[Rd] Subsetting the "ROW"s of an object

[Berry, Charles]

> On Jun 8, 2018, at 2:15 PM, Hadley Wickham <h.wickham using gmail.com> wrote:
> 
> On Fri, Jun 8, 2018 at 2:09 PM, Berry, Charles <ccberry using ucsd.edu> wrote:
>> 
>> 
>>> On Jun 8, 2018, at 1:49 PM, Hadley Wickham <h.wickham using 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 = )                 2.8µs   3.54µs   3.29µs   34.9µs
> #> 2 list(x = quote(expr = ))    169ns 219.38ns    181ns   24.2µs
> 
> (note the units)

Yes. That is good for about half the difference. And I guess the rest is getting rid of seq(). This seems a bit quicker than anything else and satisfies Herve's objections:

subset_ROW <-
      function(x,i)
  {
      dims <- dim(x)
      nd <- length(dims)
      index_list <- if (nd > 1) 2L + 2L:nd else 0
      mc <- quote(x[i])
      mc[ index_list ] <- list(quote(expr=))
      mc[[ "drop" ]] <- FALSE
      eval(mc)
  }

Chuck