How to set default value of a slot as NULL in R?

I'm new to R.

I'm trying to define a class similar to a tree node, that is , it has a left node and right node, which should be of the same class as the parent node. So I define the class as follows:

setClass('Node', representation=(left='Node',right='Node', ...))

I want to set the default value of Node to be NULL by setting a prototype, but R says the following:

  invalid class "Node" object: invalid object for slot "left" in class "bicluster": got class "NULL", should be or extend class "Node"

But if I don't speficy the default value to be NULL, then the default value will be a recursive Node of depth 4, which I think is some waste of resource.

Is my consideration unnecessary or is there a better way to do it ?

This is a revised answer.

Class unions are funky -- they effectively insert a class into the middle of an existing hierarchy, so suddenly things that extend list now extend listOrNULL.

Instead I'd create a small class hierarchy that represents a "Tree" that can either be "Empty" or "Internal". The "Internal" class will have a slot to contain data (of type "ANY"), plus the left and right links, which will be "Tree" elements.

setClass("Tree")

setClass("Empty", contains="Tree")

setClass("Internal", contains="Tree",
         representation=representation(elem="ANY", left="Tree", right="Tree"),
         prototype=prototype(left=new("Empty"), right=new("Empty")))

I'll write a constructor for my Tree, with methods for creating an empty tree, and a tree from an element plus left and right descendants.

setGeneric("Tree", function(elem, left, right) standardGeneric("Tree"),
           signature="elem")

setMethod(Tree, "missing", function(elem, left, right) new("Empty"))

setMethod(Tree, "ANY", function(elem, left, right) {
    new("Internal", elem=elem, left=left, right=right)
})

A basic operation is to insert an element x into a tree t

setGeneric("insert", function(x, t) standardGeneric("insert"))

setMethod(insert, c("ANY", "Empty"), function(x, t) {
    Tree(x, Tree(), Tree())
})

setMethod(insert, c("ANY", "Internal"), function(x, t) {
    if (x < t@elem) {
        l <- insert(x, t@left)
        r <- t@right
    } else {
        l <- t@left
        r <- insert(x, t@right)
    }
    Tree(t@elem, l, r)
})

Another operation is to test for membership

setGeneric("member", function(x, t) standardGeneric("member"))

setMethod(member, c("ANY", "Empty"), function(x, t) FALSE)

setMethod(member, c("ANY", "Internal"), function(x, t) {
    if (x < t@elem) member(x, t@left)
    else if (t@elem < x) member(x, t@right)
    else TRUE
})

An interesting, functional, property of this implementation is that it is persistent

> t <- Tree()
> t1 <- insert(10, t)
> t2 <- insert(5, t1)
> t3 <- insert(7, t2)
> t4 <- insert(15, t3)
> which(sapply(1:20, member, t4))
[1]  5  7 10 15
> which(sapply(1:20, member, t2))
[1]  5 10

This is not going to be efficient when there are a lot of updates, because of the inefficiencies of S4 class creation and because modifying a tree (e.g., adding a node) copies all nodes in the path to the new node. A different approach represents the tree as a matrix of left, right, value triples. The S4 implementation would still have poor performance, because the updates of the instance would create new instances, duplicating everything. So I'd end up at a reference class, with fields 'value' (a vector of whatever the tree is supposed to hold and a matrix of left and right relations.

At one time you needed to use setClassUnion("listOrNULL",members=c("list", "NULL")) to get a NULL into a slot that was defined as a list. I think that is now an available class. Cannot test while your setup incomplete, but defining a super-class "NodeOrNull" may get you past your initial barrier.

来源：https://stackoverflow.com/questions/7466388/how-to-set-default-value-of-a-slot-as-null-in-r

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