Counting sequential dates in R to determine the length of an event

Question

I have a dataframe containing dates when a given event occurred. Some events go on for several days, and I want to summarise each event based on its start date and its total length (in days).

I want to go from this:

Date
2020-01-01
2020-01-02
2020-01-03
2020-01-15
2020-01-20
2020-01-21

To this:

StartDate	EventLength
2020-01-01	3
2020-01-15	1
2020-01-20	2

I've tried various approaches with aggregate, ave, seq_along and lag, but I haven't managed to get a count of event length that resets when the dates aren't sequential.

Code for the example data frame in case it's helpful:

Date <- c("2020-01-01", "2020-01-02", "2020-01-03", "2020-01-15", "2020-01-20", "2020-01-21")
df <- data.frame(Date)
df$Date <- as.Date(df$Date, origin = "1970-01-01") 

Answer 1

You can split by cumsum(c(0, diff(df$Date) != 1) and then take the first date and combine it with the length assuming the dates are sorted.

do.call(rbind, lapply(split(df$Date, cumsum(c(0, diff(df$Date) != 1))),
  function(x) data.frame(StartDate=x[1], EventLength=length(x))))
#   StartDate EventLength
#0 2020-01-01           3
#1 2020-01-15           1
#2 2020-01-20           2

or another option using rle:

i <- cumsum(c(0, diff(df$Date) != 1))
data.frame(StartDate = df$Date[c(1, diff(i)) == 1], EventLength=rle(i)$lengths)
#   StartDate EventLength
#1 2020-01-01           3
#2 2020-01-15           1
#3 2020-01-20           2