Subsetting the "ROW"s of an object

I suspect this will have suboptimal performance since the TRUEs will
get recycled. (Maybe there is, or could be, ALTREP, support for
recycling)
Hadley

On Fri, Jun 8, 2018 at 10:16 AM, Berry, Charles <ccberry at ucsd.edu> wrote:

On Jun 8, 2018, at 8:45 AM, Hadley Wickham <h.wickham at gmail.com> wrote:

Hi all,

Is there a better to way to subset the ROWs (in the sense of NROW) of
an vector, matrix, data frame or array than this?

You can use TRUE to fill the subscripts for dimensions 2:nd

subset_ROW <- function(x, i) {
  nd <- length(dim(x))
  if (nd <= 1L) {
    x[i]
  } else {
    dims <- rep(list(quote(expr = )), nd - 1L)
    do.call(`[`, c(list(quote(x), quote(i)), dims, list(drop = FALSE)))
  }
}

subset_ROW <-
     function(x,i)
{
     mc <- quote(x[i])
     nd <- max(1L, length(dim(x)))
     mc[seq(4, length=nd-1L)] <- rep(list(TRUE), nd - 1L)
     mc[["drop"]] <- FALSE
     eval(mc)

}

subset_ROW(1:10, 4:6)
#> [1] 4 5 6

str(subset_ROW(array(1:10, c(10)), 2:4))
#>  int [1:3(1d)] 2 3 4
str(subset_ROW(array(1:10, c(10, 1)), 2:4))
#>  int [1:3, 1] 2 3 4
str(subset_ROW(array(1:10, c(5, 2)), 2:4))
#>  int [1:3, 1:2] 2 3 4 7 8 9
str(subset_ROW(array(1:10, c(10, 1, 1)), 2:4))
#>  int [1:3, 1, 1] 2 3 4

subset_ROW(data.frame(x = 1:10, y = 10:1), 2:4)
#>   x y
#> 2 2 9
#> 3 3 8
#> 4 4 7

HTH,

Chuck