[R] Idempotent apply

[hadley wickham]

One small problem with apply is that it is not idempotent - it always
puts the result vector in the first dimension of the result array. For
example:

a <- array(1:24, c(2,3,4))
dim(apply(a, c(1,2), I))
dim(apply(a, c(1,2), diff))

I've written a wrapper around apply that rearranges the dimensions
into the original order.

# Idempotent apply
iapply <- function (X, MARGIN, FUN, ...)  {
	dims <- c((1:length(dim(X)))[!(1:length(dim(X)) %in% MARGIN)], MARGIN)
	res <- apply(X, MARGIN, FUN)

	if (length(dim(res)) == length(dims)) {
		aperm(res, order(dims))
	} else {
		res
	}	
}

dim(iapply(a, c(1,2), I))
dim(iapply(a, c(1,2), diff))

Hopefully this may be of use to someone else.  The function isn't
completely satisfying as apply doesn't deal nicely with functions that
return higher-dimensional arrays (eg.  apply(a, 1, I)) and reduces 1-D
arrays to vectors (eg. apply(a,3,sum)).  Any comments on the code
would be greatfully appreciated.

Hadley