[R] how to select columns

[David Winsemius]

On Jan 30, 2012, at 2:30 AM, David Studer wrote:

> Hello,
> I have the following question:
>
> when creating a data.frame
> a1<-c(1,2,3)
> a2<-c(1,2,3)
> c<-data.frame(a1,a2)
> I can select columns using an index like:
> c[,1:2]
> Is this possible too when using column-names? (something like  
> c(,"a1":"a2"),
> which doesn't work):

Generally you need to use grep to convert column names to numbers for  
use within "[" operations]

df[ , grep("^a1$", names(df)):grep"^"a2$", names(df)) ]

-- 
Another David

>
> Alternative question: Is there a function to get the index of a  
> variable by
> name

That's what grep will do.

> or can I
> select certain columns using a loop? (a_1, a_2, ..., a_n)
>
> Thank you very much!
> David

David Winsemius, MD
Heritage Laboratories
West Hartford, CT