如何使用dplyr在行组之间进行划分？ [英] How to divide between groups of rows using dplyr?

问题描述

我有这个数据框：

x <- data.frame(
    name = rep(letters[1:4], each = 2),
    condition = rep(c("A", "B"), times = 4),
    value = c(2,10,4,20,8,40,20,100)
) 
#   name condition value
# 1    a         A     2
# 2    a         B    10
# 3    b         A     4
# 4    b         B    20
# 5    c         A     8
# 6    c         B    40
# 7    d         A    20
# 8    d         B   100

我想按名称分组并用 condition == B 与那些具有 condition == A 的人一起获得：

I want to group by name and divide the value of rows with condition == "B" with those with condition == "A", to get this:

data.frame(
    name = letters[1:4],
    value = c(5,5,5,5)
)
#   name value
# 1    a     5
# 2    b     5
# 3    c     5
# 4    d     5

我知道类似的东西可以使我更加接近：

I know something like this can get me pretty close:

x$value[which(x$condition == "B")]/x$value[which(x$condition == "A")]

，但是我想知道是否有一种简单的方法可以使用dplyr（我的数据框是一个玩具示例，我通过链接多个 group_by 和摘要调用）。

but I was wondering if there was an easy way to do this with dplyr (My dataframe is a toy example and I got to it by chaining multiple group_by and summarise calls).

推荐答案

尝试：

x %>% 
  group_by(name) %>%
  summarise(value = value[condition == "B"] / value[condition == "A"])

其中给出：

#Source: local data frame [4 x 2]
#
#    name value
#  (fctr) (dbl)
#1      a     5
#2      b     5
#3      c     5
#4      d     5

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