CODEWITHSUNDEEP
rvectorization
user3294195
user3294195

Reputation: 1828

Column-wise "outer-product"

I have a 20 x 10 matrix X, and I want to create a 10 x 10 x 20 array A with A[i,j,] = X[,i] * X[,j] for i,j in 1:10. Here is a simple solution using a for loop:

X <- matrix(seq(10*20),20,10)
A <- array(dim=c(10,10,20))
for (i in 1:10)
  for (j in 1:10)
    A[i,j,] <- X[,i] * X[,j]

How do I implement this with vectorization?

Thanks.

Upvotes: 1

Views: 221

Answers (1)

sbstn
sbstn

Reputation: 628

Use outer, is this what you want?

A2 = array(apply(X, 1L, function (x) outer(x,x)), dim=c(10,10,20))

# test
X <- matrix(seq(10*20),20,10)
A <- array(dim=c(10,10,20))
for (i in 1:10){
  for (j in 1:10){
    A[i,j,] <- X[,i] * X[,j]
  }
}

all.equal(A, A2) # TRUE

EDIT: updated answer after question example in question was changed.

Upvotes: 2

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