R devel - May 2002 - is.na<- coerces character vectors to be factors within dataframes (PR#1577)

I am not sure if this is a bug within is.na<- or if it lies deeper in the
dataframe construction process. Indeed, perhaps it is not a bug at all (in
which case I would suggest that the help page for NA be provided with a warning
for unsuspecting users (like me)).

When used on a character vector within a dataframe, is.na<- coerces the
vector
to factor.
> x <- data.frame(var = LETTERS[1:3])
> x$var <- as.character(x$var)
> x
  var
1   A
2   B
3   C
> is.character(x$var)
[1] TRUE
> is.na(x[[1]]) <- 2
> x
   var
1    A
2   <NA>
3    C
> is.character(x$var)
[1] FALSE
> is.factor(x$var)
[1] TRUE
>
Interestingly enough, this coersion does not occur if you refer to x$var
instead of x[[1]].
> x <- data.frame(var = LETTERS[1:3])
> x$var <- as.character(x$var)
> is.na(x$var) <- 2
> x
   var
1    A
2   <NA>
3    C
> is.character(x$var)
[1] TRUE
> is.factor(x$var)
[1] FALSE
> 
I could (ort of) imagine a story in which the coercision is the desired
behavior --  by using is.na you are implicitly taking apart a dataframe and
putting it back together and, when you make dataframes, character vectors are
coerced to factor by default. But I can't come up with a story as to why
x$var
should be handled differently then x[[1]].
> R.version
         _                   
platform sparc-sun-solaris2.6
arch     sparc               
os       solaris2.6          
system   sparc, solaris2.6   
status                       
major    1                   
minor    5.0                 
year     2002                
month    04                  
day      29                  
language R                   
> 
Thanks,

David Kane

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