在R中查找列表元素

Question

现在我正在使用R中的字符向量,我使用strsplit逐字分离.我想知道是否有一个函数可以用来检查整个列表,看看列表中是否有特定的单词,并且(如果可能的话)说出它所在的列表中的哪些元素.

恩.

a = c("a","b","c")
b= c("b","d","e")
c = c("a","e","f")

如果z=list(a,b,c),那么f("a",z)将最佳地屈服[1] 1 3,并且f("b",z)将最佳地屈服[1] 1 2

任何帮助都会很精彩.

Answer 1

正如alexwhan所说,grep是使用的功能.但是,请注意将其与列表一起使用.它没有做你认为它正在做的事情.例如:

grep("c", z)
[1] 1 2 3   # ?

grep(",", z)
[1] 1 2 3   # ???

幕后发生的事情是grep强迫其第二个参数使用角色as.character.当应用于列表时,as.character返回的是通过解压缩获得的该列表的字符表示.(Modulo unlist.)

as.character(z)
[1] "c(\"a\", \"b\", \"c\")" "c(\"b\", \"d\", \"e\")" "c(\"a\", \"e\", \"f\")"

cat(as.character(z))
c("a", "b", "c") c("b", "d", "e") c("a", "e", "f")

这就是grep正在努力的方向.

如果要grep在列表上运行,则使用更安全的方法lapply.这将返回另一个列表,您可以对其进行操作以提取您感兴趣的内容.

res <- lapply(z, function(ch) grep("a", ch))
res
[[1]]
[1] 1

[[2]]
integer(0)

[[3]]
[1] 1


# which vectors contain a search term
sapply(res, function(x) length(x) > 0)
[1]  TRUE FALSE  TRUE

Answer 2

比grep快得多:

sapply(x, function(y) x %in% y)

如果你想索引当然只是使用which():

which(sapply(x, function(y) x %in% y))

证据!

x = setNames(replicate(26, list(sample(LETTERS, 10, rep=T))), sapply(LETTERS, list))

head(x)

$A
 [1] "A" "M" "B" "X" "B" "J" "P" "L" "M" "L"

$B
 [1] "H" "G" "F" "R" "B" "E" "D" "I" "L" "R"

$C
 [1] "P" "R" "C" "N" "K" "E" "R" "S" "N" "P"

$D
 [1] "F" "B" "B" "Z" "E" "Y" "J" "R" "H" "P"

$E
 [1] "O" "P" "E" "X" "S" "Q" "S" "A" "H" "B"

$F
 [1] "Y" "P" "T" "T" "P" "N" "K" "P" "G" "P"

system.time(replicate(1000, grep("A", x)))

   user  system elapsed 
   0.11    0.00    0.11 

system.time(replicate(1000, sapply(x, function(y) "A" %in% y)))

   user  system elapsed 
   0.05    0.00    0.05 

Answer 3

您正在寻找grep():

grep("a", z)
#[1] 1 3

grep("b", z)
#[1] 1 2