my.index {mvbutils}

Arbitrary-level retrieval from and modification of recursive objects

Description

As of R 2.12, you probably don't need these at all. But, in case you do: my.index and my.index.assign are designed to replace [[ and [[<- within a function, to allow arbitrary-depth access into any recursive object. In order to avoid conflicts with system usage and/or slowdowns, it is wise to do this only inside a function definition where they are needed. A zero-length index returns the entire object, which I think is more sensible than the default behaviour (chuck a tanty). my.index.exists tests whether the indexed element actually exists. Note that these functions were written in 2001; since then, base-R has extended the default behaviour of [[ etc for recursive objects, so that my.index( thing, 1, 3, 5) can sometimes be achieved just by to thing[[c(1,3,5)]] with the system version of [[. However, at least as of R 2.10.1, the system versions still have limited "recursability".

Usage

# Use them like this, inside a function definition:
# assign( "[[", my.index); var[[i]]
# assign( "[[<-", my.index.assign); var[[i]] <- value
my.index( var, ...) # not normally called by name
my.index.assign( var, ..., value) # not normally called by name
my.index.exists( i, var)

Arguments

var

a recursive object of any mode (not just list, but e.g. call too)

value

anything

...

one or more numeric index vectors, to be concatenated

i

numeric index vector

Details

Although R allows arbitrary-level access to lists, this does not (yet) extend to call objects or certain other language objects– hence these functions. They are written entirely in R, and are probably very slow as a result. Notwithstanding EXAMPLES below, it is unwise to replace system [[ and [[<- with these replacements at a global level, i.e. outside the body of a function– these replacements do not dispatch based on object class, for example.

Note that my.index and my.index.assign distort strict R syntax, by concatenating their ... arguments before lookup. Strictly speaking, R says that x[[2,1]] should extract one element from a matrix list; however, this doesn't really seem useful because the same result can always be achieved by x[2,1][[1]]. With my.index, x[[2,1]] is the same as x[[c(2,1)]]. The convenience of automatic concatentation seemed slightly preferable (at least when I wrote these, in 2001).

my.index.exists checks whether var is "deep enough" for var[[i]] to work. Unlike the others, it does not automatically concatenate indices.

At present, there is no facility to use a mixture of character and numeric indexes, which you can in S+ via "list subscripting of lists".

Author(s)

Mark Bravington

Examples

local({
  assign( "[[", my.index)
  assign( "[[<-", my.index.assign)
  ff <- function() { a <- b + c }
  body( ff)[[2,3,2]] # as.name( "b")
  my.index.exists( c(2,3,2), body( ff)) # TRUE
  my.index.exists( c(2,3,2,1), body( ff)) # FALSE
  body( ff)[[2,3,2]] <- quote( ifelse( a>1,2,3))
  ff # function () { a <- ifelse(a > 1, 2, 3) + c }
  my.index.exists( c(2,3,2,1), body( ff)) # now TRUE
})