R help - Feb 2012 - subset select="variable with a list of names"

Hello,
I would like to make a function which extracts a subset, from a dataset, 
with only the columns that I want (specifying their names).

For example, having this matrix:
 > mydata<-matrix(c(22,1,3,2001,24,5,7,2002,26,7,8,2002,28,5,7,2003), 
byrow=TRUE, ncol=4, dimnames=list(c(1,2,3,4), 
c("age","day","month","year")))

 > mydata

   age day month year
1  22   1     3 2001
2  24   5     7 2002
3  26   7     8 2002
4  28   5     7 2003


I would like to create a function like:
x<-function(names) {subset(mydata, select=names) }

So I can choose every time which columns select, i.e. when I call:
x("age,day")

it would returns:
   age day
1  22   1
2  24   5
3  26   7
4  28   5

Obviously it is not working, and I don't know how to do to fix it. Do 
you have any suggestion?

Thank you very much

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Francisco
> Sent: Thursday, February 09, 2012 4:52 AM
> To: r-help at r-project.org
> Subject: [R] subset select="variable with a list of names"
> 
> Hello,
> I would like to make a function which extracts a subset, from a
> dataset,
> with only the columns that I want (specifying their names).
> 
> For example, having this matrix:
>  > mydata<-matrix(c(22,1,3,2001,24,5,7,2002,26,7,8,2002,28,5,7,2003),
> byrow=TRUE, ncol=4, dimnames=list(c(1,2,3,4),
> c("age","day","month","year")))
> 
>  > mydata
> 
>    age day month year
> 1  22   1     3 2001
> 2  24   5     7 2002
> 3  26   7     8 2002
> 4  28   5     7 2003
> 
> 
> I would like to create a function like:
> x<-function(names) {subset(mydata, select=names) }
> 
> So I can choose every time which columns select, i.e. when I call:
> x("age,day")
> 
> it would returns:
>    age day
> 1  22   1
> 2  24   5
> 3  26   7
> 4  28   5
> 
> Obviously it is not working, and I don't know how to do to fix it. Do
> you have any suggestion?
> 
> Thank you very much
Given your function definition, the function call needs to be

x(c("age","day"))

Whether it is good form to write a function like this, I will leave to others to
comment.


Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204