[R] POSIXct-coerced NA's not considered NA by is.na()

[jim holtman]

If you are going to convert a column from character to POSIXct, I
would assume that you would do it this way and get the results that
you want:

> x <- data.frame(times = c('2012-08-14 12:00', '2011-01-04 08:00', 'b')
+     , stringsAsFactors = FALSE
+     )
> x
             times
1 2012-08-14 12:00
2 2011-01-04 08:00
3                b
> x$newtime <- as.POSIXct(x$times, format = "%Y-%m-%d %H:%M")
> x
             times             newtime
1 2012-08-14 12:00 2012-08-14 12:00:00
2 2011-01-04 08:00 2011-01-04 08:00:00
3                b                <NA>
> is.na(x$newtime)
[1] FALSE FALSE  TRUE
>


On Fri, Aug 24, 2012 at 2:26 PM, Alexander Shenkin <ashenkin at ufl.edu> wrote:
> Thanks for your reply, Jim.
>
> On 8/24/2012 12:14 PM, jim holtman wrote:
>> I think your first problem is the coersion done by 'c' when you are
>> mixing objects of various types: you have POSIXct and character.
>
> Yes, that's something I may have confounded. Still, the warning I'm
> getting is "In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion".
>  I'm not sure how c()'s coercion works - the warning seems to indicate
> that c() is finding as.POSIXct.  That's strange though, since I would
> expect to get an error in that case, not just a warning:
>
>> as.POSIXct("b")
> Error in as.POSIXlt.character(x, tz, ...) :
>   character string is not in a standard unambiguous format
>
>> What were your expections?
>
> I was expecting that the NA resulting from the coercion would result in
> a TRUE value when being operated on by is.na().  Instead, I got:
>
>> is.na(date_vec[4])
> [1] FALSE
>
>>> x <- Sys.time()
>>> str(x)
>>  POSIXct[1:1], format: "2012-08-24 13:12:31"
>>> y <- c(x, 'b')
>>> str(y)
>>  POSIXct[1:2], format: "2012-08-24 13:12:31" NA
>> Warning message:
>> In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion
>>> dput(y)
>> structure(c("1345828351.75", "b"), class = c("POSIXct", "POSIXt"
>> ))
>>>
>>
>> Look at the 'dput' and see that what it is trying to do is to use the
>> numeric value changed to a character string as a POSIXct value.  So I
>> am not surprised by the error since it is probably not what you
>> wanted.  Did you intend to use 'list' instead of 'c'?
>
> I'm a bit confused about how you get that from dput.  Here's what I see:
>
>> dput(date_vec)
> structure(c("1345831833", "1345831834", NA, "b"), class = c("POSIXct",
> "POSIXt"))
>
> Regardless, I do get the same strange is.na() behavior from the following:
>
>> bad_date = "b"
>> class(bad_date) = "POSIXct"
>> bad_date
> [1] NA
> Warning message:
> In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion
>> is.na(bad_date) # shouldn't this be TRUE?
> [1] FALSE
>
> As nasty as it may be, shouldn't something showing up as "NA" result in
> TRUE when being tested by is.na()?
>
> Just to put some context around this, I was investigating this issue as
> I was thinking about converting dataframe columns to dates, and
> detecting errors in that conversion.
>
> thanks,
> Allie
>
>>
>> On Fri, Aug 24, 2012 at 9:47 AM, Alexander Shenkin <ashenkin at ufl.edu> wrote:
>>> Hello folks,
>>>
>>> I found a strangeness while experimenting with POSIXct vectors and
>>> lists.  It seems that coerced NA's aren't "real" NAs, at least as
>>> considered by is.na()?
>>>
>>>> date_vec = c(as.POSIXct(now()), as.POSIXct(now()+1),NA,"b")
>>>> date_vec
>>> [1] "2012-08-22 15:00:46 COT" "2012-08-22 15:00:47 COT" NA
>>>
>>> [4] NA
>>> Warning message:
>>> In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion
>>>> date_vec[4]
>>> [1] NA
>>> Warning message:
>>> In as.POSIXlt.POSIXct(x, tz) : NAs introduced by coercion
>>>> is.na(date_vec[4])
>>> [1] FALSE
>>>> is.na(date_vec[3])
>>> [1] TRUE
>>>> is.POSIXct(date_vec[4])
>>> [1] TRUE
>



-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.