group_by() into fill() not working as expected

Question

I'm trying to do a Last Observation Carried Forward operation on some poorly formatted data using dplyr and tidyr. It isn't working as I'd expect.

library(dplyr)
library(tidyr)

df <- data.frame(id=c(1,1,2,2,3,3),
                 email=c('[email protected]', NA, '[email protected]', NA, NA, NA))
df2 <- df %>% group_by(id) %>% fill(email)

This results in:

Source: local data frame [6 x 2]
Groups: id [3]

     id         email
  (dbl)        (fctr)
1     1 [email protected]
2     1 [email protected]
3     2 [email protected]
4     2 [email protected]
5     3 [email protected]
6     3 [email protected]

I expect it to be:

Source: local data frame [6 x 2]
Groups: id [3]

     id         email
  (dbl)        (fctr)
1     1 [email protected]
2     1 [email protected]
3     2 [email protected]
4     2 [email protected]
5     3 NA
6     3 NA

The reason I expect it to be the latter is because of group_by's documentation saying, "The group_by function takes an existing tbl and converts it into a grouped tbl where operations are performed "by group"." The group in this case is determined by the id variable, and the following operation is fill(email). However, it's pretty clearly NOT doing that.

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And before anybody asks, it makes no difference if the fields are both character instead of numeric or factor.

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UPDATE @aosmith pointed out this open issue on Github. I'm going to say that there won't be a proper solution to this problem until that issue is resolved. Everything else would just be a workaround. So, if somebody makes a successful PR addressing that issue and posts it here, I'd be happy to mark it as the solution.

Answer 1

Luckily you can still use zoo::na.locf for this:

df %>% 
    group_by(id) %>% 
    mutate(email = zoo::na.locf(email, na.rm = FALSE))  
# Source: local data frame [6 x 2]
# Groups: id [3]
# 
#      id         email
#   (dbl)        (fctr)
# 1     1 [email protected]
# 2     1 [email protected]
# 3     2 [email protected]
# 4     2 [email protected]
# 5     3            NA
# 6     3            NA

Answer 2

Looks like this has been fixed in the development version of tidyr. You now get the expected result per id using fill from tidyr_0.3.1.9000.

df %>% group_by(id) %>% fill(email)

Source: local data frame [6 x 2]
Groups: id [3]

     id         email
  (dbl)        (fctr)
1     1 [email protected]
2     1 [email protected]
3     2 [email protected]
4     2 [email protected]
5     3            NA
6     3            NA