[R] dataframes without data

[Wolfgang Koller]

 Hi!

Yesterday I posted the following problem:

> There seems to be some inconsistency in the following behaviour:
> 
> > MYDATA <- data.frame(b=c("A","B","C"),x=c(1,2,3))
> > MYDATA
>   b x
> 1 A 1
> 2 B 2
> 3 C 3
> > MYDATAnew <- MYDATA[0:0,]
> > MYDATAnew
> [1] b x
> <0 rows> (or 0-length row.names)
> > MYDATAnew <- rbind(MYDATA[0:0,],MYDATA)
> > MYDATAnew
>   b x
> 1 1 1
> 2 2 2
> 3 3 3
> 
> Does R forget the type of the columns of a data.frame when the
> data.frame contains no data??

>From the following experiment I conclude, that the answer to this
question is "no" and that the problem lies in rbind() (maybe this is a
bug?):

> AAA <- data.frame(b=c("A","B","C"),x=c(1,2,3))
> str(AAA)
`data.frame':  3 obs. of  2 variables:
 $ b: Factor class ; 3 levels: A,B: 1 2 3
  ..- attr(*, "class")= chr "factor"
 $ x: num  1 2 3
> BBB <- AAA[0:0,]
> str(BBB)
`data.frame':  0 obs. of  2 variables:
 $ b: Factor class ; 3 levels: A,B: 
  ..- attr(*, "class")= chr "factor"
 $ x: num 
> CCC <- rbind(BBB,AAA)
> str(CCC)
`data.frame':  3 obs. of  2 variables:
 $ b: int  1 2 3
 $ x: num  1 2 3
> DDD <- rbind(AAA,BBB)
Error: missing value where logical needed

Do I do something wrong, when I intend to use dataframes in that way?

I should also mention, that Olivier Houix (thanks!) suggested to me to
do:

> AAA <- data.frame(b=c("A","B","C"),x=c(1,2,3))
> CCC <- rbind(AAA[1:1,],AAA)

But then I have to cut away the first row again:
> CCC <-CCC[-1,]
> CCC
  b x
2 A 1
3 B 2
4 C 3

This has a side effect on the row-numbers. Also, I hoped that there
would be a more elegant solution.

Thanks for suggestions, that help me understand better the basic
concepts of R!

Wolfgang Koller


----------------------------------------------------------
Wolfgang Koller,  koller2 at fgr.wu-wien.ac.at
Forschungsinstitut fuer Europafragen
Wirtschaftsuniversitaet Wien
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