a[M] gives the same as your `cobbled together' code.

On Wed, Sep 28, 2011 at 6:35 AM, robin hankin <hankin.robin@gmail.com>wrote:

> hello everyone.
>
> Look at the following R idiom:
>
>  a <- array(1:30,c(3,5,2))
>  M <- (matrix(1:15,c(3,5)) %% 4) < 2
>  a[M,] <- 0
>
> Now, I think that "a[M,]" has an unambiguous meaning (to a human).
> However, the last line doesn't work as desired, but I expected it
> to...and it recently took me an indecent amount of time to debug an
> analogous case.  Just to be explicit, I would expect a[M,] to extract
> a[i,j,] where M[i,j] is TRUE.  (Extract.Rd is perfectly clear here, and R
> is
> behaving as documented).
>
> The best I could cobble together was the following:
>
>  ind <- which(M,arr.ind=TRUE)
>  n <- 3
>  ind <-
> cbind(kronecker(ind,rep(1,dim(a)[n])),rep(seq_len(dim(a)[n]),nrow(ind)))
>  a[ind] <- 0
>
>
> but the intent is hardly clear, certainly compared to "a[M,]"
>
> I've been pondering how to implement such indexing, and its
> generalization.
>
> Suppose 'a' is a seven-dimensional array, and M1 a matrix and M2 a
> three-dimensional array (both Boolean).  Then "a[,M1,,M2]" is a
> natural generalization of the above.  I would want a[,M1,,M2] to
> extract a[i1,i2,i3,i4,i5,i6,i7] where M1[i2,i3] and M[i5,i6,i7] are
> TRUE.
>
> One would need all(dim(a)[2:3] == dim(M1)) and all(dim(a)[5:7] ==
> dim(M2)) for consistency.
>
> Can any R-devel subscribers advise?
>
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> hankin.robin@gmail.com
>
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>

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