Outer with Three Vectors

Here's my shot at it.  The gouter function can be enhanced further (e.g., as
outer() does with dimnames), but I think the basic functionality is there.
You can basically pass in any number of vectors you want, but you need to
wrap them in a single list.  outer() allows arrays, but gouter() below will
only work with list of vectors.

gouter <- function(x, FUN, ...) {
  xgrid <- as.list(do.call("expand.grid", x))
  names(xgrid) <- NULL
  xdim <- sapply(x, length)
  array(do.call(deparse(substitute(FUN)), c(xgrid, list(...))),
                dim=sapply(x, length), dimnames=x)
}

Here's a simple test:

f <- function(x, y, z) x + y + z
x1 <- 1:3
x2 <- 4:5
x3 <- 6:9
gouter(list(x1, x2, x3), f)

, , 6

   4  5
1 11 12
2 12 13
3 13 14

, , 7

   4  5
1 12 13
2 13 14
3 14 15

, , 8

   4  5
1 13 14
2 14 15
3 15 16

, , 9

   4  5
1 14 15
2 15 16
3 16 17

HTH,
Andy

From: Wolfgang Viechtbauer

Hello,

outer() is great for avoiding things like:

for (val1 in val1s) {
  for (val2 in val2s)) {
    x[i,j] <- somefunction(val1, val2)
  }
}

The same can be obtained with:

outer(val1s, val2s, somefunction)

But what if there are three (or more) sets of values to loop over? Any
way of avoiding the loops then?

Thanks,

-- 
Wolfgang Viechtbauer

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