Dear list,
When using as.POSIXlt with times measured down to microseconds the default
format.POSIXlt seems to cause some possibly undesirable behaviour:
According to the code in format.POSIXlt the maximum accuracy of printing
fractional seconds is 1 microsecond, but if I do;
options( digits.secs = 6 )
as.POSIXlt( 1.000002 , tz="", origin="1970-01-01")
as.POSIXlt( 1.999998 , tz="", origin="1970-01-01")
as.POSIXlt( 1.999999 , tz="", origin="1970-01-01")
I return respectively:
[1] "1970-01-01 01:00:01.000002 BST"
[1] "1970-01-01 01:00:01.999998 BST"
[1] "1970-01-01 01:00:01 BST"
If options( digits.secs = 6 ) should I not expect to be able to print 1.999999
seconds? This seems to be caused by the following code fragment in
format.POSIXlt:
np <- getOption("digits.secs")
if (is.null(np))
np <- 0L
else np <- min(6L, np)
if (np >= 1L)
for (i in seq_len(np) - 1L) if (all(abs(secs - round(secs,
i)) < 1e-06)) {
np <- i
break
}
Specifically
for (i in seq_len(np) - 1L) if (all(abs(secs - round(secs,
i)) < 1e-06))
Which in the case of 1.999999 seconds will give:
options( scipen = 10 )
np <- 6
sapply( seq_len(np) - 1L , function(x) abs(1.999999 - round(1.999999, x)) )
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001
The logical test all( ... < 1e-06) should evaluate to FALSE but due to
floating point precision it evaluates TRUE:
sprintf( "%.20f" , abs(1. 999999 - round(1. 999999,5)))
[1] "0.00000099999999991773"
If instead of:
for (i in seq_len(np) - 1L) if (all(abs(secs - round(secs,
i)) < 1e-06))
in format.POSIXlt we had a comparison value that was half the minimum increment:
for (i in seq_len(np) - 1L) if (all(abs(secs - round(secs,
i)) < 5e-07))
This behaviour disappears:
mod.format.POSIXlt( as.POSIXlt( 1.999999 , tz="", origin="1970-01-01") )
[1] "1970-01-01 01:00:01.999999"
But I am unsure if the original behaviour is what I should expect given the
documentation (I have read it and I can't see a reason to expect 1.999999 to
round down to 1). And also if changing the formatting function would have other
undesirable consequences?
My sessionInfo():
R version 3.0.0 (2013-04-03)
Platform: x86_64-w64-mingw32/x64 (64-bit)
locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
Thank you,
Simon
------------------------------------
Simon O'Hanlon
Postgraduate Researcher
Helminth Ecology Research Group
Department of Infectious Disease Epidemiology
Imperial College London
St. Mary's Hospital, Norfolk Place,
London, W2 1PG, UK
Office: +44 (0) 20 759 43229
------------------------------------
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