[R] Transforming boolean subsetting into index subsetting

[Bert Gunter]

?which

-- Bert

On Fri, Jun 28, 2013 at 2:58 PM, Julio Sergio <juliosergio at gmail.com> wrote:
> One of the techniques to subset, a vector for instance, is the following:
>
>   V <- c(18, 8, 5, 41, 8, 7)
>   V
>   ## [1] 18  8  5 41  8  7
>   ( I <- abs(V - 9) <= 3 )
>   ## [1] FALSE  TRUE FALSE FALSE  TRUE  TRUE
>   V[I]
>   ## [1] 8 8 7
>
> However, sometimes we are interested in the indexes of the elements where the
> condition holds. Is there an easy way to transform the I vector into an
> indexes vector similar to:  I == c(2,5,6) ?
>
> I know that I can traverse the I vector with a for() loop collecting the
> indexes, I just wonder if such an operation can be avoided.
>
> Thanks,
>
>   -Sergio.
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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