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

[Martin Maechler]

>>>>> 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).

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