[Rd] ISOdate/ISOdatetime performance suggestions, other date/time questions

[Sklyar, Oleg (MI London)]

Dear list:

working with date/times I have come across a problem that ISOdate and
ISOdatetime are too slow on large vectors of data. I was surprised just
until I looked at the implementation and the man page: "ISOdatetime and
ISOdate are convenience wrappers for strptime". In other terms, they
convert data to character representation first in order to create a
POSIXlt object that is then converted to POSIXct. And POSIXct, i.e. the
number of seconds since 1970, is really what one wants most often.

Obviously this is not a bug, but it is really a suboptimal
implementation of a pretty important function as the example below
shows.

Now my questions are:

- any chance that the implementation can be changed in R (suggested,
well tz needs to be added)?
- is there a better pure-R (no-C) way than that shown below to convert
to POSIXct?
- any idea why in the example below fooling R into thinking a list is
POSIXlt is faster than just creating a POSIXlt by rep or seq? It's not a
huge difference, but still. Unfortunately seq on POSIXlt returns POSIXct
anyway, so the class of 'origin' is set correctly.
- any idea why seq is faster than rep when applied on POSIXct? There is
hardly anything simpler than on double values...

Thanks in advance for your comments,
Oleg

It's common in finance to work with time stamps stored in a form like
%Y%m%d.%H%M%OS, e.g. 20080410.140444 for now, this is what 'ts' in the
example below is:

ts = 1e4*trunc(rnorm(50000,2008,2)) + 1e2*trunc(runif(50000,1,12)) + 
     trunc(runif(50000,1,28)) + 1e-2*trunc(runif(50000,1,24)) +
     1e-4*trunc(runif(50000,1,60)) + 1e-6*runif(50000,1,60)

posix.viaISOdate = function(x) {
    date = trunc(x at .Data)
    time = round(1e6*x at .Data%%1,2)
    rtime = round(time)
    z = list(sec=rtime%%1e2 + time%%1,
            min=(rtime%/%1e2)%%1e2,
            hour=rtime%/%1e4,
            mday=date%%100,
            mon=(date%/%100)%%100,
            year=date%/%10000)
    ISOdate(z$year,z$mon,z$mday,z$hour,z$min,z$sec) # to POSIXct
}

## This is just a test of how is it faster to create a long POSIXlt
object
## before another implementations are given

origin = as.POSIXct("1970-01-01") 

mean(sapply(1:25,function(i) system.time(
    as.POSIXlt(rep(origin,600000))
))[1,])
# [1] 0.3972

mean(sapply(1:25,function(i) system.time(
    as.POSIXlt(seq(origin, origin, length.out=600000))
))[1,])
# [1] 0.30528


posix.viaPOSIXlt1 = function(x) {
    origin = as.POSIXct("1970-01-01") 
    z = as.POSIXlt(seq(origin, origin, length.out=length(x)))
    date = trunc(x at .Data)
    time = round(1e6*x at .Data%%1,2)
    rtime = round(time)
    z$sec=rtime%%1e2 + time%%1
    z$min=(rtime%/%1e2)%%1e2
    z$hour=rtime%/%1e4
    z$mday=date%%100
    z$mon=(date%/%100)%%100-1
    z$year=date%/%10000-1900
    as.double(z) # to POSIXct
}

posix.vialist = function(x) {
    date = trunc(x at .Data)
    time = round(1e6*x at .Data%%1,2)
    rtime = round(time)
    na = rep(0.0,length(x))
    z = list(sec=rtime%%1e2 + time%%1,
        min=(rtime%/%1e2)%%1e2,
        hour=rtime%/%1e4,
        mday=date%%100,
        mon=(date%/%100)%%100-1,
        year=date%/%10000-1900,
        wday=na,yday=na,isdst=na)
    class(z) = c("POSIXt","POSIXlt")
    as.double(z) # to POSIXct
}

v1 = posix.viaISOdate(ts)
v2 = posix.viaPOSIXlt1(ts)
v3 = posix.vialist(ts)

all(v1==v2 & v2==v3)
# [1] TRUE

mean(sapply(1:25,function(i) system.time(
    system.time(posix.viaISOdate(ts))
))[1,])
# [1] 1.54244

mean(sapply(1:25,function(i) system.time(
    system.time(posix.viaPOSIXlt1(ts))
))[1,])
# [1] 0.37624

mean(sapply(1:25,function(i) system.time(
    system.time(posix.vialist(ts))
))[1,])
# [1] 0.35488




sessionInfo()
R version 2.6.2 (2008-02-08)
x86_64-unknown-linux-gnu

locale:
LC_CTYPE=en_GB.UTF-8;LC_NUMERIC=C;LC_TIME=en_GB.UTF-8;LC_COLLATE=C;LC_MO
NETARY=en_GB.UTF-8;LC_MESSAGES=en_GB.UTF-8;LC_PAPER=en_GB.UTF-8;LC_NAME=
C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_GB.UTF-8;LC_IDENTIFICATI
ON=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] rcompgen_0.1-17

Dr Oleg Sklyar
Technology Group
Man Investments Ltd
+44 (0)20 7144 3803
osklyar at maninvestments.com


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