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How to filter n consecutive days from a start date in a vector of dates across multiple years in R?

April 4, 2023

I’m stucked trying to create a code that filters 5 consecutive days from a vector of dates starting from a given start date, across multiple years.

Example:

v = seq.Date(as.Date("2000-01-01"), as.Date("2020-12-31"), 1)

initial_date = as.Date("01-01") #(mm:dd)

Expected output:

2000-01-01 #(YYYY:mm:dd)
2000-01-02
2000-01-03
2000-01-04
2000-01-05
2001-01-01
2001-01-02
2001-01-03
2001-01-04
2001-01-05
...
2020-01-01
2020-01-02
2020-01-03
2020-01-04
2020-01-05

If the initial date is, for example, 12-31 (mm:dd), the result should be:

Any tips?

>Solution :

This function converts your "mm-dd" to a Date to find the next 5 days, then uses strftime(format = "%m-%d") to compare dates while ignoring the year.

get_days <- function(dates, mmdd, n = 5) {
  wanted <- strftime(as.Date(paste0("1900-", mmdd)) + seq(n) - 1,"%m-%d")
  dates <- dates[match(wanted[[1]], strftime(dates, "%m-%d")):length(dates)]
  dates[strftime(dates, "%m-%d") %in% wanted]
}

get_days(v, "12-31")
#  [1] "2000-12-31" "2001-01-01" "2001-01-02" "2001-01-03" "2001-01-04" "2001-12-31"
#  [7] "2002-01-01" "2002-01-02" "2002-01-03" "2002-01-04" "2002-12-31" "2003-01-01"
#  ...
# [91] "2018-12-31" "2019-01-01" "2019-01-02" "2019-01-03" "2019-01-04" "2019-12-31"
# [97] "2020-01-01"

The second line using match() excludes elements of v where the "index" "mm-dd" is before the start of the vector. e.g., "2000-01-01" is excluded since "1999-12-31" isn’t in v.