r - 更改级别时如何保持 NA

Question

我构建了一个包含 NA 的因子向量。

my_vec <- factor(c(NA,"a","b"),exclude=NULL)
levels(my_vec)
# [1] "a" "b" NA 

我更改其中一个级别。

levels(my_vec)[levels(my_vec) == "b"] <- "c"

NA 消失。

levels(my_vec)
# [1] "a" "c"

我怎样才能保留它？

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编辑

@rawr 提供了一个在大多数情况下都可以工作的很好的解决方案，它适用于我之前的具体示例，但不适用于我将在下面展示的那个@Hack-R 有一个使用 addNA 的实用选项，我可以让它工作，但我宁愿一个完全通用的解决方案

看到这个普遍的问题

my_vec <- factor(c(NA,"a","b1","b2"),levels = c("a",NA,"b1","b2"),exclude=NULL)
levels(my_vec)
[1] "a"  NA   "b1" "b2"
levels(my_vec)[levels(my_vec) %in% c("b1","b2")] <- "c"
levels(my_vec)
[1] "a" "c"      # NA disppeared

@rawr 的解决方案:

my_vec <- factor(c(NA,"a","b1","b2"),levels = c("a",NA,"b1","b2"),exclude=NULL)
levels(my_vec)
[1] "a"  NA   "b1" "b2"
attr(my_vec, 'levels')[levels(my_vec) %in% c("b1","b2")] <- "c"
levels(my_vec)
droplevels(my_vec)
[1] "a" NA  "c" "c" # c is duplicated

@Hack-R 的解决方案:

my_vec <- factor(c(NA,"a","b1","b2"),levels = c("a",NA,"b1","b2"),exclude=NULL)
levels(my_vec)
[1] "a"  NA   "b1" "b2"
levels(my_vec)[levels(my_vec) %in% c("b1","b2")] <- "c"
my_vec <- addNA(my_vec)
levels(my_vec)
[1] "a" "c" NA     # NA is in the end

我想要 levels(my_vec) == c("a",NA,"c")

Answer 1

您必须引用 NA，否则 R 会将其视为空值而不是因子水平。默认情况下，因子级别按字母顺序排序，但显然这并不总是有用，因此您可以通过将新的列表顺序传递给 levels()

来指定不同的顺序

require(plyr)
my_vec <- factor(c("NA","a","b1","b2"))
vec2 <- revalue(my_vec,c("b1"="c","b2"="c"))

#now reorder levels

my_vec2 <- factor(vec2, levels(vec2)[c(1,3,2)])

Levels: a NA c