[R] Inverse function to 'which'?

[Peter Holzer]

Henrik Bengtsson writes:
 > Is there an inverse function to 'which' available, which creates a logical
 > vector/matrix from indices? I wrote my own, but if there is a standard
 > function I would like to use that instead. Example:
 > 
 > x <- c(TRUE, TRUE, FALSE, TRUE, FALSE, FALSE)
 > unwhich(which(x))            # [1] TRUE TRUE FALSE TRUE
 > unwhich(which(x), length(x)) # [1] TRUE TRUE FALSE TRUE FALSE FALSE
 > 
 > y <- matrix(x, nrow=3, ncol=2)
 > unwhich(which(y, arr.ind=TRUE))          # "Minimum" size
 > unwhich(which(y, arr.ind=TRUE), dim(y))  # Same dimensions as y

The following function does what you want:

unwhich <- function (which, dim = max(which)) {
  y <- array(logical(length(which)), dim = dim)
  y[which] <- TRUE
  y
}

Cheers, Peter
 
 > Thanks
 > 
 > Henrik Bengtsson
 > h e n r i k b @ b r a j u . c o m
 > 
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