get back for R

sir-mixalot.R

.back_ix <- function (x, ix) {
  stopifnot(is.numeric(ix), all(abs(ix) <= length(x)))
  ix <- ix[ix != 0]
  neg <- ix < 0
  ix <- length(x) + 1 - abs(ix)
  ix[neg] <- -1 * ix[neg]
  ix
}

back <- function (x, ix) {
  x[.back_ix(x, ix)]
}

`back<-` <- function (x, ix, value) {
  x[.back_ix(x, ix)] <- value
  x
}


letters[length(letters)]                         # so unreadable
letters[(length(letters) - 3):(length(letters))] # even worse
letters[length(letters) - 3:0]                   # still too long

tail(letters, 4)        # ok
tail(letters, 2:4)      # boring
tail(letters, 1) <- "Z" # boooring

rev(letters)[1:3]                     # ok...
rev(letters)[1:3] <- c("Z", "Y", "X") # why can't we have nice things :-(


back(letters, 4)
back(letters, 1:4) # obviously
back(letters, 2:4) # not a problem
back(letters, 4:1) # just count from the back

back(letters, 4) <- "W"
back(letters, 2:3) <- c("Y", "X")
letters # as god intended


letters[-(1:21)]
back(letters, -(1:21)) # of course!

letters[0]
back(letters, 0)

letters[30]
back(letters, 30) # don't get over-excited