[R] Taking specific/timed differences in a zoo timeseries

[Gabor Grothendieck]

Try this:

library(zoo)
# temp <- ... from post asking question

# create a day sequence, dt, with no missing days
# and create a 0 width series with those times.
# merge that with original series giving original
# series plus a bunch of times having NA values.
# Use na.locf to fill in those values with the last
# non-missing so far.

rng <- range(time(temp))
dt <- seq(rng[1], rng[2], "day")

temp.m <- na.locf(merge(temp, zoo(, dt)))

# create a lagged time scale and subtract the
# lagged series from original

dt.lag <- as.Date(as.yearmon(dt)+1/12) + as.numeric(format(dt, "%d")) - 1
temp - zoo(coredata(temp.m), dt.lag)


Using your data the output from the last line is:

> temp - zoo(coredata(temp.m), dt.lag)
2009-10-05 2009-10-06 2009-10-07 2009-10-08 2009-10-09 2009-10-13 2009-10-14
        -5         -6          3          2         -2          2          1


On Wed, Oct 14, 2009 at 10:39 AM, Sergey Goriatchev <sergeyg at gmail.com> wrote:
> Hello everyone.
>
> I have a specific problem that I have difficulties to solve.
> Assume I have a zoo object:
>
> set.seed(12345)
> data <- round(runif(27)*10+runif(27)*5, 0)
> dates <- as.Date(c("09/03/09", "09/04/09", "09/07/09", "09/09/09",
> "09/10/09", "09/11/09", "09/14/09", "09/16/09", "09/17/09",
> "09/18/09", "09/21/09", "09/22/09", "09/23/09",
> "09/24/09", "09/25/09", "09/28/09", "09/29/09", "09/30/09",
> "10/01/09", "10/02/09", "10/05/09", "10/06/09", "10/07/09",
> "10/08/09", "10/09/09", "10/13/09", "10/14/09"), "%m/%d/%y")
> temp <- zoo(data, order.by=dates)
>
> What I need to do is to take differences between say October 14th and
> September 14, then October 13th and September 13th, that is 1 month
> difference independent of number of days inbetween. And when there is
> no matching date in an earlier month, like here where there is no
> September 13th, the date should be the first preceding date, that is
> September 11th in this example. How can I do that?
>
> The above is just an example, my zoo object is very big and I need to
> take differences between years, that is between October 14th, 2009 and
> October 14th, 2008, then Oct.13, 2009 and Oct.13, 2008, and so on.
> Also, the time index of my zoo object has format "10/14/09" (that is
> Oct.14, 2009), and that is the format I need to operate with and do
> not want to change. In the example I reformated just so that I can
> create a zoo object.
>
> Could some friendly person please show me how to do such a calculation?
>
> Thank you in advance!
>
> Best,
> Sergey
>
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