R: Find values of previous/next rows of group in data.table

Asked Nov 23 '16 at 17:36

Active Nov 24 '16 at 08:13

Viewed 354 times

I have a data.table with orders. And I want to calculate intervals between orders for each customer. So, dataset looks like:

N   order_id  user_id  order_date
1:  1         1        2016-11-18
2:  2         2        2016-11-20
3:  3         3        2016-11-21
4:  4         1        2016-11-22
5:  5         2        2016-11-23

What I need is to find a column previous_order_date which will show the date of a previous order for current customer. So my data.table will look like this:

N   order_id  user_id  order_date  previous_order_date
1:  1         1        2016-11-18  NA
2:  2         2        2016-11-20  NA
3:  3         3        2016-11-21  NA
4:  4         1        2016-11-22  2016-11-18
5:  5         2        2016-11-23  2016-11-20

Any suggestions? Thanks a lot!

edited Nov 24 '16 at 08:13

Has QUIT--Anony-Mousse

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asked Nov 23 '16 at 17:36

matvey.co

4

You can use `shift`: `dt[, past_date:=shift(order_date), by=user_id]`. – lmo Nov 23 '16 at 17:39
Thanks! That's what I was looking for. PS. I had old version of data.table, *shift()* only works from 1.9.5 – matvey.co Nov 23 '16 at 18:01

R: Find values of previous/next rows of group in data.table

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