mvbutils (version 2.8.232)

my.index: 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".

Examples

# NOT RUN {
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
})
# }