Calculating time difference by ID

Question

I have data like this:

Incident.ID.. = c(rep(\"INCFI0000029582\",4), rep(\"INCFI0000029587\",4))
date = c(\"2014-09-25 08:39:45\", \"2014-09-25 08:39:48\", \         


        

Answer 1

With base R you could simply wrap it up in ave

ave(as.numeric(as.POSIXct(date)), Incident.ID.., FUN = padded.diff) 

Or using data.table (as per @akruns comment)

library(data.table) 
setDT(df)[, date.diff := padded.diff(as.POSIXct(date)), by = Incident.ID..]

Answer 2

Here is an example using dplyr and lubridate

library(dplyr)
library(lubridate)
df %>%
    group_by(Incident.ID..) %>%
    mutate(diff = c(0, diff(ymd_hms(date))))

Source: local data frame [8 x 3]
Groups: Incident.ID..

    Incident.ID..                date    diff
1 INCFI0000029582 2014-09-25 08:39:45       0
2 INCFI0000029582 2014-09-25 08:39:48       3
3 INCFI0000029582 2014-09-25 08:40:44      56
4 INCFI0000029582 2014-10-10 23:04:00 1347796
5 INCFI0000029587 2014-09-25 08:33:32       0
6 INCFI0000029587 2014-09-25 08:34:41      69
7 INCFI0000029587 2014-09-25 08:35:24      43
8 INCFI0000029587 2014-10-10 23:04:00 1348116