R语言实现对数据框按某一列分组求组内平均值

aggregate(.~ID,data=这个数据框名字，mean) 

pcm_df$duplicated <- duplicated(paste(pcm_df$OUT_MAT_NO, pcm_df$Posit, sep = "_"), fromLast = TRUE)
pcm_df <- subset(pcm_df, !duplicated)
pcm_df$duplicated <- NULL

## S3 method for class 'data.frame'
aggregate(x, by, FUN, ..., simplify = TRUE, drop = TRUE)

type<-c("a","b","c","a","c","d","b","a","c","b")
value<-c(53,15,8,99,76,22,46,56,34,54)
df<-data.frame(type,value)
df
  type value
1   a  53
2   b  15
3   c   8
4   a  99
5   c  76
6   d  22
7   b  46
8   a  56
9   c  34
10  b  54

 aggregate(df$value, by=list(type=df$type),sum)
 type  x
1  a 208
2  b 115
3  c 118
4  d 22

aggregate(df$value, by=list(type=df$type),mean)
 type    x
1  a 69.33333
2  b 38.33333
3  c 39.33333
4  d 22.00000

aggregate(df$value, by=list(type=df$type),length)
 type x
1  a 3
2  b 3
3  c 3
4  d 1

type_2 <-c("F","M","M","F","F","M","M","F","M","M")
df <- data.frame(df, type_2)
df
  type value type_2
1   a  53   F
2   b  15   M
3   c   8   M
4   a  99   F
5   c  76   F
6   d  22   M
7   b  46   M
8   a  56   F
9   c  34   M
10  b  54   M
aggregate(x=df$value, by=list(df$type,df$type_2),sum)
 Group.1 Group.2  x
1    a    F 208
2    c    F 76
3    b    M 115
4    c    M 42
5    d    M 22