[R] Vectors of years, months, and days to dates?

[Patrick Connolly]

On Tue, 08-Jun-2004 at 02:57PM +1200, Richard A. O'Keefe wrote:

|> Rolf Turner <rolf at math.unb.ca> attacked:
|> 		Have you ***any*** evidence that R's procedure degrades
|> 		performance, under any circumstances?  (Apparently not.) In
|> 		that case why are you going on and on about it?
|> 	
|> I did the following:
|> 
|>     library(chron)
|>     ymd.to.POSIXlt <-
|>         function (y, m, d) as.POSIXlt(chron(julian(y=y, x=m, d=d)))
|>     n <- 100000
|>     y <- sample(1970:2004, n, replace=TRUE)
|>     m <- sample(1:12,      n, replace=TRUE)
|>     d <- sample(1:28,      n, replace=TRUE)
|>     system.time(ymd.to.POSIXlt(y, m, d))
|>     [1]  8.78  0.10 31.76  0.00  0.00
|>     system.time(as.POSIXlt(paste(y,m,d, sep="-")))
|>     [1] 14.64  0.13 53.30  0.00  0.00
|> 
|> This was on a 500MHz SunBlade100, a slow machine by today's standards,
|> using R 1.9.0.  There is all the evidence one could reasonably ask for
|> that going through paste() instead of providing y, m, d as numeric
|> vectors *does* degrade performance under at least some circumstances.

Ain't that just the darnest thing?

I tried on a somewhat faster machine and got this:


>     system.time(ymd.to.POSIXlt(y, m, d))
[1] 2.06 0.22 2.29 0.00 0.00
>     system.time(as.POSIXlt(paste(y,m,d, sep="-")))
[1] 2.82 0.04 2.86 0.00 0.00

Negligible difference, I'd have thought, but I tried a few more times


>     system.time(ymd.to.POSIXlt(y, m, d))
[1] 1.12 0.11 1.23 0.00 0.00
>     system.time(as.POSIXlt(paste(y,m,d, sep="-")))
[1] 2.69 0.04 2.73 0.00 0.00
>     system.time(ymd.to.POSIXlt(y, m, d))
[1] 0.98 0.02 1.00 0.00 0.00
>     system.time(as.POSIXlt(paste(y,m,d, sep="-")))
[1] 2.82 0.00 2.82 0.00 0.00
>     system.time(ymd.to.POSIXlt(y, m, d))
[1] 0.98 0.04 1.02 0.00 0.00
>     system.time(as.POSIXlt(paste(y,m,d, sep="-")))
[1] 2.65 0.03 2.68 0.00 0.00
> 


ymd.to.POSIXlt benefits from repeated, but the as.POSIXlt method
doesn't.  I even tried it again with a new R process and got the same
sort of thing?

What makes the difference?

-- 
Patrick Connolly
HortResearch
Mt Albert
Auckland
New Zealand 
Ph: +64-9 815 4200 x 7188
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