Dplyr : how to find the first-non missing string by groups?

dplyr, r

Solution

Summarise will give one entry per group, here, finding the first non-missing using `which`

data %>%
  group_by(group) %>%
  summarise(first_non_missing = names[which(!is.na(names))[1]])

gives

  group first_non_missing
  <chr>             <chr>
1     A              fred
2     B              josh

If you still want all of the rows, replace `summarise` with `mutate`.

Problem

Consider the following simple example ``` group <-c('A','A','A','B','B','B','B') names<- c(NA,'fred',NA,'josh','josh',NA,NA) data=data_frame(group,names) > data # A tibble: 7 × 2 group names <chr> <chr> 1 A <NA> 2 A fred 3 A <NA> 4 B josh 5 B josh 6 B <NA> 7 B <NA> ``` Here, I would like to get, for each `group` the first non missing name in `names`. How can I do that? The solution below using `coalesce` and `first` fail. ``` data %>% group_by(group) %>% mutate(first_non_missing = first(names), first_non_missing_alt = coalesce(names)) %>% ungroup() # A tibble: 7 × 4 group names first_non_missing first_non_missing_alt <chr> <chr> <chr> <chr> 1 A <NA> <NA> <NA> 2 A fred <NA> fred 3 A <NA> <NA> <NA> 4 B josh josh josh 5 B josh josh josh 6 B <NA> josh <NA> 7 B <NA> josh <NA> ``` Indeed, for group `A`, `first_non_missing` should be `fred` for all three observations.. Many thanks!

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