HI,
   I have a dataframe with two variable A, B. I transform the two variable
and name them as C, D and save it in a dataframe  dfcd. However, I wonder
why can't I call them by dfcd$C and dfcd$D?

   Thanks,

Miao

> A=c(1,2,3)
> B=c(4,6,7)
> dfab<-data.frame(A,B)
> C=dfab["A"]*2
> D=dfab["B"]*3
> dfcd<-data.frame(C,D)
> dfcd
  A  B
1 2 12
2 4 18
3 6 21
> dfcd$C
NULL
> dfcd$A
[1] 2 4 6

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