How to properly combine POSIXct objects with NULL in R?

Question

When using R's primitive c function, combining the current datetime with NULL gives:

> class(c(NULL,Sys.time()))
[1] "numeric"

But when passing NULL last:

> class(c(Sys.time(),NULL))
[1] "POSIXct" "POSIXt"

Is this intended? The documentation for c reads: "The output type is determined from the highest type of the components in the hierarchy NULL < raw < logical < integer < double < complex < character < list < expression." but there is no mention of order being relevant.

Is there a better (more consistent) way to combine objects of class POSIXct with NULL into a vector? Or should one always explicitly test for NULL and handle it as a seperate case?

score 2 · Accepted Answer · answered Dec 06 '16 at 13:27

This is because c acts as an S3 generic, and there is a POSIXct method for it:

> c.POSIXct
function (..., recursive = FALSE) 
.POSIXct(c(unlist(lapply(list(...), unclass))))
<bytecode: 0x000001ab3e6efe58>
<environment: namespace:base>

(S3 methods dispatch on the class of the first argument, so c(POSIXct, NULL) will call c.POSIXct, whereas c(NULL, POSIXct) will call the default method.)

How to properly combine POSIXct objects with NULL in R?

1 Answers1