[Rd] silent recycling in logical indexing
bbolker at gmail.com
Thu Jan 4 22:44:24 CET 2018
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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