Return most frequent string value for each group [duplicate]

Answer 1

The key is to start grouping by both a and b to compute the frequencies and then take only the most frequent per group of a, for example like this:

df %>% 
  count(a, b) %>%
  slice(which.max(n))

Source: local data frame [2 x 3]
Groups: a

  a b n
1 1 B 2
2 2 B 2

Of course there are other approaches, so this is only one possible "key".