R help - Jan 2012 - how to select columns

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Le lundi 30 janvier 2012 ? 08:30 +0100, David Studer a ?crit
:
> 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)
Read the R intro, or any tutorial on R. You can just do:
c[,c("a1", "a2")]

(And I think you don't understand what ":" does, read the manual.
At
least, it doesn't work like your attempt c(,"a1":"a2")
would imply.)
> Alternative question: Is there a function to get the index of a variable by
> name or can I
> select certain columns using a loop? (a_1, a_2, ..., a_n)
No need for a loop:
which(colnames(c) == "a1")


Cheers

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