[R] as.Date() results depend on order of data within vector?

[Gavin Simpson]

On Sun, 2007-01-07 at 12:01 +0000, Mark Wardle wrote:
> Dear all,
> 
> The as.Date() function appears to give different results depending on
> the order of the vector passed into it.
> 
> d1 = c("1900-01-01", "2007-01-01","","2001-05-03")
> d2 = c("", "1900-01-01", "2007-01-01","2001-05-03")
> as.Date(d1)	# gives correct results
> as.Date(d2)	# fails with error (* see below)
> 
> This problem does not arise if the dates are NA rather than an empty
> string, but my data is coming via RODBC and I still don't have NAs
> passed across properly.
> 
> I might add that I initially noticed this behaviour when using RODBC's
> sqlQuery() function call, and I initially had difficulty explaining why
> one column of dates was passed correctly, but another failed. The
> failing column was a "date of death" column where it was NA ("") for
> most patients.
> 
> I've come up with two workarounds that work. The first is to sort the
> data at the SQL level, ensuring the initial record is not null. The
> second is to use sqlQuery() with as.is=T option, and then do the sorting
> and conversion afterwards.

Why not just tell R what the format the dates are in, using the "format"
argument to as.Date?

> d1 = c("1900-01-01", "2007-01-01","","2001-05-03")
> d2 = c("", "1900-01-01", "2007-01-01","2001-05-03")
> as.Date(d1, "%Y-%m-%d")
[1] "1900-01-01" "2007-01-01" NA           "2001-05-03"
> as.Date(d2, "%Y-%m-%d")
[1] NA           "1900-01-01" "2007-01-01" "2001-05-03"

> 
> Is the behaviour of as.Date() shown above as expected/designed?

I don't know about expected/designed, but I would have thought
explicitly stating the date format would be the most fool-proof way of
making sure R did what you wanted, and the easiest way to work around
your "problem".

HTH

G

> 
> Many thanks,
> 
> Mark
> 
> 
> (*) "Error in fromchar(x) : character string is not in a standard
> unambiguous format"
> 
> sessionInfo():
> R version 2.4.0 (2006-10-03) powerpc-apple-darwin8.7.0 locale:
> C/en_GB.UTF-8/C/C/C/C
> attached base packages:
> [1] "methods"   "stats"     "graphics"  "grDevices" "utils"
> "datasets" "base"
> 
> other attached packages:
> rcompletion       RODBC
>    "0.0-12"     "1.1-7"
> 
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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Gavin Simpson                     [t] +44 (0)20 7679 0522
ECRC                              [f] +44 (0)20 7679 0565
UCL Department of Geography
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