On 2012-08-18 11:03, Martin Maechler 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
For this purpose my private function library has a function withnames():
withnames(): Extract from data frame as a named vector
Description: Extracts data from a data frame; if the result is a vector
(i.e. we extracted a single column and did not specify 'drop=FALSE')
it is assigned names derived from the row names of the data frame.
Usage: withnames(expr)
Arguments: expr: R expression.
Details: 'expr' is evaluated in an environment in which the extractor
functions '$.data.frame', '[.data.frame', and '[[.data.frame' are
replaced by versions that attach the data frame's row names to an
extracted vector.
Value: 'expr', evaluated as described above.
## Code
withnames<-function(expr) {
eval(substitute(expr),
list(
`[.data.frame` = function(x,i,...) {
out<-x[i,...]
if (is.null(dim(out))) names(out)<-row.names(x)[i]
return(out)},
`[[.data.frame` = function(x,...) {
out<-x[[...]]
if (is.null(dim(out))) names(out)<-row.names(x)
return(out)},
`$.data.frame` = function(x,name) {
out<-x[[name, exact=FALSE]]
if (is.null(dim(out))) names(out)<-row.names(x)
return(out)}
),
enclos=parent.frame())
}
## Examples
dd <- data.frame(aa=1:6, bb=letters[c(1,3,2,3,3,1)],
row.names=LETTERS[1:6])
dd
dd$aa # Unnamed vector
withnames(dd$aa) # Named vector
withnames(dd[["aa"]]) # Named vector
withnames(dd[2:4,"aa"]) # Named vector
withnames(dd$bb) # Factor with names
withnames(outer(dd$a,dd$a)) # Both dimensions have names
## But now I am looking for a version that will play nicely with with():
withnames(with(dd, aa)) # No names!
with(dd, withnames(aa)) # No names!
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
