matrix - Multiplying two matrices in R

Question

I have 2 matrices.

The first one: [1,2,3]

and the second one:

[3,1,2
 2,1,3
 3,2,1]

I'm looking for a way to multiply them.

The result is supposed to be: [11, 13, 10]

In R, mat1%*%mat2 don't work.

Answer 1

You need the transpose of the second matrix to get the result you wanted:

> v1 <- c(1,2,3)
> v2 <- matrix(c(3,1,2,2,1,3,3,2,1), ncol = 3, byrow = TRUE)
> v1 %*% t(v2)
     [,1] [,2] [,3]
[1,]   11   13   10

Or potentially quicker (see ?crossprod) if the real problem is larger:

> tcrossprod(v1, v2)
     [,1] [,2] [,3]
[1,]   11   13   10