[R] add only the 1st of May with POSIXct

[Enrico Schumann]

On Tue, 28 May 2024, Stefano Sofia writes:

> Dear R-list users,
>
> From an initial and a final date I create a sequence of days using POSIXct.
>
> If this interval covers all or only in part the months from May to October, I need to get rid of the days from the 2nd of May to the 31st of October:
>
>
> a <- as.POSIXct("2002-11-01", format = "%Y-%m-%d", tz="Etc/GMT-1")
>
> b <- as.POSIXct("2004-06-01", format = "%Y-%m-%d", tz="Etc/GMT-1")
>
> mydf <- data.frame(data_POSIX=seq(as.POSIXct(paste(format(a, "%Y-%m-%d"), "09:00:00", sep=""), format="%Y-%m-%d %H:%M:%S", tz="Etc/GMT-1"), as.POSIXct(paste(format(b, "%Y-%m-%d"), "09:00:00", sep=""), format="%Y-%m-%d %H:%M:%S", tz="Etc/GMT-1"), by="1 day"))
>
>
> If I execute
>
> as.data.frame(mydf[format(mydf$data_POSIX,"%m") %in% c("11", "12", "01", "02", "03", "04"), ])
>
> the interval will be
>
> from 2002-11-01 09:00:00 to 2003-04-30 09:00:00
>
> and from 2003-11-01 09:00:00 to 2004-04-30 09:00:00
>
>
> but I need also 2003-05-01 09:00:00 and 2004-05-01 09:00:00
>
>
> How can I solve this problem?
>
>
> Thank you for your attention and your help
>
> Stefano
>

I think this could be simplified a bit:

    a <- as.POSIXct("2002-11-01 09", format = "%Y-%m-%d %H", tz="Etc/GMT-1")
    b <- as.POSIXct("2004-06-01 09", format = "%Y-%m-%d %H", tz="Etc/GMT-1")

Create your sequence:

    S <- seq(a, b , by = "1 day")
    month.day <- format(S, "%m-%d")

Now subset S for those days that you want:

    S[month.day <= "05-01" | month.day > "10-31"]


-- 
Enrico Schumann
Lucerne, Switzerland
http://enricoschumann.net