[R] is.constant 2

[Gabor Grothendieck]

Christian Hoffmann <christian.hoffmann <at> wsl.ch> writes:

> 
> >>x <- c(1, 2, NA)
>  >>>>is.constant(x)
>  >
>  >>
>  >> [1] TRUE
>  >>
>  >> For data such as c(1, 1, 1, NA), I should think the safest answer 
> should be
>  >> NA, because one really doesn't know whether that last number is 1 or 
> not.
>  >>
>  >> Andy
>  >>
> 
>  >My version is
>  >is.constant <-  function(x) {
>  >   if (is.numeric(x) & !any(is.na(x)))  identical(min(x), max(x)) else 
>  >FALSE
>  >}
> 
> Since the issue of factors surfaced, I improved my function:
> 
> is.constant <-  function(x) {
>    if (is.factor(x)) (length(attributes(x)$levels)==1) && 
> (!any(is.na(as.character(x))))
>    else (is.numeric(x) && !any(is.na(x)) && identical(min(x), max(x)))
> }

Suggest you use an S3 generic and a separate methods for factor, and
in the future, other classes.  Also to make it more consistent with
other R functions have an na.rm= argument which defaults to TRUE.
If na.rm = FALSE then it should return NA  there are any NAs in
the same way that sum(c(1,2,NA), na.rm = FALSE) returns NA. There is 
some question of how to handle zero length arguments (or ones
that become zero length after removing NAs).

is.constant <- function(x, ...) UseMethod("is.constant")
is.constant.factor <- function(x, na.rm = TRUE) ... code for factor ...
is.constant.default <- function(x, na.rm = TRUE) ... code for default ...