data.frame(cbind(VAR = nv)); df
**> > VAR
**> > a 1
**> > d 17
**> > e 101
**> >
**> > Now how, can I get 'nv' back from 'df' ? I.e., how to get
**> >
**> >> identical(nv, .......)
**> > [1] TRUE
**> >
**> > where ...... only uses 'df' (and no non-standard R packages)?
**> >
**> > As said, I know a simple solution (*), but I'm sure it is not
**> > obvious to most R users and probably not even to the majority of
**> > R-devel readers... OTOH, people like Bill Dunlap will not take
**> > long to provide it or a better one.
**> >
**> > (*) In my solution, the above '.......' consists of 17 letters.
**> > I'll post it later today (CEST time) ... or confirm
**> > that someone else has done so.
**> >
**> > Martin
**> >
**> > ______________________________________________
**> > R-devel_at_r-project.org mailing list
**> > https://stat.ethz.ch/mailman/listinfo/r-devel
**>
**> ______________________________________________
**> R-devel_at_r-project.org mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-devel
*

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