防止取消列表删除NULL值 [英] Prevent unlist to drop NULL values

问题描述

我有一个列表向量，并且在它们上使用了unlist.向量中的某些元素是NULL，而unlist似乎正在删除它们.

I have a vector of lists and I use unlist on them. Some of the elements in the vectors are NULL and unlist seems to be dropping them.

如何防止这种情况?

这是一个简单的(非)工作示例，显示了unlist

Here's a simple (non) working example showing this unwanted feature of unlist

a = c(list("p1"=2, "p2"=5), 
      list("p1"=3, "p2"=4), 
      list("p1"=NULL, "p2"=NULL), 
      list("p1"=4, "p2"=5))
unlist(a)
 p1 p2 p1 p2 p1 p2 
 2  5  3  4  4  5 

推荐答案

此处的问题是，向量中间不能包含NULL.例如:

The issue here is that you can't have NULL in the middle of a vector. For example:

> c(1,NULL,3)
[1] 1 3

虽然中间可以有NA.您可以将其转换为字符，然后再转换为数字，这会自动将NULL值转换为NA(带有警告):

You can have NA in the middle though. You could could convert it to character and then back to numeric, which automatically converts the NULL values to NA (with a warning):

> b <- as.numeric(as.character(a))
Warning message:
NAs introduced by coercion 

然后重新输入名称，因为先前的操作已将其删除:

then put the names back in, because they've been dropped by the previous operation:

> names(b) <- names(a)
> b
p1 p2 p1 p2 p1 p2 p1 p2 
2  5  3  4 NA NA  4  5 `

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