dplyr：在 mutate 命令中使用过滤器 group_by [重复]

Question

我想在数据表中添加一列，其中包含 y 的每个值除以 x（1 或 2）中相应条件的平均值，其中 x2 = 1。对于以下 x = 1 的数据，y 应该是除以 1.4，其中 x=2 y 应除以 1。

dt1 <- data.table(x=c("1","1","1","1","1","1","1","1","1","1","2","2","2","2","2","2","2","2","2","2"),
       x2=c("1","1","2","2","2","2","3","3","3","3","1","1","2","2","2","2","3","3","3","3"), 
       y=c(1.41,1.39,1.9,2.1,0.9,1.1,3.1,2.9,3.9,4.1,0.9,1.1,1.9,2.1,0.9,1.1,3.1,2.9,3.9,4.1))

我可以将 x * x2 = 1 的平均值写入一个新文件。

mean <- dt1 %>% filter(x2==1) %>% group_by(x) %>% summarise(mean(y))

但我不知道如何网格化指示命令调用正确的值。 dt1 %>% mutate(z = y/a reference to 'mean')

我想创建一个新列，其中填充了我想要除以的值，但我再次无法弄清楚如何从命令中调用分组条件。

t <- dt1 %>% mutate(T=ifelse(x==1,(filter(x2==1) %>% group_by(x=1) %>%
     summarise(mean(y))),ifelse(x==1,(filter(x2==2) %>% group_by(x=2) %>% 
     summarise(mean(y))),NA)

我并没有只使用 dplyr，但我最近一直在使用它。我愿意接受最简单的解决方案。

Answer 1

试试

  left_join(dt1,
            dt1 %>% 
                 filter(x2==1) %>%
                 group_by(x) %>%
                 summarise(a=mean(y)), by='x') %>%
                 mutate(z=y/a)%>%
                 head()

  #  x x2    y   a         z
  #1 1  1 1.41 1.4 1.0071429
  #2 1  1 1.39 1.4 0.9928571
  #3 1  2 1.90 1.4 1.3571429
  #4 1  2 2.10 1.4 1.5000000
  #5 1  2 0.90 1.4 0.6428571
  #6 1  2 1.10 1.4 0.7857143

或使用data.table

library(data.table)
dt2 <- dt1[x2==1,list(a=mean(y)) , by=x]
setkey(dt1, x)
res <- dt1[dt2][,z:=y/a]
head(res)
#   x x2    y   a         z
#1: 1  1 1.41 1.4 1.0071429
#2: 1  1 1.39 1.4 0.9928571
#3: 1  2 1.90 1.4 1.3571429
#4: 1  2 2.10 1.4 1.5000000
#5: 1  2 0.90 1.4 0.6428571
#6: 1  2 1.10 1.4 0.7857143

更新

@aosmith 建议的dplyr 更紧凑的选项是

  dt1 %>%
      group_by(x) %>% 
      mutate(a=mean(y[x2==1]), z=y/a)