[R] help needed on data management

[Alfred Xuan]

Hi, 

I am quite new in R, but want to try the
following data aggregation in R. The data has
three columns, and the fourth is what i am trying
to get. I created this in SAS, and would like to
learn to do it in R.

Here is some snapshot of the dataset. I have a
list of different users, and on different dates
(sorted), they had played several rounds of
plays. Date here is coded ascendingly. On each
date per user, I want to sum the total number of
plays for a span of 3 days counting backward. For
example, on the date of 16528 for user 1, the
back3 should the sum of 9+3+8=20. For those dates
that can only go back for less than 3 days, sum
the number of plays on available dates. For
example, on date 16527 for userid 1, there are
only 2 days, so the sum is 3+8=11.

Userid	plays	date	back3
1	8	16526	8
1	3	16527	11
1	9	16528	20
1	4	16529	16
1	3	16531	16
1	12	16532	19
1	3	16533	18
1	16	16534	31
1	6	16535	25
1	16	16536	38
1	10	16537	32
2	30	16528	30
2	16	16529	46
2	15	16530	61
2	11	16531	42
2	9	16532	35
2	11	16533	31
2	24	16534	44
2	21	16535	56
2	37	16536	82
2	6	16537	64
2	58	16538	101
2	34	16539	98
2	19	16540	111
2	18	16541	71
2	23	16542	60
2	35	16543	76
2	10	16544	68
2	7	16545	52
2	1	16546	18
2	15	16547	23
2	10	16548	26
2	12	16549	37
2	20	16550	42
2	18	16551	50
2	12	16552	50
2	8	16553	38
2	8	16554	28
2	32	16555	48
2	12	16556	52
2	13	16557	57
2	25	16610	50
3	1	16574	1
3	1	16576	2
3	1	16577	3
3	1	16578	3
3	2	16587	4
3	2	16588	5
3	1	16590	5
3	4	16602	7
3	1	16603	6
3	1	16624	6
3	6	16625	8
3	3	16626	10
3	4	16653	13
3	3	16654	10

Any help is appreciated.
Alfred


 
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