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

Jeff Newmiller jdnewm|| @end|ng |rom dcn@d@v|@@c@@u@
Mon Sep 21 19:06:52 CEST 2020

The base R as.difftime function is perfectly usable to create this offset without pulling in lubridate.

On September 21, 2020 8:06:51 AM PDT, Eric Berger <ericjberger using gmail.com> wrote:
>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.
>day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M",
>day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M",
>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$data_POSIXminus9,"%y-%m-%d")
>df2X <- aggregate(diff ~ dayX, df1, sum)
>On Mon, Sep 21, 2020 at 5:30 PM Stefano Sofia
><stefano.sofia using regione.marche.it> wrote:
>> Dear R-list members,
>> I have semi-hourly snowfall data.
>> I should sum the semi-hourly increments (only the positive ones, but
>this is not described in my example) day by day, not from 00 to 24 but
>from 9 to 9.
>> I am able to use the diff function, create a list of days and use the
>function aggregate, but it works only from 0 to 24. Any suggestion for
>an efficient way to do it?
>> Here my code:
>> day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M",
>> day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M",
>> df1 <- data.frame(data_POSIX=seq(day_1, day_2, by="30 min"))
>> df1$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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Sent from my phone. Please excuse my brevity.

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