[R] NAs in indices

[Charles C. Berry]

On Sun, 2 Sep 2007, Muenchen, Robert A (Bob) wrote:

> Hi All,
>
> I'm fiddling with an program to read a text file containing periods that
> SAS uses for missing values. I know that if I had the original SAS data
> set instead of a text file, R would handle this conversion for me.
>
> Data frames do not allow missing values in their indices but vectors do.
> Why is that? A search of the error message points out the problem and
> solution but not why they differ. A simplified program that demonstrates
> the issue is below.
>
> Thanks,
> Bob
>
> # Here's a data frame that has both periods and NAs.
> # I want sex to remain character for now.
>
> sex=c("m","f",".",NA)
> x=c(1,2,3,NA)
> myDF <- data.frame(sex,x,stringsAsFactors=F)
> rm(sex,x)
> myDF
>
> # Substituting NA into data frame does not work
> # due to NAs in the indices. The error message is:
> # missing values are not allowed in subscripted assignments of data
> frames
>
> myDF[ myDF$sex==".", "sex" ] <- NA
> myDF
>
> # This works because myDF$sex is a vector and vectors allow NAs in
> indexes.
> # Why don't data frames allow this?
>
> myDF$sex[ myDF$sex=="." ] <- NA
> myDF


R version 2.5.1  'allows' it.


> df <- as.data.frame(diag(3)[,-1])
> df[ df[,1]==1 ] <- NA
> df

but the result may not be what you were expecting. See

 	 ?"[.data.frame"

(esp. Details) for more info on why it does not 'work' as you expected.


Also, since you mention a 'text file' I suggest you look at

 	 ?read.table

or

 	?scan

where you will see that

 	dots.are.NA <- read.table("my.file", na.strings = '.' )

may help you.

Chuck

>
> =========================================================
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901