返回每个组的最常见字符串值

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

我想按变量分组a并返回最常用的值b

我想要的结果看起来像

  a b
1 1 B
2 2 B

在dplyr它会是这样的

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

我dplyr只提到可视化问题.

Answer 1

关键是要开始按两者分组a并b计算频率,然后只采用每组最频繁的频率a,例如:

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

当然还有其他方法,所以这只是一个可能的"关键".

Answer 2

对我有用或更简单的是：

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