Dear Rxperts...

I would like to conditionally include an element (as a column) in a
dataframe. Please see the  sample code below:

a1 <-
data.frame(P=rep(1,10),Qr=LETTERS[1:10],b=letters[1:10],R=rep(c("A","B"),each=5))

 lc1 <- list(C1 = "P",C2 =
ifelse(is.element("Q",names(a1)),"Q",ifelse(is.element("b",names(a1)),"b",NULL)),C3="R")
 lc2 <- list(C1 = "P",C2 =
ifelse(is.element("Q",names(a1)),"Q",ifelse(is.element("b",names(a1)),"b",NULL)),C3="Ra")

a2 <- subset(a1, sel=unlist(lc1)) # this works
a3 <- subset(a1, sel=unlist(lc2)) # this doesn't

Is there a way to dynamically include columns in a dataframe?

Regards,
santosh

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