Overloading subscript operator "["

Question

I am trying to overload the subscript operator ("[") for a custom class I've created. I am trying to figure out how to deal with the following issues.

How can you figure out if the operator is on lhs or rhs ? i.e. a[x] = foo vs foo = a[x]
When subscripting the entire dimension like foo = a[,x] how can I identify the first parameter ?
When using a[seq(x,y)] it seems to be expanding the entire sequence. Is there an easy way to get the first, step and last values without expanding ?

EDIT: My first point has received multiple answers. In the process I've figured out the answer to the second one. You can use the "missing" function to figure out which parameters are present.

Here is a sample code:

setMethod("[", signature(x="myClass"),
          function(x, i, j, k, l) {
              if (missing(i)) { i = 0 }
              if (missing(j)) { j = 0 }
              if (missing(k)) { k = 0 }
              if (missing(l)) { l = 0 }
          })

I have accepted an answer to this question as point number 3 is of least priority to me.

For the third bullet point: it could be done by parsing the expression, but it sounds a bit surprising (the behavior of `a[seq(x, y)]` and `v = seq(x, y); a[v]` would be different) — David Robinson, Mar 14 '14 at 14:36
@DavidRobinson Can you elaborate more on what you mean by `parsing the expression` ? — Pavan Yalamanchili, Mar 14 '14 at 15:10

score 11 · Answer 1 · edited Mar 14 '14 at 15:14

11

Discover the Generic so you know what you are aiming to implement :

getGeneric('[')
# standardGeneric for "[" defined from package "base"
# function (x, i, j, ..., drop = TRUE)

And :

getGeneric('[<-')
# standardGeneric for "[<-" defined from package "base"
# function (x, i, j, ..., value)

Then you implement it like this for example :

`[<-.foo` <-
function(x, i, j, value) 
{
       ....

}

edited Mar 14 '14 at 15:14

Josh O'Brien

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answered Mar 14 '14 at 14:53

agstudy

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score 8 · Accepted Answer · edited Mar 14 '14 at 15:00

8

For the first bullet point there are two functions to overload:

[
[<-

The first function returns the value and the second sets the value. See the documentation for Extract.data.frame{base}

edited Mar 14 '14 at 15:00

csgillespie

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answered Mar 14 '14 at 14:45

f.jamitzky

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Ari B. Friedman · Answer 3 · 2014-03-14T15:40:18.043

See the source for [.data.frame as an example. You've got an x, an i, and a j, in that order.

> `[.data.frame`
function (x, i, j, ..... )
> `[<-.data.frame`
function (x, i, j, value)

You can do something similar:

obj <- structure(runif(10),class="myclass")
`[.myclass` <- function(x,i=NULL,j=NULL) {
  message("First parameter is ", i, "\n")
  message("Second parameter is ", j, "\n")
    sum(x)
}
obj[c(1,2,3)]

`[<-.myclass` <- function(x,i=NULL,j=NULL,value) {
    res <- unclass(x)
    res[i] <- value
    res
}
obj[1] <- 1

Order is irrelevant. The i's and j's will be right:

2 -> obj[2]
> obj
 [1] 1.0000000 2.0000000 0.3466835 0.3337749 0.4763512 0.8921983 0.8643395 0.3899895 0.7773207 0.9606180

score 1 · Answer 4 · answered Mar 14 '14 at 16:12

As for the third bullet point, you could use something like this:

seq_parse <- function(s) {
    the_s <- deparse(substitute(s))
    env <- new.env()
    assign("seq", function(...) return(list(...)), envir=env)
    seq_params <- try(eval(parse(text=the_s), envir=env), silent=TRUE)
    if(!inherits(seq_params, "try-error")) {
        cat("parsing successful \n")
        return(seq_params)
    } else {
        stop("Failed to parse.")
    }
}

This function either returns the arguments to seq,

> seq_parse(seq(from=1,to=10,by=0.5))
working with params to seq_params 
$from
[1] 1

$to
[1] 10

$by
[1] 0.5

or the conventional evaluated argument,

> seq_parse(1:5)
parsing successful 
[1] 1 2 3 4 5

However, I am not sure if that is good practice. Also you have to interpret carefully the parameters to seq, they could be unnamed, and instead of to there could be length.out etc.

Overloading subscript operator "["

4 Answers4