Calculating the difference between consecutive rows by group using dplyr?

Question

I have a dataframe of ids and timestamps. I'd like to calculate the difference between each sequential timestamp for an individual id.

My dataframe looks like this:

id  time
Alpha   1
Alpha   4
Alpha   7
Beta    5
Beta    10

I'm trying to add a column like time.difference below:

id  time    time.difference
Alpha   1   NA
Alpha   4   3
Alpha   7   4
Beta    5   NA
Beta    10  5

Is there a clean way to do this using dplyr? (or tidyr or something else that's easier to read than vanilla R?)

Answer 1

Like this:

dat %>% 
  group_by(id) %>% 
  mutate(time.difference = time - lag(time))

Answer 2

using data.table

library(data.table)
library(dplyr)
setDT(dat)[, time.difference := time - lag(time, 1L), by = id]