[Rd] silent recycling in logical indexing

[Ben Bolker]

Hmm.

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 ...
fortunes::fortune("7ms")


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:
>
>> mat<-diag(1:3)
>> expr1 <- quote(mat[])
>> expr1[[3]] <- TRUE
>> expr1[[4]] <- 2
>> eval(expr1)
> [1] 0 2 0
>> mat[,2]
> [1] 0 2 0
>
> HTH,
>
> Chuck