R how to identify distance of last occurrence

Question

I want to calculate how long its been since something occurred.

Given the following, you can see that the light is on some of the time, but not all of the time. I wa

Answer 1

This should do it:

d[, distance := 1:.N - 1, by = cumsum(light)]

or this:

d[, distance := .I - .I[1], by = cumsum(light)]

And if you want to actually count number of days as opposed to row-distance, you could use:

d[, distance := as.numeric(as.POSIXct(date, format = "%m/%d/%Y") -
                           as.POSIXct(date[1], format = "%m/%d/%Y"),
                           units = 'days'),
    by = cumsum(light)]

Answer 2

I would suggest creating a grouping column based on when there is a switch from FALSE to TRUE:

# create group column
d[c(light), group := cumsum(light)]
d[is.na(group), group:=0L]
d[, group := cumsum(group)]
d

Then simply tally by group, using cumsum and negating light:

d[, distance := cumsum(!light), by=group]

# remove the group column for cleanliness
d[, group := NULL]

Results:

d

         date light distance
1: 2013-06-01  TRUE        0
2: 2013-06-02 FALSE        1
3: 2013-06-03 FALSE        2
4: 2013-06-04  TRUE        0
5: 2013-06-05  TRUE        0
6: 2013-06-06 FALSE        1
7: 2013-06-07 FALSE        2
8: 2013-06-08  TRUE        0

I added a few rows

Answer 3

An approach using run length encoding (rle) and sequence (which is a wrapper for unlist(lapply(nvec, seq_len))

d[, distance := sequence(rle(light)$lengths)][(light), distance := 0]