R help - Jun 2010 - generate list of variable names

Hi!

Would anyone know how to generate a list of variable names from a data frame by
the class of the variable?

I have large tables with different numbers of columns and am trying to script
some rote analyses. There are several categorizing variables (factors) and many
response variables (integers and numeric). I want to extract a list of
classifier column names in one list and response variable names in another list,
then run for-loops to calculate various statistics on the response variables in
terms of the classifier variables. I thought something like this might work (but
didn't):


tmp<-dataframe
for (i in 1:length(tmp)){
	if(class(tmp[[i]]=="factor") else tmp[[i]]<-NULL
}

factorlist<-names(tmp)

...

One problem is that the scope of the counter changes as I drop columns, but
there seems to be other problems as well

Any suggestions would be welcome. Thanks!

Jon

Soli Deo Gloria

Jon Erik Ween, MD, MS
Scientist, Kunin-Lunenfeld Applied Research Unit 
Director, Stroke Clinic, Brain Health Clinic, Baycrest Centre
Assistant Professor, Dept. of Medicine, Div. of Neurology
    University of Toronto Faculty of Medicine

Kimel Family Building, 6th Floor, Room 644 
Baycrest Centre
3560 Bathurst Street 
Toronto, Ontario M6A 2E1
Canada 

Phone: 416-785-2500 x3648
Fax: 416-785-2484
Email: jween@klaru-baycrest.on.ca


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Jon Erik Ween wrote:
> Hi!
> 
> Would anyone know how to generate a list of variable names from a
> data frame by the class of the variable?
a start...

df <- data.frame(f1 = factor(1:10),
                  f2 = factor(1:10),
                  n1 = 1:10,
                  n2 = 1:10)


sapply(df, class)
> 
> I have large tables with different numbers of columns and am trying
> to script some rote analyses. There are several categorizing
> variables (factors) and many response variables (integers and
> numeric). I want to extract a list of classifier column names in one
> list and response variable names in another list, then run for-loops
> to calculate various statistics on the response variables in terms of
> the classifier variables. I thought something like this might work
> (but didn't):
Reproducible example needed.  All this can surely be done more elegantly 
with lapply/mapply instead of for-loops.