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

[Hervé Pagès]

On 06/08/2018 02:15 PM, Hadley Wickham 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)

That's a good one. Need to change this in S4Vectors::default_extractROWS()
and other places. Thanks!

H.

> 
> Hadley
> 
> 

-- 
Hervé Pagès

Program in Computational Biology
Division of Public Health Sciences
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