r - 向量子集与列表子集

Question

给定:

actors_vector <- c("Jack Nicholson", "Shelley Duvall", "Danny Lloyd", 
                   "Scatman Crothers", "Barry Nelson")

reviews_factor <- factor(c("Good", "OK", "Good", "Perfect", 
                           "Bad", "Perfect", "Good"), 
                         levels = c("Bad", "OK", "Good", "Perfect"),
                         ordered = TRUE)

shining_list <- list(title = "The Shining", 
                     actors = actors_vector,
                     reviews = reviews_factor)

shining_list
$title
[1] "The Shining"

$actors
[1] "Jack Nicholson"   "Shelley Duvall"   "Danny Lloyd"      "Scatman Crothers"
[5] "Barry Nelson"    

$reviews
[1] Good    OK      Good    Perfect Bad     Perfect Good   
Levels: Bad < OK < Good < Perfect

$boxoffice
               US Non-US
First release  39     47
Director's cut 18     14

为什么shining_list[[3]][3]和shining_list$reviews[3]返回:

[1] Good
Levels: Bad < OK < Good < Perfect

而 shining_list[[c(3,3)]] 返回:

[1] 3

这是关于向量子集与列表子集的部分，地址:DataCamp .

Answer 1

这很可能是因为因子不是向量:

reviews <- factor(c("Good", "OK", "Good", "Perfect", "Bad", "Perfect", "Good"), 
                  levels=c("Bad", "OK", "Good", "Perfect"), ordered=TRUE)
is.vector(reviews)
## [1] FALSE

在内部，因子级别存储为整数向量，并在顶部定义了一些结构:

unclass(reviews)
## [1] 3 2 3 4 1 4 3
## attr(,"levels")
## [1] "Bad"     "OK"      "Good"    "Perfect"

在某些情况下，此结构会崩溃，您只剩下整数表示形式。我认为你的例子就是其中之一，其他几个例子是:

c(reviews[3], reviews[4])
## [1] 3 4
ifelse(TRUE, reviews[1], reviews[2])
## [1] 3