Subsetting the "ROW"s of an object

On Fri, Jun 8, 2018 at 2:09 PM, Berry, Charles <ccberry at ucsd.edu> wrote:

On Jun 8, 2018, at 1:49 PM, Hadley Wickham <h.wickham at gmail.com> wrote:

Hmmm, yes, there must be some special case in the C code to avoid
recycling a length-1 logical vector:

Here is a version that (I think) handles Herve's issue of arrays having one or more 0 dimensions.

subset_ROW <-
     function(x,i)
{
     dims <- dim(x)
     index_list <- which(dims[-1] != 0L) + 3
     mc <- quote(x[i])
     nd <- max(1L, length(dims))
     mc[ index_list ] <- list(TRUE)
     mc[[ nd + 3L ]] <- FALSE
     names( mc )[ nd+3L ] <- "drop"
     eval(mc)
}

Curiously enough the timing is *much* better for this implementation than for the first version I sent.

Constructing a version of `mc' that looks like `x[i,,,,drop=FALSE]' can be done with `alist(a=)' in place of `list(TRUE)' in the earlier version but seems to slow things down noticeably. It requires almost twice (!!) as much time as the version above.

I think that's probably because alist() is a slow way to generate a
missing symbol:

bench::mark(
   alist(x = ),
   list(x = quote(expr = )),
   check = FALSE
)[1:5]
#> # A tibble: 2 x 5
#>   expression                    min     mean   median      max
#>   <chr>                    <bch:tm> <bch:tm> <bch:tm> <bch:tm>
#> 1 alist(x = )                 2.8?s   3.54?s   3.29?s   34.9?s
#> 2 list(x = quote(expr = ))    169ns 219.38ns    181ns   24.2?s

(note the units)

Hadley