I have a set of data with precipitation totals (Precip) for every day. I collected data at multiple pools. I need to calculate the number of days it has been since the last time it raied >12mm. If it rained over 12mm that day then it get a ‘0’.

## creates example dataframe
Pool <- c("A","A","A","A","A","A","A","A","A","A",
        "B","B","B","B","B","B","B","B","B","B")
DATE <- as.Date(c("2005-01-01","2005-01-02","2005-01-03","2005-01-04","2005-01-05",
                        "2005-01-06","2005-01-07","2005-01-08","2005-01-09","2005-01-10",
                        "2005-01-01","2005-01-02","2005-01-03","2005-01-04","2005-01-05",
                        "2005-01-06","2005-01-07","2005-01-08","2005-01-09","2005-01-10"))
Precip <- c(0,0,3,18,4,3,13,8,3,0,13,0,3,13,0,3,10,8,13,0))

df <- data.frame(Pool, DATE, Precip)

I need the following dataframe:

   Pool       DATE Precip  Days_since12
1     A 2005-01-01      0     NA
2     A 2005-01-02      0     NA
3     A 2005-01-03      3     NA
4     A 2005-01-04     18     NA
5     A 2005-01-05      4     1
6     A 2005-01-06      3     2
7     A 2005-01-07     13     0
8     A 2005-01-08      8     1
9     A 2005-01-09      3     2
10    A 2005-01-10      0     3
11    B 2005-01-01     13     0
12    B 2005-01-02      0     1
13    B 2005-01-03      3     2
14    B 2005-01-04     13     0
15    B 2005-01-05      0     1
16    B 2005-01-06      3     2
17    B 2005-01-07     10     3
18    B 2005-01-08      8     4
19    B 2005-01-09     13     0
20    B 2005-01-10      0     1
 

I can easily add a column to indicate if the precip was over 12 or not:

df2 <- df %>%
  group_by(Pool) %>%
  mutate(pre12=ifelse(Precip>12,1,0))

But I am then not sure how to calculate the number of days between the DATE and the previous date when pre12==1