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)

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)

Thank you very much!
David

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