R help - Aug 2011 - Obtaining variable's names from a list of variables

Say I have a list of variables, 

listVar <- list(age,sex)

I am looking for a way to either

1- create a vector c("age","sex") from it, or
2- get the names one by one in a for loop such as these

    a)  for (i in 1:length(listVar)) rownames(result)[i] <- ???

    b)  for(i in listVar) print (variable's name)


Any help much appreciated.
	[[alternative HTML version deleted]]

Hi,
there is no direct way, since
listVar <- list(age,sex)
creates a unnamed list, as can be seen by
names(listVar) #or
str(listVar)

You can do sth like
listVar <- list(age=age,sex=sex) # or
listVar2 <- list(age,sex)
names(listVar2)<-c("age","sex")

and afterwards access them using names().

Or you write your own list function using its call to name the returned
object, as in

my.list<-function(...){
 tmp<-list(...)
 names(tmp)<-all.names(match.call())[-1]
 tmp
}
attach(iris)
a<-my.list(Sepal.Length,Sepal.Width)

hth.

Am 17.08.2011 08:46, schrieb Monsieur Do:
> Say I have a list of variables, 
> 
> listVar <- list(age,sex)
> 
> I am looking for a way to either
> 
> 1- create a vector c("age","sex") from it, or
> 2- get the names one by one in a for loop such as these
> 
>     a)  for (i in 1:length(listVar)) rownames(result)[i] <- ???
> 
>     b)  for(i in listVar) print (variable's name)
> 
> 
> Any help much appreciated.
> 	[[alternative HTML version deleted]]
> 
> 
> 
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
-- 
Eik Vettorazzi
Institut f?r Medizinische Biometrie und Epidemiologie
Universit?tsklinikum Hamburg-Eppendorf

Martinistr. 52
20246 Hamburg

T ++49/40/7410-58243
F ++49/40/7410-57790

On Aug 17, 2011, at 1:46 AM, Monsieur Do wrote:
> Say I have a list of variables, 
> 
> listVar <- list(age,sex)
> 
> I am looking for a way to either
> 
> 1- create a vector c("age","sex") from it, or
> 2- get the names one by one in a for loop such as these
> 
>     a)  for (i in 1:length(listVar)) rownames(result)[i] <- ???
> 
>     b)  for(i in listVar) print (variable's name)
> 
> 
> Any help much appreciated.
Based upon the way in which you created 'listVar', there really is not a
way to recover the variable names, since they are not retained:

age <- 1:2
sex <- c("M", "F")
listVar <- list(age, sex)
> listVar
[[1]]
[1] 1 2

[[2]]
[1] "M" "F"
> names(listVar)
NULL


On the other hand, if you use:

listVar <- list(age = age, sex = sex)
> listVar
$age
[1] 1 2

$sex
[1] "M" "F"
> names(listVar)
[1] "age" "sex"

HTH,

Marc Schwartz

Thank you for your answers. Problem solved. Eik's cue to
all.names(match.call())[-1]?was particularly enlightning!

Do
?
> De?: Eik Vettorazzi
[mailto:E.Vettorazzi at uke.uni-hamburg.de]
> Envoy??: 17 ao?t 2011 08:46
> ??: Monsieur Do
> Cc?: r-help at r-project.org
> Objet?: Re: [R] Obtaining variable's names
from a list of variables
> 
> Hi,
> there is no
direct way, since
> listVar
<- list(age,sex)
> creates a
unnamed list, as can be seen by
> names(listVar)
#or
> str(listVar)
> 
> You can do
sth like
> listVar
<- list(age=age,sex=sex) # or
> listVar2
<- list(age,sex)
> names(listVar2)<-c("age","sex")
> 
> and
afterwards access them using names().
> 
> Or you
write your own list function using its call to name the returned
object,
> as in
> 
> my.list<-function(...){
> ?tmp<-list(...)
> ?names(tmp)<-all.names(match.call())[-1]
> ?tmp
> }
> attach(iris)
> a<-my.list(Sepal.Length,Sepal.Width)
> 
> hth.
> 
> Am
17.08.2011 08:46, schrieb Monsieur Do:
> > Say I
have a list of variables,
> >
> >
listVar <- list(age,sex)
> >
> > I am
looking for a way to either
> >
> > 1-
create a vector c("age","sex") from it,
or
> > 2- get
the names one by one in a for loop such as these
> >
> >???? a)? for (i in 1:length(listVar)) rownames(result)[i] <- ???
> >
> >???? b)? for(i in listVar) print (variable's name)
> >
> >
> > Any
help much appreciated.
> > ??????? [[alternative HTML version deleted]]
> >
> >
> >
> >
> >
______________________________________________
> > R-help at r-project.orgmailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE
do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > and
provide commented, minimal, self-contained, reproducible
code.
> 
> --
> Eik
Vettorazzi
> Institut
f?r Medizinische Biometrie und Epidemiologie
Universit?tsklinikum
> Hamburg-Eppendorf
> 
> Martinistr.
52
> 20246
Hamburg
> 
> T
++49/40/7410-58243
> F
++49/40/7410-57790