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

[Bert Gunter]

Or to expand just a hair on Joshua's suggestion, is the following what you want:

> x <- 1:10
> names(x) <- letters[1:10]
> x
 a  b  c  d  e  f  g  h  i  j
 1  2  3  4  5  6  7  8  9 10
> df <- data.frame(x=x,y=LETTERS[1:10],row.names=names(x))
> df
   x y
a  1 A
b  2 B
c  3 C
d  4 D
e  5 E
f  6 F
g  7 G
h  8 H
i  9 I
j 10 J
> y <- t(df[,1,drop=FALSE])[1,]
> y
 a  b  c  d  e  f  g  h  i  j
 1  2  3  4  5  6  7  8  9 10
> identical(x,y)
[1] TRUE

Cheers,
Bert


On Sat, Aug 18, 2012 at 8:16 AM, Joshua Ulrich <josh.m.ulrich at gmail.com> wrote:
> 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
>
> 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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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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