Count occurrence of IDs within the last x days in R

Question

This is my first post on stackoverflow, so please forgive me if my post is not detailed enough.

I have a data table with two columns (date and group ID). At the current date, I want to count the number of group occurrences that have occurred within the last x days. For my example below, we can say the last 30 days.

date = c("2014-04-01", "2014-04-12", "2014-04-07", "2014-05-03", "2014-04-14", "2014-05-04", "2014-03-31", "2014-04-18", "2014-04-23", "2014-04-01")
group = c("G","G","F","G","E","E","H","H","H","A")
dt = data.table(cbind(group,date))

   group       date
1:     G 2014-04-01
2:     G 2014-04-12
3:     F 2014-04-07
4:     G 2014-05-03
5:     E 2014-04-14              
6:     E 2014-05-04
7:     H 2014-03-31
8:     H 2014-04-18
9:     H 2014-04-23
10:    A 2014-04-01

So, my desired new column would look like this:

   group       date   count
1:     G 2014-04-01       0
2:     G 2014-04-12       1
3:     F 2014-04-07       0
4:     G 2014-05-03       1 (not including first G since it is outside 30 days)      
5:     E 2014-04-14       0       
6:     E 2014-05-04       1
7:     H 2014-03-31       0
8:     H 2014-04-18       1
9:     H 2014-04-23       2
10:    A 2014-04-01       0 

I was able to use dplyr to perform a non-window count on counting the occurrences of the group at the current date, but I am struggling to find a way to do a 30 day count. For the non-window count, I did the following:

dt = data.table(dt %>%
 group_by(group) %>%
 mutate(count = row_number() - 1))

    group       date count
 1:     G 2014-04-01     0
 2:     G 2014-04-12     1
 3:     F 2014-04-07     0
 4:     G 2014-05-03     2
 5:     E 2014-04-14     0
 6:     E 2014-05-04     1
 7:     H 2014-03-31     0
 8:     H 2014-04-18     1
 9:     H 2014-04-23     2
10:     A 2014-04-01     0

This is a small sample of the dataset, where the entire dataset contain a few million rows, so I would need something efficient. Any tips or suggestions would be greatly appreciated. Thank you in advance!

Answer 1

A data.table option

dt[, date := as.Date(date)][, count := cumsum(date <= first(date) + 30) - 1, group]

gives

> dt
    group       date count
 1:     G 2014-04-01     0
 2:     G 2014-04-12     1
 3:     F 2014-04-07     0
 4:     G 2014-05-03     1
 5:     E 2014-04-14     0
 6:     E 2014-05-04     1
 7:     H 2014-03-31     0
 8:     H 2014-04-18     1
 9:     H 2014-04-23     2
10:     A 2014-04-01     0

---

A dplyr option following similar idea

dt %>%
  mutate(date = as.Date(date)) %>%
  group_by(group) %>%
  mutate(count = cumsum(date <= first(date) + 30) - 1) %>%
  ungroup()