[R] is.na behavior

[Richard M. Heiberger]

It is in context of determining if an input argument for a graph title
is missing or null or na.  In any of those cases the function defines
a main title.
If the incoming title is not one of those, then I use the incoming title.
When the incoming title is an expression I see the warning.

library(lattice)

simple <- function(x, y, main) {
  if (missing(main) || is.null(main) || is.na(main))
     main <-"abcd"
  xyplot(y ~ x, main=main)
}

simple(1, 2)
simple(1, 2, main=expression("defg"))

## In the real case the constructed title is not a simple character
## string, but the result of function call with several incoming
## arguments and several computed arguments.  It is of a complexity
## that making it the default in the calling sequence would
## unnecessarily complicate the calling sequence.

On Wed, Nov 18, 2015 at 10:04 PM, William Dunlap <wdunlap at tibco.com> wrote:
> You can convert the expression to a list and use is.na on that:
>    > e <- expression(1+NA, NA, 7, function(x)x+1)
>    > is.na(as.list(e))
>    [1] FALSE  TRUE FALSE FALSE
> and you can do the same for a call object
>    > is.na(as.list(quote(func(arg1, tag2=NA, tag3=log(NA)))))
>                 tag2  tag3
>    FALSE FALSE  TRUE FALSE
>
> However, what is your motivation for wanting to apply is.na to an expression?
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>
> On Wed, Nov 18, 2015 at 5:54 PM, Richard M. Heiberger <rmh at temple.edu> wrote:
>> What is the rationale for the following warning in R-3.2.2?
>>
>>> is.na(expression(abcd))
>> [1] FALSE
>> Warning message:
>> In is.na(expression(abcd)) :
>>   is.na() applied to non-(list or vector) of type 'expression'
>>
>>         [[alternative HTML version deleted]]
>>
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