title: How to check the existence of elements of list in r
body: <p>How can I check if an element of list exists or not. This is a list of lists so for example I want to check whether the third element l1[[3]] exists or not. I have tried is.null(l1[["3"]]) 
  but it returns false no matter whether it exists or not and if I use is.null(l1[[3]]) it will give the error of subscript out of bind in case it does not exists but not TRUE. 
  How should I che </p>