用NA索引整数向量 [英] Indexing integer vector with NA

问题描述

我很难理解这一点.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

如果我使用单个 NA 对其进行索引，则结果的长度为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] 正在建立索引，但是我不理解

So this cannot be the solution. My second approach is that x[NA] is indexing but then I do not understand

1. 这为什么给我五个 NA's
2. NA作为索引的含义. x [1] 给您第一个值，但是 x [NA] 的结果应该是什么?

1. Why this gives me five NA's
2. What NA as an index means. x[1] gives you the first value but what should be the result of x[NA]?

推荐答案

比较您的代码:

> 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，它是整数.从?"[" 您可以看到，在 i 是逻辑的情况下，它被回收到 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:

仅对于[-索引:i，j，...可以是逻辑向量，指示要选择的元素/切片.如有必要，可将此类载体回收匹配相应的范围.i，j，...也可以是负数整数，指示要排除在选择之外的元素/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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