R: Outer, Matrices and vectorizing

Question

I'd like to understand better how outer works and how to vectorize functions. Below is a minimal example for what I am trying to do:I have a set of numbers 2,3,4. for each combination (a,b) of create a diagonal matrix with a a b b b on the diagonal, and then do something with it, e.g. calculating its determinant (this is just for demonstration purposes). The results of the calculation should be written in a 3 by 3 matrix, one field for each combination.

The code below isn't working - apparently, outer (or my.func) doesn't understand that I don't want the whole lambdas vector to be applied - you can see that this is the case when you uncomment the print command included.

lambdas <- c(1:2)
my.func <- function(lambda1,lambda2){
# print(diag(c(rep(lambda1,2),rep(lambda2,2))))
 det(diag(c(rep(lambda1,2),rep(lambda2,2))))
}
det.summary <- outer(lambdas,lambdas, FUN="my.func")

How do I need to modify my function or the call of outer so things behave like I'd like to?

I guess I need to vectorize my function somehow, but I don't know how, and in which way the outer call would be processed differently.

Edit: I've changed size of the matrices to make it a bit less messy. I'd like to generate 4 diagonal 4 by 4 matrices, with the following diagonals; in are brackets the corresponding parameters lambda1, lambda2:

1 1 1 1 (1,1), 1 1 2 2 (1,2), 2 2 1 1 (2,1), 2 2 2 2 (2,2).

Then, I want to calculate their determinants (which is an arbitrary choice here) and put the results into a matrix, whose first column corresponds to lambda1=1, the second to lambda1=2, and the rows correspond to the choice of lambda2. det.summary should be a 2 by to matrix with the following values:

1 4
4 16

as these are the determinants of the diagonal matrices listed above.

I've read this a couple of times and I still have no idea what you trying to do. Can you show your desired result? — thelatemail

BrodieG BrodieG · Accepted Answer · 2014-01-30T02:30:03

What do you know, there is a Vectorize function (capital "V")!

outer(lambdas,lambdas, Vectorize(my.func))
#      [,1] [,2]
# [1,]    1    4
# [2,]    4   16

As you figured out (and as it took me a while to figure out) outer requires the function to be vectorized. In some ways, it is the opposite of the *pply functions which effectively vectorize an operation by feeding the operator/function each value in turn. But this is easily dealt with, as shown above.

R: Outer, Matrices and vectorizing

1 Answers