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

Question

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!

Answer 1

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.