Extract non null elements from a list in R

问题

I have a list like this:

    x = list(a = 1:4, b = 3:10, c = NULL)
    x
    #$a
    #[1] 1 2 3 4
    #
    #$b
    #[1]  3  4  5  6  7  8  9 10
    #
    #$c
    #NULL

and I want to extract all elements that are not null. How can this be done? Thanks.

回答1:

Here's another option:

Filter(Negate(is.null), x)

回答2:

What about:

x[!unlist(lapply(x, is.null))]

---

Here is a brief description of what is going on.

1. lapply tells us which elements are NULL

   R> lapply(x, is.null)
   $a
   [1] FALSE
   
   $b
   [1] FALSE
   
   $c
   [1] TRUE
   

2. Next we convect the list into a vector:

   R> unlist(lapply(x, is.null)) 
   a     b     c 
   FALSE FALSE  TRUE 
   

3. Then we switch TRUE to FALSE:

   R> !unlist(lapply(x, is.null))
       a     b     c 
   TRUE  TRUE FALSE 
   

4. Finally, we select the elements using the usual notation:

   x[!unlist(lapply(x, is.null))]
   

回答3:

x[!sapply(x,is.null)]

This generalizes to any logical statement about the list, just sub in the logic for "is.null".

回答4:

Simpler and likely quicker than the above, the following works for lists of any non-recursive (in the sense of is.recursive) values:

example_1_LST <- list(NULL, a=1.0, b=Matrix::Matrix(), c=NULL, d=4L)
example_2_LST <- as.list(unlist(example_1_LST, recursive=FALSE))

str(example_2_LST) prints:

List of 3
 $ a: num 1
 $ b:Formal class 'lsyMatrix' [package "Matrix"] with 5 slots
  .. ..@ x       : logi NA
  .. ..@ Dim     : int [1:2] 1 1
  .. ..@ Dimnames:List of 2
  .. .. ..$ : NULL
  .. .. ..$ : NULL
  .. ..@ uplo    : chr "U"
  .. ..@ factors : list()
 $ d: int 4

来源：https://stackoverflow.com/questions/16896376/extract-non-null-elements-from-a-list-in-r