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     #     #$c     #NULL 

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

Answer 1

Here's another option:

Filter(Negate(is.null), x) 

Answer 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))]