R help - May 2011 - Use paste function to select column of data

Hello,

I want to build a function to call up a column of a data.frame by the names of
the columns.  I have column names that are sequentially named (col1, col2,
etc.). How do I change a character expression into something that will be
understood as a data.frame column.  For example:

 example<-data.frame(cbind(col1=1:10, col2=21:30, col3=41:50))
  call.fun<-function(t){
    x<-paste("col",t, sep="")  ## Change this so that it
is the data, not a character expression
     example$x}

call.fun(t=2)

 Within the real function, I will continue do calculations on the column of
data.  My problem is that I am either getting a character expression or NULL
from my function.

Thanks for your help on what is probably a very simple question.

John
	[[alternative HTML version deleted]]

On 2011-05-18 12:12, John Poulsen wrote:
> Hello,
>
> I want to build a function to call up a column of a data.frame by the names
of the columns.  I have column names that are sequentially named (col1, col2,
etc.). How do I change a character expression into something that will be
understood as a data.frame column.  For example:
>
>   example<-data.frame(cbind(col1=1:10, col2=21:30, col3=41:50))
>    call.fun<-function(t){
>      x<-paste("col",t, sep="")  ## Change this so
that it is the data, not a character expression
>       example$x}
>
> call.fun(t=2)
Get out of the dollar habit.
Replace your

   example$x

with

   example[[x]]

or with

   example[, x]

Peter Ehlers
>
>   Within the real function, I will continue do calculations on the column
of data.  My problem is that I am either getting a character expression or NULL
from my function.
>
> Thanks for your help on what is probably a very simple question.
>
> John
> 	[[alternative HTML version deleted]]
>
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On May 18, 2011, at 3:12 PM, John Poulsen wrote:
> Hello,
>
> I want to build a function to call up a column of a data.frame by  
> the names of the columns.  I have column names that are sequentially  
> named (col1, col2, etc.). How do I change a character expression  
> into something that will be understood as a data.frame column.  For  
> example:
>
> example<-data.frame(cbind(col1=1:10, col2=21:30, col3=41:50))
>  call.fun<-function(t){
>    x<-paste("col",t, sep="")  ## Change this so that
it is the data,
> not a character expression
# right.... don't use the "$" operator, instead  use
"[["
         example[[x]]    }
>
> call.fun(t=2)
>
> Within the real function, I will continue do calculations on the  
> column of data.  My problem is that I am either getting a character  
> expression or NULL from my function.
>
> Thanks for your help on what is probably a very simple question.
>
> John
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT