[R] seq.POSIXt() with short time intervals

[Don MacQueen]

I was surprised when seq.POSIXt() returned a single value rather than 
a vector, from inputs that I thought were reasonable. Here's an 
example to illustrate:

>  t0 <- ISOdatetime(2002,6,24,0,0,10)

## expected a sequence of 16 times 1 second apart
>  seq.POSIXt(from=t0,to=t0+15,by='1 sec')
[1] "2002-06-24 00:00:10 PDT"

## traces to this call
>  seq.default(1024902010,1024902025,by=1)
[1] 1024902010
>

## which is due to the following expressions in seq.default()
      del <- to - from
      ## a few lines omitted
      dd <- abs(del)/max(abs(to), abs(from))
      if (dd < sqrt(.Machine$double.eps)) return(from)

This looks like a trap to catch bad input, and return something 
reasonable (or at least not crash).

It's a given that seq.default() pre-dates seq.POSIXt() by many years, 
and the test for relative range undoubtedly has good reasons, so it's 
probably not a good idea to change seq.default().

There's nothing inherently wrong with such a time sequence, and in 
fact, if t0 = ISOdatetime(1970,6,24,0,0,10) instead, seq.POSIXt() 
does return a vector.

I'd like to see seq.POSIXt() return a full sequence in such cases, 
regardless of the starting point, so what I'll do is think about it, 
and see if I can't come up with a good way to modify seq.POSIXt() to 
do that. In the meantime, I'm interested in any comments or 
suggestions anyone might have.

Turns out that with the base time, t0, as above, t0+15 is right at 
the threshold of the relative range test:
## compare with t0=t0+16
## this call returns what was expected
>  seq.POSIXt(from=t0, to=t0+16, by='1 sec')
  [1] "2002-06-24 00:00:10 PDT" "2002-06-24 00:00:11 PDT" "2002-06-24 
00:00:12 PDT" "2002-06-24 00:00:13 PDT"
  [5] "2002-06-24 00:00:14 PDT" "2002-06-24 00:00:15 PDT" "2002-06-24 
00:00:16 PDT" "2002-06-24 00:00:17 PDT"
  [9] "2002-06-24 00:00:18 PDT" "2002-06-24 00:00:19 PDT" "2002-06-24 
00:00:20 PDT" "2002-06-24 00:00:21 PDT"
[13] "2002-06-24 00:00:22 PDT" "2002-06-24 00:00:23 PDT" "2002-06-24 
00:00:24 PDT" "2002-06-24 00:00:25 PDT"
[17] "2002-06-24 00:00:26 PDT"


I've found this on two platforms:
>  version
          _
platform sparc-sun-solaris2.7
arch     sparc
os       solaris2.7
system   sparc, solaris2.7
status
major    1
minor    4.1
year     2002
month    01
day      30
language R

platform powerpc-apple-darwin5.3
arch     powerpc
os       darwin5.3
system   powerpc, darwin5.3
status
major    1
minor    4.1     (also 4.0)
year     2001
month    12
day      19
language R

-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
--------------------------------------
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