lead 或 lag 函数以获得多个值，而不仅仅是第 n 个值

Question

我有一个小标题，每行都有一个单词列表。我想从搜索关键字的函数创建一个新变量，如果找到关键字，则创建一个由关键字 plus-and-minus 3 个单词组成的字符串。

下面的代码是 close，但是，它没有抓取关键字前后的 所有三个 词，而是抓取单个词3 ahead/behind.

df <- tibble(words = c("it", "was", "the", "best", "of", "times", 
                       "it", "was", "the", "worst", "of", "times"))
df <- df %>% mutate(chunks = ifelse(words=="times", 
                                    paste(lag(words, 3), 
                                          words, 
                                          lead(words, 3), sep = " "),
                                    NA))

最直观的解决方案是 lag 函数是否可以执行如下操作：lead(words, 1:3) 但这不起作用。

显然我可以很快地手动完成此操作 (paste(lead(words,3), lead(words,2), lead(words,1),...lag(words,3))，但我最终实际上希望能够获取关键字 plus-and-minus 50 字——太多 hand-code。

如果 tidyverse 中存在解决方案将是理想的，但任何解决方案都会有所帮助。任何帮助将不胜感激。

Answer 1

一个选项是 sapply:

library(dplyr)

df %>%
  mutate(
    chunks = ifelse(
      words == "times",
      sapply(
        1:nrow(.),
        function(x) paste(words[pmax(1, x - 3):pmin(x + 3, nrow(.))], collapse = " ")
        ),
      NA
      )
  )

输出：

# A tibble: 12 x 2
   words chunks                      
   <chr> <chr>                       
 1 it    NA                          
 2 was   NA                          
 3 the   NA                          
 4 best  NA                          
 5 of    NA                          
 6 times the best of times it was the
 7 it    NA                          
 8 was   NA                          
 9 the   NA                          
10 worst NA                          
11 of    NA                          
12 times the worst of times

虽然不是明确的 lead 或 lag 功能，但它通常也可以达到目的。

Answer 2

类似于@arg0naut 但没有 dplyr:

r  = 1:nrow(df)
w  = which(df$words == "times")
wm = lapply(w, function(wi) intersect(r, seq(wi-3L, wi+3L)))

df$chunks <- NA_character_
df$chunks[w] <- tapply(df$words[unlist(wm)], rep(w, lengths(wm)), FUN = paste, collapse=" ")

# A tibble: 12 x 2
   words chunks                      
   <chr> <chr>                       
 1 it    <NA>                        
 2 was   <NA>                        
 3 the   <NA>                        
 4 best  <NA>                        
 5 of    <NA>                        
 6 times the best of times it was the
 7 it    <NA>                        
 8 was   <NA>                        
 9 the   <NA>                        
10 worst <NA>                        
11 of    <NA>                        
12 times the worst of times

data.table翻译：

library(data.table)
DT = data.table(df)

w = DT["times", on="words", which=TRUE]
wm = lapply(w, function(wi) intersect(r, seq(wi-3L, wi+3L)))

DT[w, chunks := DT[unlist(wm), paste(words, collapse=" "), by=rep(w, lengths(wm))]$V1]

Answer 3

data.table::shift 接受 n（滞后）参数的向量并输出一个列表，因此您可以将其与 do.call(paste 列表元素一起使用。但是，除非您使用的是 data.table 版本 >= 1.12，否则我认为它不会让您混合正负 n 值（如下所示）。

有数据table:

library(data.table)
setDT(df)

df[, chunks := trimws(ifelse(words != "times", NA, do.call(paste, shift(words, 3:-3, ''))))]

#     words                       chunks
#  1:    it                         <NA>
#  2:   was                         <NA>
#  3:   the                         <NA>
#  4:  best                         <NA>
#  5:    of                         <NA>
#  6: times the best of times it was the
#  7:    it                         <NA>
#  8:   was                         <NA>
#  9:   the                         <NA>
# 10: worst                         <NA>
# 11:    of                         <NA>
# 12: times           the worst of times

使用 dplyr 且仅将 data.table 用于 shift 函数：

library(dplyr)

df %>% 
  mutate(chunks = do.call(paste, data.table::shift(words, 3:-3, fill = '')),
         chunks = trimws(ifelse(words != "times", NA, chunks)))

# # A tibble: 12 x 2
#    words chunks                      
#    <chr> <chr>                       
#  1 it    NA                          
#  2 was   NA                          
#  3 the   NA                          
#  4 best  NA                          
#  5 of    NA                          
#  6 times the best of times it was the
#  7 it    NA                          
#  8 was   NA                          
#  9 the   NA                          
# 10 worst NA                          
# 11 of    NA                          
# 12 times the worst of times

Answer 4

这是使用 lag 和 lead

的另一个 tidyverse 解决方案

laglead_f <- function(what, range)
    setNames(paste(what, "(., ", range, ", default = '')"), paste(what, range))

df %>%
    mutate_at(vars(words), funs_(c(laglead_f("lag", 3:0), laglead_f("lead", 1:3)))) %>%
    unite(chunks, -words, sep = " ") %>%
    mutate(chunks = ifelse(words == "times", trimws(chunks), NA))
## A tibble: 12 x 2
#   words chunks
#   <chr> <chr>
# 1 it    NA
# 2 was   NA
# 3 the   NA
# 4 best  NA
# 5 of    NA
# 6 times the best of times it was the
# 7 it    NA
# 8 was   NA
# 9 the   NA
#10 worst NA
#11 of    NA
#12 times the worst of times

想法是将来自三个 lagged 和 leading 向量的值存储在具有 mutate_at 和命名函数的新列中，unite 这些列和然后根据您的条件过滤条目 words == "times".

lead 或 lag 函数以获得多个值，而不仅仅是第 n 个值

lead or lag function to get several values, not just the nth

r

lag

lead

dplyr