Find number of days between observations in R

Question

I have a dataset in time series (an observation per day). What I would like to do is find the number of days until an observation is above zero for each day in the dataset.

A sample of the dataset looks like this:

Date        Runoff
01/01/1980  0
02/01/1980  0
03/01/1980  0
04/01/1980  0
05/01/1980  4.5
06/01/1980  2
07/01/1980  0
08/01/1980  0
09/01/1980  0
10/01/1980  0
11/01/1980  0
12/01/1980  0
13/01/1980  1.2
14/01/1980  0 
15/01/1980  0
16/01/1980  0
17/01/1980  0
18/01/1980  0.8

And what I would like to get to is this:

Date        Runoff   No_Days
01/01/1980  0        4
02/01/1980  0        3
03/01/1980  0        2
04/01/1980  0        1
05/01/1980  4.5      0
06/01/1980  2        0
07/01/1980  0        6
08/01/1980  0        5
09/01/1980  0        4
10/01/1980  0        3
11/01/1980  0        2
12/01/1980  0        1  
13/01/1980  1.2      0
14/01/1980  0        4      
15/01/1980  0        3
16/01/1980  0        2
17/01/1980  0        1
18/01/1980  0.8      0

So as you can see if there is runoff on a particular date then there are 0 days until runoff, if there is a runoff event the next day then there is 1 day until runoff etc.

I've been using R for a while and this is the first time I haven't been able to find a solution hence this is my first R related question on here so please go easy on me!

If I've missed anything don't hesitate to let it be known.

Thank you,

J

Answer 1

I was also thinking of rle and seq_len:

df$No_Days <- unlist(sapply(rle(df$Runoff)$lengths, 
                            function(x) 
                              if (x>1) 
                                rev(seq_len(x)) 
                              else 0))

Answer 2

Here's a more R way. I am using rle and a bit of hack

dates.df$No_Days <-unlist(lapply(rle(dates.df$Runoff)$lengths,function(x) rev(seq(x:1))))
#now the hackish part. For those runoffs that are greater than zero
dates.df$No_Days <-ifelse(dates.df$Runoff>0,0,dates.df$No_Days)