Apply a function to array element indices
Given a function f() that takes a vector of indices, and an
array of arbitrary dimensions, apply f() to the elements of a
do.index(a, f, ...)
a |
Array |
f |
Function that takes a vector argument of the same length as
|
... |
Further arguments supplied to |
Returns a matrix of the same dimensions as a
Tamas Papp suggests the one-liner
function(a, f, ...){array(apply(as.matrix(expand.grid(lapply(dim(a),seq_len),KEEP.OUT.ATTRS=FALSE)),1,f,...),dim(a))}
which is functionally identical to do.index(); but
it is no faster than the version implemented in the package, and (IMO)
is harder to read.
Further note that function arow() is much much faster than
do.index(); it is often possible to rephrase a call to
do.index() as a call to arow(); do this where possible
unless the additional code opacity outweighs the speed savings.
Robin K. S. Hankin, with improvements by Gabor Grothendieck and Martin Maechler, via the R help list
a <- array(0,c(2,3,4))
b <- array(rpois(60,1),c(3,4,5))
f1 <- function(x){sum(x)}
f2 <- function(x){sum((x-1)^2)}
f3 <- function(x){b[t(x)]}
f4 <- function(x){sum(x)%%2}
f5 <- function(x,u){x[u]}
do.index(a,f1) # should match arow(a,1)+arow(a,2)+arow(a,3)
do.index(a,f2)
do.index(a,f3) # same as apltake(b,dim(a))
do.index(a,f4) # Male/female toilets at NOC
do.index(a,f5,2) # same as arow(a,2)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.