[Rd] Date class shows Inf as NA; this confuses the use of is.na()

[Joris Meys]

Hi Werner,

on ?is.na it says:

> The default method for anyNA handles atomic vectors without a class and
NULL.

I hear you, and it is confusing to say the least. Looking deeper, the
culprit seems to be in the conversion of a Date to POSIXlt prior to the
formatting:

> x <- as.Date(Inf,origin = '1970-01-01')
> is.na(as.POSIXlt(x))
[1] TRUE

Given this implicit conversion, I'd argue that as.Date should really return
NA as well when passed an infinite value. The other option is to provide an
is.na method for the Date class, which is -given is.na is an internal
generic- rather trivial:

> is.na.Date <- function(x) is.na(as.POSIXlt(x))
> is.na(x)
[1] TRUE

This might be a workaround for your current problem without needing changes
to R itself. But this will give a "wrong" answer in the sense that this
still works:

> Sys.Date() - x
Time difference of -Inf days

I personally would go for NA as the "correct" date for an infinite value,
but given that this will have implications in other areas, there is a
possibility of breaking code and it should be investigated a bit further
imho.
Cheers
Joris




On Fri, Jun 8, 2018 at 11:21 PM, Werner Grundlingh <wgrundlingh using gmail.com>
wrote:

> Indeed. as_date is from lubridate, but the same holds for as.Date.
>
> The output and it's interpretation should be consistent, otherwise it leads
> to confusion when programming. I understand that the difference exists
> after asking a question on Stack Overflow:
>   https://stackoverflow.com/q/50766089/914686
> This understanding is never mentioned in the documentation - that an Inf
> date is actually represented as NA:
>   https://www.rdocumentation.org/packages/base/versions/3.
> 5.0/topics/as.Date
> So I'm of the impression that the display should be fixed as a first option
> (thereby providing clarity/transparency in terms of back-end and output),
> or the documentation amended (to highlight this) as a second option.
>
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-- 
Joris Meys
Statistical consultant

Department of Data Analysis and Mathematical Modelling
Ghent University
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