[Rd] Behaviour of read.table with empty columns

[John Fox]

Dear r-devel list members,

I stumbled across the following behaviour of read.table() recently: Suppose
that I have the data

a  " " ""
"" ""  ""

in a file or copied to the clipboard, and issue the command

> DF <- read.table("clipboard")
> DF
  V1 V2 V3
1  a NA NA
2    NA NA

> is.na(DF)
        V1   V2   V3
[1,] FALSE TRUE TRUE
[2,] FALSE TRUE TRUE

I was surprised by the NAs. Note that they occur only when a column consists
entirely of empty strings or strings composed of blanks. 

On the other hand

> data.frame(A=c("", "", ""))
  A
1  
2  
3  

works as I would have expected.

A work-around for me was

> DF[is.na(DF)] <- ""
> DF
  V1 V2 V3
1  a      
2         

But, as I said, I found the behaviour of read.table() puzzling. 

All this is with R 2.5.0 on a Windows XP Pro SP 2 system.

Comments?

Thanks,
 John

--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox