如何在R语言DataFrame中提取含error行对应的前置ID行

首先修正你的DataFrame定义，date列的括号要改成c()，否则R会报错：

df <- data.frame(
  date = c(1,1,1,1,rep(2, 36)),
  action = c("ID=1", "foo","bah", "error",
            "ID=2", "foo","bah", "success",
            "ID=3", "foo","bah", "error",
            "ID=4", "foo","bah", "success",
            "ID=5", "foo","bah", "success",
            "ID=6", "foo","bah", "error",
            "ID=7", "foo","bah", "error",
            "ID=8", "foo","bah", "success",
            "ID=9", "foo","bah", "success",
            "ID=10", "foo","bah", "success"
            )
)

你的现有代码逻辑错误：过滤出action为error的行后，无论怎么切片，得到的都是error本身的行，而不是对应的ID行。下面提供两种可行方案：

方案一：分组筛选（dplyr） #

利用每组4行的规律，先分组，再筛选出组内最后一行是error的组，取组内第一行（即ID行）：

library(dplyr)

df %>%
  mutate(group_id = rep(1:(nrow(.)/4), each = 4)) %>%
  group_by(group_id) %>%
  filter(last(action) == "error") %>%
  slice(1) %>%
  ungroup() %>%
  select(date, action)

方案二：索引定位 #

直接找到所有error行的位置，减去3得到对应ID行的索引，再提取数据：

error_rows <- which(df$action == "error")
result <- df[error_rows - 3, c("date", "action")]

两种方案都能得到你想要的结果：

date action
1    1   ID=1
2    2   ID=3
3    2   ID=6
4    2   ID=7

内容的提问来源于stack exchange，提问作者Anthony W