R语言提取指定行：Sequ非NA行及其对应前序NA行

首先给出原始数据：

df <- structure(list(Utterance = c("(5.127)", ">like I don't understand< sorry like how old's your mom¿", 
                                   "(0.855)", "eh six:ty:::-one=", "(0.101)", "(0.487)", "[((v: gasps)) she said] ~no you're [not?]~", 
                                   "[((v: gasps)) she said] ~no you're [not?]~", "~<[NO YOU'RE] NOT (.) you can't go !in!>~", 
                                   "(0.260)", "show her [your boobs] next time"), 
                     Q = c(NA, "q_wh", "", "", NA, NA, "q_really", "", "", NA, NA), 
                     Sequ = c(NA, 1L, 1L, 1L, NA, NA, 0L, 0L, 0L, NA, NA)), class = "data.frame", row.names = c(NA, -11L))

需求说明 #

需要提取两类行：

• Sequ列不为NA的所有行；
• 每个连续Sequ非NA行块的前一行（该行Sequ为NA）。

最初尝试的问题 #

最初定义的函数仅能提取前序NA行和每个非NA块的第一行，无法获取全部目标行：

QA_sequ <- function(value) {
  inds <- which(!is.na(value) & lag(is.na(value)))  
  sort(unique(c(inds-1, inds)))
}

library(dplyr)
df %>% 
  slice(QA_sequ(Sequ))

输出结果仅包含部分目标行，不符合预期。

简洁解决方案 #

直接使用dplyr的逻辑筛选，一步到位：

library(dplyr)

df %>%
  filter(
    !is.na(Sequ) | 
    (is.na(Sequ) & !is.na(lead(Sequ)))
  )

逻辑说明 #

• !is.na(Sequ)：直接保留所有Sequ列非NA的行；
• (is.na(Sequ) & !is.na(lead(Sequ)))：保留那些自身Sequ为NA，但下一行Sequ非NA的行（即每个连续非NA块的前导NA行）。

执行后即可得到预期的结果：

Utterance        Q Sequ
1                                                   (5.127)     <NA>   NA
2  >like I don't understand< sorry like how old's your mom¿     q_wh    1
3                                                   (0.855)             1
4                                         eh six:ty:::-one=             1
5                                                   (0.487)     <NA>   NA
6                [((v: gasps)) she said] ~no you're [not?]~ q_really    0
7                [((v: gasps)) she said] ~no you're [not?]~             0
8                 ~<[NO YOU'RE] NOT (.) you can't go !in!>~             0

内容的提问来源于stack exchange，提问作者Chris Ruehlemann