如何按组获取变量的所有最小值？

Question

我有一个数据框:

df<-data.frame(P = c("A","A","A", "B","B","B", "C", "C", "C"), 
               index = c("ind1","ind2","ind3","ind1","ind2","ind3","ind1","ind2","ind3"),
               var = c(2,1,1,8,5,4,2,8,6))

我想获得每个值的所有最小值S var和它们的相关index值P.我可以做这个:

DT <- data.table(df)
DT[  ,.SD[which.min(var)], by = P]

它只给出一个最小值var(第一个)P:

   P index  var
1: A  ind2   1
2: B  ind3   4
3: C  ind1   2

而且我想:

   P index  var
1: A  ind2   1
2: A  ind3   1
2: B  ind3   4
3: C  ind1   2

想法？

Answer 1

使用dplyr,您可以使用以下之一:

library(dplyr)
DT %>% group_by(P) %>% filter(var == min(var))  # or %in% instead of ==
#Source: local data table [4 x 3]
#Groups: P
#
#       P  index   var
#  (fctr) (fctr) (dbl)
#1      A   ind2     1
#2      A   ind3     1
#3      B   ind3     4
#4      C   ind1     2

要么

DT %>% group_by(P) %>% top_n(1, desc(var)) # top_n() returns multiple rows in case of ties
#Source: local data table [4 x 3]
#Groups: P
#
#       P  index   var
#  (fctr) (fctr) (dbl)
#1      A   ind2     1
#2      A   ind3     1
#3      B   ind3     4
#4      C   ind1     2

要么

DT %>% group_by(P) %>% filter(min_rank(var) == 1)
#Source: local data table [4 x 3]
#Groups: P
#
#       P  index   var
#  (fctr) (fctr) (dbl)
#1      A   ind2     1
#2      A   ind3     1
#3      B   ind3     4
#4      C   ind1     2