[Rd] array extraction

From: robin hankin <hankin.robin_at_gmail.com>
Date: Wed, 28 Sep 2011 09:35:08 +1300


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_at_gmail.com

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Received on Tue 27 Sep 2011 - 20:37:33 GMT

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