[R] evaluate the daily mean with date in "POSIXct" "POSIXt" class

[Jim Lemon]

Hi Stefano,
I might be missing something, but try this:

MteBove<-read.table(text="posix_date  posix_time  snow    temp
 2010-01-19 23:30:00 45 NA
 2010-01-20 00:30:00 10 2.7
 2010-01-20 03:00:00 45 NA
 2010-01-20 03:30:00 44 NA
 2010-01-20 04:00:00 44 NA
 2010-01-20 04:30:00 44 NA
 2010-01-20 05:00:00 44 NA
 2010-01-20 05:30:00 45 NA
 2010-01-20 06:00:00 44 NA
 2010-01-20 06:30:00 45 NA
 2010-01-20 23:30:00 45 NA
 2010-01-21 00:30:00 10 2.7
 2010-01-21 03:00:00 45 NA
 2010-01-21 03:30:00 44 NA
 2010-01-21 04:00:00 44 NA
 2010-01-21 04:30:00 44 NA
 2010-01-21 05:00:00 44 NA
 2010-01-21 05:30:00 45 NA
 2010-01-21 06:00:00 44 NA
 2010-01-21 06:30:00 45 NA",
 header=TRUE)
MteBove$day<-as.Date(MteBove$posix_date,"%Y-%m-%d")
by(MteBove$snow,MteBove$day,mean)

and if you have NA values, add na.rm=TRUE to the last line.

Jim


On Tue, Jun 7, 2016 at 4:55 PM, Stefano Sofia
<stefano.sofia at regione.marche.it> wrote:
> Dear R-list users,
> I have the following data frame, called MteBove:
>
> posix_date    snow    temp
> ...
> 2010-01-20 23:30:00 45 NA
> 2010-01-02 00:30:00 10-2.7
> 2010-01-20 03:00:00 45 NA
> 2010-01-20 03:30:00 44 NA
> 2010-01-20 04:00:00 44 NA
> 2010-01-20 04:30:00 44 NA
> 2010-01-20 05:00:00 44 NA
> 2010-01-20 05:30:00 45 NA
> 2010-01-20 06:00:00 44 NA
> 2010-01-20 06:30:00 45 NA
> ...
>
>> sapply(MteBove, class)
> gives
> $posix_date
> [1] "POSIXct" "POSIXt"
>
> $snow
> [1] "numeric"
>
> $temp
> [1] "numeric"
>
> There are semi-hourly data (i.e. 48 data each day) from the first of November to the first of May of each Winter season, and I need the daily mean for snow and temp.
> First I created a subset for each Winter season
> I tried to create a new column with only the day, like
>
>> MteBove$day <- trunc(MteBove$posix_date, "days")
>
> and then
>
>> list_days <- unique(MteBove$day)
>> means <- rapply(list_days, function(x) mean(MteBove[MteBove$day == x, ], na.rm=T))
>
> with the following result:
> [1] "means IS"
>   sec   min  hour  mday   mon  year  wday  yday isdst
>    NA    NA    NA    NA    NA    NA    NA    NA    NA
>
> I tried some changes always in this direction, with no success.
> Is there an efficient way to do that?
> Could somebody give me an hint about it?
>
> Thank you for your attention and your help
> Stefano Sofia
>
>
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