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

[Winston Chang]

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
>
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