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

[Hadley Wickham]

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.

But aren't you making life difficult for yourself by not using I ?

df <- data.frame(VAR = I(nv))
str(df[[1]])

(which isn't quite identically because it now has the AsIs class)

Hadley

-- 
Assistant Professor
Department of Statistics / Rice University
http://had.co.nz/