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I want an easy way to access the last values of objects. I have implemented a function that allows for Python-like negative indexing (count-backwards-and-keep-only-those-elements instead of R's remove-those-elements-and-keep-all-other-elements). There are some questions about this, but no one attempted to do this:

Basically, I want to redefine [ and [[, so that one can user a special operator (e.g. _) to use Python-like indexing. For instance, like this:

iris[1, ] #first row
iris[-1, ] #everything but first row
iris[_1, ] #last row, equivalent to tail(iris, 1)
iris[, _1] #last column, equivalent to iris[ncol(iris)]

Is it possible? I know it is possible to redefine e.g. + to introduce character concatenation like other languages have (see this answer).

In case it's not possible, here's my workaround function:

extract_last = function(x, margin_1, margin_2, drop = FALSE) {
  #check types
  if (!(is.vector(x) || is.matrix(x) || is.data.frame(x))) stop("x was an unsupported type (not a vector, matrix or data.frame)!")

  #vector
  if (is.vector(x)) return(rev(x)[margin_1])

  #get dims
  x_dims = dim(x)

  #make indices
  if (missing("margin_1")) {
    margin_1 = 1:x_dims[1]
  } else {
    margin_1 = (x_dims[1] + 1) - margin_1
  }
  if (missing("margin_2")) {
    margin_2 = 1:x_dims[2]
  } else {
    margin_2 = (x_dims[2] + 1) - margin_2
  }

  #subset
  return(x[margin_1, margin_2, drop = drop])
}

#tests
extract_last(iris, 1) == tail(iris, 1)
extract_last(iris, 10:1) == tail(iris, 10)
extract_last(iris, , 1) == iris[5]
extract_last(iris, , 2:1) == iris[4:5]
extract_last(iris, 10:1, 1) == iris[141:150, 5, drop = FALSE]
Improve this question
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  • 5
    Is iris[nrow(iris), ] or iris[, ncol(iris)] too much typing? Commented Feb 2, 2016 at 16:48
  • 1
    I don't think this can be done in general (i.e., not only for a specific class). E.g., you can't really define [ methods for the implicit classes (character , numeric, ...). Commented Feb 2, 2016 at 16:55
  • You could write a function called `` (see [this chapter written by Hadley Wickam), but I wouldn't advise it for so many reasons. I'd argue the easiest solution is T <- function(...){a <- list(...); do.call(tail,a)}.. this is pretty short and does your stuff: a <- matrix(1:9,3,3); T(a[1,],1); T(a,1). Commented Feb 2, 2016 at 17:06
  • 1
    By using backticks `[`. So something like `[` <- function(x,y,z=NULL){if (!is.null(z)){print("this was a bad idea")} else {do.call(.Primitive("["),list(x,y))}}; b <- 1:10; b[3]; b[2,3]. [And in case you can't figure it out, use rm(`[`) to get rid of it again.] Commented Feb 2, 2016 at 17:29
  • 1
    I agree that it would be kinda cool if R added a keyword like, say, "last" so that foo[1:(last-5)] would skip the last 5 elements, but in the long run it's really not that hard to use ncol or nrow as Roman suggested. If you're going to be accessing the same array a lot, just define a couple variables nrowmat <- nrow(mat) and use them. Commented Feb 2, 2016 at 17:54
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