NA在`i`表达的data.table（可能的bug） [英] NA in `i` expression of data.table (possible bug)

问题描述

当 i 包含 NA 时，不返回该特定行。我不知道这是预期的行为还是它？

When i contains NA, that particular row is not returned. I am not sure this is the intended behavior or is it?

require(data.table)
x = data.table(a=c(NA, 1:3, NA))    
x[a>0]       
   a
1: 1
2: 2
3: 3

x[!(a>0)]
    a
1: NA
2: NA

x[a<0]   
Empty data.table (0 rows) of 1 col: a

x[!(a<0)]
    a
1: NA
2:  1
3:  2
4:  3
5: NA

 > sessionInfo()
 R version 2.15.2 (2012-10-26)
 Platform: x86_64-unknown-linux-gnu (64-bit)

 locale:
  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
  [7] LC_PAPER=C                 LC_NAME=C                 
  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
  [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

 attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     

 other attached packages:
  [1] data.table_1.8.8

推荐答案

正如@flodel指出的，问题可以简化为为什么这不是 TRUE ：

As @flodel points out, the question can be simplified to, Why is this not TRUE:

identical(x[as.logical(a)], x[!!as.logical(a)])   # note the double bangs

答案在于data.table如何处理 i 中的 NA 以及如何处理 ！在 i 中。两者都接受特殊待遇。问题真的出现在两者的结合。

The answer lies in how data.table handles NA in i and how it handles ! in i. Both of which receive special treatment. The problem really arises in the combination of the two.

• NA 位于 i 被视为 FALSE 。
将 i 中的

• ！视为否定。

• NA's in i are treated as FALSE.
• ! in i are treated as a negation.

这在 ?. data.table Grothendieck在另一个答案中指出）。
相关部分为：

This is well documented in ?.data.table (as G. Grothendieck points out in another answer). The relevant portions being:

整数和逻辑向量的工作方式与[.data.frame。除了在逻辑i中的NAs被视为FALSE，并且单个NA逻辑不被回收以匹配行数，因为它在[.data.frame。

...

所有类型的'i'可以加前缀！。这表示应该执行非加入或非选择。在整个data.table文档中，我们引用i的类型，我们指的是'！'后面的'i'类型，如果存在。

integer and logical vectors work the same way they do in [.data.frame. Other than NAs in logical i are treated as FALSE and a single NA logical is not recycled to match the number of rows, as it is in [.data.frame.
...
All types of 'i' may be prefixed with !. This signals a not-join or not-select should be performed. Throughout data.table documentation, where we refer to the type of 'i', we mean the type of 'i' after the '!', if present.

如果你看看 [。data.table 的代码，！如果存在，则是

If you look at the code for [.data.table, the way ! are handled, if present, is by

1. 删除前面的！


2. 解释剩余的 i


3. 否定解释

处理 NA 的方法是将这些值设置为 FALSE 。

然而 - 非常重要的是 - 这发生在上面的步骤2中。

The way NAs are handled is by setting those values to FALSE.
However -- and very importantly -- this happens within step 2 above.

因此，真正发生的是当 i 包含 NA AND i 前缀为！，则NA有效地解释为 TRUE 。虽然在技术上，这是记录，我不知道这是否如预期。

Thus, what is really happening is that when i contains NA AND i is prefixed by !, then the NA's are effectively interpreted as TRUE. While technically, this is as documented, I am not sure if this is as intended.

当然，最后一个问题是@ flodel的观点：为什么 .logical（a）] 不同于 x [!! as.logical（a）] ？原因是只有第一 bang获得特殊处理。第二个bang被 R 解释为正常。

Of course, there is the final question of @flodel's point: Why is x[as.logical(a)] not the same as x[!!as.logical(a)]? The reason for this is that only the first bang gets special treatment. The second bang is interpreted as normal by R.

由于！NA 仍然是 NA （NA）的解释的修改是：

Since !NA is still NA, the sequence of modification for the interpretation of !!(NA) is:

!!(NA)  
!( !(NA) )  
!(  NA   )
!( FALSE )
TRUE

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