Question

I’m trying to wrap my head around the basic concepts of the R programming language and am finding it difficult since R is geared towards statistics instead of general-purpose programming. I can’t find anything similar to pointers/references. How would you implement a linked list, search tree, etc. within the R language?

Note: I understand that if you’re actually rolling your own self-referential data structures in R, there’s probably a better way to accomplish what you’re trying to accomplish. However, I believe an answer will help me understand the overall structure and concepts of the language better.

Edit: With regard to Matt Shotwell’s comment, the point of this question is that I’m looking to write linked lists and trees cleanly, within R, not as an extension written in C or some other language. Doing it as an extension or by mucking with arcane details of the interpreter defeats the purpose.

Answer 1

A linked list in R can be represented as a vector, typically a list. You don’t need to write special code to reference the next and previous items, because R does it for you via indexing.

To add a new item to the list, just keep track of its length and assign to the next in line.

lst <- list() # creates an empty (length zero) list
lst[[1]] <- 1 # automagically extends the lst
lst[[2]] <- 2 # ditto

This can be inefficient for long lists because of the way R handles memory. If possible, create the list in advance, and assign their contents as they become available.

lst <- list(1, 2, 3, 4, 5)    # a list of 5 items

lst <- vector("list", 10000)  # 10000 NULLs
lst[[1]] <- 1
lst[[10000]] <- 10000  # lst now contains 1, NULL, ..., NULL, 10000

Deleting an item from the list can be accomplished with negative indexes.

lst <- list(1, 2, 3, 4, 5)
lst <- lst[-2]  # now contains 1, 3, 4, 5

A tree is just a list containing other lists.

tree <- list(list(1, 2), list(3, list(4, 5)))

# left child: list(1, 2)
tree[[1]]

# right child
tree[[2]]

# right child of right child:list(4, 5)
tree[[2]][[2]]

By default there is no built-in enforcing of the structure, eg only two children per node for a binary tree. More structured approaches are available via S4 classes, but this will do the job in a pinch.