用 NA 索引整数向量 [英] Indexing integer vector with NA
问题描述
我在理解这一点时遇到了问题.我有一个长度为 5 的整数向量:
I have problems understanding this. I have an integer vector of length 5:
x <- 1:5
如果我用单个 NA
索引它,结果长度为 5:
If I index it with a single NA
, the result is of length 5:
x[NA]
# [1] NA NA NA NA NA
我的第一个想法是 R 检查 1-5 是否为 NA
但
My first idea was that R checks whether 1-5 is NA
but
x <- c(NA, 2, 4)
x[NA]
# NA NA NA.
所以这不是解决方案.我的第二种方法是 x[NA]
正在编制索引,但我不明白
So this cannot be the solution. My second approach is that x[NA]
is indexing but then I do not understand
- 为什么这给了我五个
NA
- NA 作为索引的含义.
x[1]
给你第一个值,但是x[NA]
的结果应该是什么?
- Why this gives me five
NA's
- What NA as an index means.
x[1]
gives you the first value but what should be the result ofx[NA]
?
推荐答案
比较你的代码:
> x <- 1:5; x[NA]
[1] NA NA NA NA NA
与
> x <- 1:5; x[NA_integer_]
[1] NA
在第一种情况下,NA
是逻辑类型(class(NA)
显示),而在第二种情况下,它是一个整数.从?"["
可以看出,在i
为逻辑的情况下,循环到x
的长度:
In the first case, NA
is of type logical (class(NA)
shows), whereas in the second it's an integer. From ?"["
you can see that in the case of i
being logical, it is recycled to the length of x
:
对于 [-indexing only: i, j, ... 可以是逻辑向量,表示要选择的元素/切片.如有必要,此类载体将被回收匹配相应的范围.i, j, ... 也可以是负数整数,表示要从选择中删除的元素/切片.
For [-indexing only: i, j, ... can be logical vectors, indicating elements/slices to select. Such vectors are recycled if necessary to match the corresponding extent. i, j, ... can also be negative integers, indicating elements/slices to leave out of the selection.
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