[Rd] silent recycling in logical indexing
bbolker at gmail.com
Thu Jan 4 22:46:03 CET 2018
PS I'm tempted to insert a warning at this point and see how often it
actually gets triggered ...
On Thu, Jan 4, 2018 at 4:44 PM, Ben Bolker <bbolker at gmail.com> wrote:
> Chuck: I don't see how this example represents
> incomplete/incommensurate recycling. Doesn't TRUE replicate from
> length-1 to length-3 in this case (mat[c(TRUE,FALSE),2] would be an
> example of incomplete recycling)?
> William: clever, but maybe too clever unless you really need the
> speed? (The clever way is 8 times faster in the following case ...)
> x <- rep(1,1e6)
> rbenchmark::benchmark(x[c(FALSE,TRUE,FALSE)],x[seq_along(x) %% 3 == 2])
> On the other hand, it takes 0.025 vs 0.003 seconds per iteration ...
> On Thu, Jan 4, 2018 at 4:09 PM, Berry, Charles <ccberry at ucsd.edu> wrote:
>>> On Jan 4, 2018, at 11:56 AM, Ben Bolker <bbolker at gmail.com> wrote:
>>> Sorry if this has been covered here somewhere in the past, but ...
>>> Does anyone know why logical vectors are *silently* recycled, even
>>> when they are incommensurate lengths, when doing logical indexing?
>> It is convenient to use a single `TRUE' in programmatic manipulation of subscripts in the same manner as using an empty subscript interactively:
>>> expr1 <- quote(mat)
>>> expr1[] <- TRUE
>>> expr1[] <- 2
>>  0 2 0
>>  0 2 0
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