Re: [R] reverse array indexing

[Peter Wolf]

Richard A. O'Keefe wrote:

Jerome Asselin <[EMAIL PROTECTED]> suggests this:
        arr <- array(rnorm(27),c(3,3,3))
        dimarr <- dim(arr)
        tmparr <- array(1:prod(dimarr),dimarr)
        sapply(c(3),function(x,tmparr) which(tmparr==x,T),tmparr=tmparr)
        sapply(c(3,17,13,5),function(x,tmparr) which(tmparr==x,T),tmparr=tmparr)
        
Of course, in R we can simplify the last two lines to
   sapply(<<argument goes here>>, function(x) which(tmparr==x,T))
However, wearing my "computer scientist" hat, I have to wonder about costs.
This is basically the equivalent of the APL "decode" operator.
Let's define

   index.decode <- function (index, array) {
        dimarr <- dim(arr)
        tmparr <- array(1:prod(dimarr), dimarr)
        sapply(index, function(x) which(tmparr == x, T))
   }
The result is a matrix with C=length(index) columns
and R=length(dim(array)) rows.  ...
 

I think you mean the APL encode operator?

> index<-1:8
> encode(index-1,c(2,2,2))+1
    [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    1    1    1    1    2    2    2    2
[2,]    1    1    2    2    1    1    2    2
[3,]    1    2    1    2    1    2    1    2
download code of encode from:  
http://www.wiwi.uni-bielefeld.de/~wolf/software/R-wtools/decodeencode.rev

Peter Wolf

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