在R中查找列表元素 [英] Finding Elements of Lists in R

问题描述

现在，我正在使用R中的字符向量，我使用strsplit逐词分隔.我想知道是否有一个函数可以用来检查整个列表，查看列表中是否有特定单词，并(如果可能)说出列表中的哪个元素.

Right now I'm working with a character vector in R, that i use strsplit to separate word by word. I'm wondering if there's a function that I can use to check the whole list, and see if a specific word is in the list, and (if possible) say which elements of the list it is in.

例如.

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

如果为z=list(a,b,c)，则f("a",z)最佳生成[1] 1 3，而f("b",z)最佳生成[1] 1 2

If z=list(a,b,c), then f("a",z) would optimally yield [1] 1 3, and f("b",z) would optimally yield [1] 1 2

任何帮助都会很棒.

推荐答案

正如alexwhan所说，grep是要使用的函数.但是，请小心将其与列表一起使用.它没有做您可能认为正在做的事情.例如:

As alexwhan says, grep is the function to use. However, be careful about using it with a list. It isn't doing what you might think it's doing. For example:

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

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

幕后发生的事情是grep使用as.character将其第二个参数强制转换为character.当应用于列表时，as.character返回的是该列表通过解析得到的字符表示形式. (取消公开列表的模版.)

What's happening behind the scenes is that grep coerces its 2nd argument to character, using as.character. When applied to a list, what as.character returns is the character representation of that list as obtained by deparsing it. (Modulo an 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")

这是grep正在处理的.

如果要在列表上运行grep，更安全的方法是使用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.

If you want to run grep on a list, a safer method is to use lapply. This returns another list, which you can operate on to extract what you're interested in.

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

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