R: removing NULL elements from a list

Question

mylist <- list(NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,      123, NULL, 456)  > mylist [[1]] NULL  [[2]] NULL  [[3]] NULL  [[4]] NULL  [[5]] NULL  [[6]] NULL  [[7]] NULL  [[8]] NULL  [[9]] NULL  [[10]] NULL  [[11]] [1] 123  [[12]] NULL  [[13]] [1] 456 

My list has 13 elements, 11 of which are NULL. I would like to remove them, but preserve the indices of the elements that are nonempty.

mylist2 = mylist[-which(sapply(mylist, is.null))] > mylist2 [[1]] [1] 123  [[2]] [1] 456 

This removes the NULL elements just fine, but I don't want the nonempty elements to be reindexed, i.e, I want mylist2 to look something like this, where the indices of the nonempty entries are preserved.

> mylist2 [[11]] [1] 123  [[13]] [1] 456 

Answer 1

The closest you'll be able to get is to first name the list elements and then remove the NULLs.

names(x) <- seq_along(x)  ## Using some higher-order convenience functions Filter(Negate(is.null), x) # $`11` # [1] 123 #  # $`13` # [1] 456  # Or, using a slightly more standard R idiom x[sapply(x, is.null)] <- NULL x # $`11` # [1] 123 #  # $`13` # [1] 456 

Answer 2

Simply use mylist[lengths(mylist) != 0].

Function lengths() was introduced in R 3.2.0 (April 2015).