[R] aggregate semi-hourly data not 00-24 but 9-9

[Eric Berger]

Hi Stefano,
If you mean from 9am on one day to 9am on the following day, you can
do a trick. Simply subtract 9hrs from each timestamp and then you want
midnight to midnight for these adjusted times, which you can get using
the method you followed.

I googled and found that lubridate::hours() can be used to add or
subtract hours from a POSIXct.

library(lubridate)

day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M", tz="Etc/GMT-1")
day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M", tz="Etc/GMT-1")
df1 <- data.frame(data_POSIX=seq(day_1, day_2, by="30 min"))
df1$hs <- rnorm(nrow(df1), 40, 10)
df1$diff[2:nrow(df1)] <- diff(df1$hs)

df1$data_POSIXminus9 <- df1$data_POSIX - lubridate::hours(9)
df1$dayX <- format(df1$data_POSIXminus9,"%y-%m-%d")
df2X <- aggregate(diff ~ dayX, df1, sum)
df2X

HTH,
Eric

On Mon, Sep 21, 2020 at 5:30 PM Stefano Sofia
<stefano.sofia using regione.marche.it> wrote:
>
> Dear R-list members,
> I have semi-hourly snowfall data.
> I should sum the semi-hourly increments (only the positive ones, but this is not described in my example) day by day, not from 00 to 24 but from 9 to 9.
>
> I am able to use the diff function, create a list of days and use the function aggregate, but it works only from 0 to 24. Any suggestion for an efficient way to do it?
> Here my code:
> day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M", tz="Etc/GMT-1")
> day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M", tz="Etc/GMT-1")
> df1 <- data.frame(data_POSIX=seq(day_1, day_2, by="30 min"))
> df1$hs <- rnorm(nrows(df1), 40, 10)
> df1$diff[2:nrow(df1)] <- diff(df1$hs)
> df1$day <- format(df$data_POSIX,"%y-%m-%d")
> df2 <- aggregate(diff ~ day, df, sum)
>
> Thank you for your help
> Stefano
>
>          (oo)
> --oOO--( )--OOo----------------
> Stefano Sofia PhD
> Civil Protection - Marche Region
> Meteo Section
> Snow Section
> Via del Colle Ameno 5
> 60126 Torrette di Ancona, Ancona
> Uff: 071 806 7743
> E-mail: stefano.sofia using regione.marche.it
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