Hi Stefano,
I read in your date-time as two separate fields for convenience. You
can split your single field at the space to get the same result.
ssdf<-read.table(text="date_POSIX time_POSIX vmax
2018-02-01 00:00:00 27
2018-02-01 00:10:00 41
2018-02-01 00:20:00 46
2018-02-01 00:30:00 39
2018-02-01 00:40:00 34
2018-02-01 00:50:00 32",
header=TRUE,stringsAsFactors=FALSE)
# get the time of day as seconds from the time field
ssdf$seconds<-as.numeric(strptime(ssdf$time_POSIX,"%H:%M:%S"))
# subtract whatever current date strptime guesses for the date
ssdf$seconds<-ssdf$seconds-min(ssdf$seconds)
# create an AM/PM variable
ssdf$ampm<-ifelse(ssdf$seconds > 0 & ssdf$seconds <= 1800,"am","pm")
means<-aggregate(vmax~ampm,ssdf,mean)
Jim
On Thu, Dec 3, 2020 at 4:55 AM Stefano Sofia
<stefano.so...@regione.marche.it> wrote:
>
> Dear list users,
> I have wind data with frequency of 10 minutes (three years data). For
> simplicity let me use only max wind speed.
> I need to reduce the frequency to 30 minutes, at 00 (taking the mean of
> data at 40, 50 and 00 minutes) and at 30 (taking the mean of data at 10, 20
> and 30 minutes) of each hour.
>
> The simple code here reported works well, but the column "interval" groups
> data forward, not backward:
>
> init_day <- as.POSIXct("2018-02-01-00-00", format="%Y-%m-%d-%H-%M",
> tz="Etc/GMT-1")
> fin_day <- as.POSIXct("2018-02-01-02-00", format="%Y-%m-%d-%H-%M",
> tz="Etc/GMT-1")
> mydf <- data.frame(data_POSIX=seq(init_day, fin_day, by="10 mins"))
> mydf$vmax <- round(rnorm(13, 35, 10))
> mydf$interval <- cut(mydf$data_POSIX, , breaks="30 min")
> means <- aggregate(vmax ~ interval, mydf, mean)
>
> data_POSIX vmax interval
> 1 2018-02-01 00:00:00 27 2018-02-01 00:00:00
> 2 2018-02-01 00:10:00 41 2018-02-01 00:00:00
> 3 2018-02-01 00:20:00 46 2018-02-01 00:00:00
> 4 2018-02-01 00:30:00 39 2018-02-01 00:30:00
> 5 2018-02-01 00:40:00 34 2018-02-01 00:30:00
> 6 2018-02-01 00:50:00 32 2018-02-01 00:30:00
> ...
>
> I should work with
>
> data_POSIX vmax interval
> 1 2018-02-01 00:00:00 27 2018-02-01 00:00:00
> 2 2018-02-01 00:10:00 41 2018-02-01 00:30:00
> 3 2018-02-01 00:20:00 46 2018-02-01 00:30:00
> 4 2018-02-01 00:30:00 39 2018-02-01 00:30:00
> 5 2018-02-01 00:40:00 34 2018-02-01 00:00:00
> 6 2018-02-01 00:50:00 32 2018-02-01 00:00:00
> ...
>
>
> Is there a way to modify this code to groupp data correctly? (I would prefer
> using only the base package)
>
> 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.so...@regione.marche.it
> ---Oo---------oO----------------
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