[R] Calculating Weeks Since Last Event

[Abhinaba Roy]

Hi,

I have an input data

> dput (input)

structure(list(ScanDate = structure(c(16433, 16440, 16447, 16454,
16461, 16468, 16475, 16482, 16489, 16496, 16503, 16510, 16517,
16524, 16531, 16538, 16545, 16552, 16559, 16566, 16573, 16580,
16587, 16594, 16601, 16608, 16615, 16622), class = "Date"), OnPromotion =
c(0,
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 0,
0, 0, 1, 1, 1, 1)), .Names = c("ScanDate", "OnPromotion"), sorted =
"ScanDate", class = c("data.table",
"data.frame"), row.names = c(NA, -28L))

I am looking for an output

> dput(output)

structure(list(ScanDate = structure(c(16433, 16440, 16447, 16454,
16461, 16468, 16475, 16482, 16489, 16496, 16503, 16510, 16517,
16524, 16531, 16538, 16545, 16552, 16559, 16566, 16573, 16580,
16587, 16594, 16601, 16608, 16615, 16622), class = "Date"), OnPromotion =
c(0,
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 0,
0, 0, 1, 1, 1, 1), Weeks_Since_Last_Promo = c(0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 3, 4, 1,
1, 1)), .Names = c("ScanDate", "OnPromotion", "Weeks_Since_Last_Promo"
), sorted = "ScanDate", class = c("data.table", "data.frame"), row.names =
c(NA,
-28L))

The logic :

The data is weekly.

I want to calculate the number of weeks elapsed since the last promotion
(OnPromotion : 1 indicates promotion for that week and 0 indicates no
promotion).

As, there are no promotion initially we set the value for
'Weeks_Since_Last_Promo' to 0 (zero). The first promo occurs on
'2015-03-02' and 'Weeks_Since_Last_Promo' is still 0. Moving to
'2015-03-09' there was a promotion the week before and so 1 week elapsed
after the last promo.

If we look at '2015-06-15' then there was a promo 4 weeks back in the week
of '2015-05-18' and so 'Weeks_Since_Last_Promo' = 4.

How can we do it in R?

Thanks,
Abhinaba

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