[R] how to select columns

Milan Bouchet-Valat nalimilan at club.fr
Mon Jan 30 09:54:07 CET 2012

Le lundi 30 janvier 2012 à 08:30 +0100, David Studer a écrit :
> 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")


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