Generalized Outer Products
DESCRIPTION:
Performs an outer product operation given two arrays (or vectors) and, optionally, a function.
USAGE: 
outer(X, Y, FUN="*", ...)


X %o% Y #operator form
REQUIRED ARGUMENTS:
X,Y:
first and second arguments to the function FUN. Missing values (NAs) are allowed if FUN accepts them.
OPTIONAL ARGUMENTS:
FUN:
an S-PLUS function that takes at least two vectors as arguments and returns a single value. This may also be a character string giving the name of such a function.
...:
other arguments to FUN, if needed. The names of the arguments, if any, should be those meaningful to FUN.
VALUE:
array, whose dimension vector is the concatenation of the dimension vectors of X and Y, and such that FUN(X[i,j,k,...], Y[a,b,c,...]) is the value of the [i,j,k,...,a,b,c,...] element. (Vectors are considered to be one-dimensional arrays.)
DETAILS:
outer forms two arrays corresponding to the data in X and Y, each of which has a dim attribute formed by concatenating the dim attributes of X and Y. It then calls FUN just once with these two arrays as arguments. Therefore, FUN should be a function that operates elementwise on vectors or arrays and expects (at least) two arguments.
SEE ALSO:
crossprod , Matrix-product .
EXAMPLES: 
z <- x %o% y   # The outer product array
  # dim(z) == c(dim(as.array(x)), dim(as.array(y)))


 # simulate a two-way table
outer(c(3.1, 4.5, 2.8, 5.2), c(2.7, 3.1, 2.8),"+") + matrix(rnorm(12), nrow=4)


z <- outer(months, years, paste) # All month, year combinations pasted.