[R] is.na behavior

[Richard M. Heiberger]

Maybe the way to rephrase my question is to ask why there is not
an is.na.expression method that does that task for me?

> is.na(as.list(expression("defg")))
[1] FALSE
> is.na(expression("defg"))
[1] FALSE
Warning message:
In is.na(expression("defg")) :
  is.na() applied to non-(list or vector) of type 'expression'
>

On Wed, Nov 18, 2015 at 10:22 PM, Richard M. Heiberger <rmh at temple.edu> wrote:
> 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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