How to keep POSIXct format when appending string to date item in list?

Question

Here the time is added to list and list converted to string:

str(list(Sys.time()))

Output is :

  POSIXct[1:1], format: "2017-11-10 21:22:56"

How to paste string to list item and keep format ? This should be outputted :

  List of 1
  $ : Time : POSIXct[1:1], format: "2017-11-10 21:22:56"

I've tried :

str(list(paste("time" , Sys.time())))
str(list(c("time" , Sys.time())))

But the outputs are :

> str(list(paste("time" , Sys.time())))
List of 1
$ : chr "time 2017-11-10 21:23:23"
> str(list(c("time" , Sys.time())))
List of 1
$ : chr [1:2] "time" "1510349011.98052"

Answer 1

str(list(paste("time" , Sys.time())))

This is a list of length 1, the 1 element is from paste, which is going to return a character string, because that's what pastes job is. So str reports a list of length 1 of chr type.

str(list(c("time" , Sys.time())))

This is also a list of length 1, the 1 element is a vector (created by c) of the string "time" and a POSIXct object from Sys.time(). Vectors can only store one kind of thing, so R has to convert everything to characters.

Interestingly the way POSIXct elements in a vector are converted to chr depends on what's the first element in the vector:

> str(c("this",Sys.time()))
 chr [1:2] "this" "1510353128.84358"

> str(c(Sys.time(),"this"))
 POSIXct[1:2], format: "2017-11-10 22:33:13" NA
 Warning message:
 In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion

Because R is using the first element to figure out which conversion method to use. If the first element is a character, it uses as.character.default which converts POSIXt objects to numbers because deep down they are just numbers, and as.character.default doesn't understand POSIXt timestamps:

> as.character.default(Sys.time())
[1] "1510353378.21108"

If the first element, or all the elements, are POSIX objects then you do get a formatted timestamp:

> as.character.POSIXct(Sys.time())
Error in as.character.POSIXct(Sys.time()) : 
  could not find function "as.character.POSIXct"
> as.character.POSIXt(Sys.time())
[1] "2017-11-10 22:36:30"

This fails:

> str(c(Sys.time(),"this"))
 POSIXct[1:2], format: "2017-11-10 22:33:13" NA
 Warning message:
 In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion

because its trying to call as.POSIXlt.POSIXct on a string "this":

> as.POSIXlt.POSIXct("this")
[1] NA
Warning message:
In as.POSIXlt.POSIXct("this") : NAs introduced by coercion
> 

Clear? Probably not. Basically you should figure out what you want and format data elements into character before you start pasting strings together.