[Rd] Quiz: How to get a "named column" from a data frame

[Joshua Wiley]

On Sat, Aug 18, 2012 at 9:33 AM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
>>>>>> Joshua Ulrich <josh.m.ulrich at gmail.com>
>>>>>>     on Sat, 18 Aug 2012 10:16:09 -0500 writes:
>
>     > I don't know if this is better, but it's the most obvious/shortest I
>     > could come up with.  Transpose the data.frame column to a 'row' vector
>     > and drop the dimensions.
>
>     R> identical(nv, drop(t(df)))
>     > [1] TRUE
>
> Yes, that's definitely shorter,
> congratulations!
>
> One gotta is that I'd want a solution that also works when the
> df has more columns than just one...
>
> Your idea to use  t(.) is nice and "perfect" insofar as it
> coerces the data frame to a matrix, and that's really the clue:
>
> Where as  df[,1]  is losing the names,
> the matrix indexing is not.
> So your solution can be changed into
>
>      t(df)[1,]
>
> which is even shorter...
> and slightly less efficient, at least conceptually, than mine, which has
> been
>
>    as.matrix(df)[,1]
>
> Now, the remaining question is:  Shouldn't there be something
> more natural to achieve that?
> (There is not, currently, AFAIK).

Perhaps a data frame method for as.vector?

as.vector.data.frame <- function(x, ...) as.matrix(x)[,1]
as.vector(df[1])

or an additional argument to `[.data.frame` like keep.names, which
defaults to FALSE to maintain current behavior but can optionally be
TRUE.

Cheers,

Josh

>
> Martin
>
>
>     > Best,
>     > --
>     > Joshua Ulrich  |  about.me/joshuaulrich
>     > FOSS Trading  |  www.fosstrading.com
>
>
>     > 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
>
> ______________________________________________
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/