Extract non null elements from a list in R

Question

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
    #
    #$         


        

Answer 1

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))]