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

[Rui Barradas]

Às 16:23 de 28/05/2024, Stefano Sofia escreveu:
> 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
> 
> 
> 
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Hello,

First of all, 'a' and 'b' are already objects of class "POSIXct", you 
don't need to repeat the code creating them when creating mydf.

As for the question, see the code below.


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(a, b, by = "1 day"))

# get the years from the data
years <- format(c(a, b), "%Y") |> as.integer()
# this creates a sequence with all the years
years <- Reduce(`:`, years)

# coerce to "Date"
from <- ISOdate(years, 5L, 2L, tz = "Etc/GMT-1")
to <- ISOdate(years, 10L, 30L, tz = "Etc/GMT-1")

# this logical index keeps only the dates between May, 2nd and Nov 1st.
keep <- data.frame(from, to) |>
   apply(1L, \(x) x[1L] <= mydf$data_POSIX & mydf$data_POSIX <= x[2L]) |>
   rowSums() > 0L

mydf[keep, , drop = FALSE]



Hope this helps,

Rui Barradas


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