Return most frequent string value for each group [duplicate]

Question

a <- c(rep(1:2,3))
b <- c("A","A","B","B","B","B")
df <- data.frame(a,b)

> str(b)
chr [1:6] "A" "A" "B" "B" "B" "B"

  a b
1 1 A
2 2 A
3 1 B
4 2 B
5 1 B
6 2 B

I want to group by variable a and return the most frequent value of b

My desired result would look like

  a b
1 1 B
2 2 B

In dplyr it would be something like

df %>% group_by(a) %>% summarize (b = most.frequent(b))

I mentioned dplyr only to visualize the problem.

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".

Answer 2

What works for me or is simpler is:

df %>% group_by(a) %>% count(b) %>% top_n(1) # includes ties

library(data.table)
DT<-as.data.table(df)
DT[ , .N, by=.(a, b)][
  order(-N), 
  .SD[ N == max(N) ]
  ,by=a]                     # includes ties

Answer 3

by() each value of a, create a table() of b and extract the names() of the largest entry in that table():

> with(df,by(b,a,function(xx)names(which.max(table(xx)))))
a: 1
[1] "B"
------------------------
a: 2
[1] "B"

You can wrap this in as.table() to get a prettier output, although it still does not exactly match your desired result:

> as.table(with(df,by(b,a,function(xx)names(which.max(table(xx))))))
a
1 2 
B B