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

Answer 1

mat1%%mat2 Actuall y works , this gives [ 16 9 11 ] but you want mat1 %% t(mat2). This means transpose of second matrix, then u can get [11 13 10 ]

Rcode:

mat1 = matrix(c(1,2,3),nrow=1,ncol=3,byrow=TRUE)
mat2 = matrix(c(3,1,2,2,1,3,3,2,1), nrow=3,ncol=3,byrow=TRUE)
print(mat1)
print(mat2 )
#matrix Multiplication
print(mat1 %*% mat2 )
# matrix multiply with second matrix with transpose
# Note of using  function t()
print(mat1 %*% t(mat2 ))

Answer 2

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

Answer 3

It's difficult to say what the best answer here is because the notation in the question isn't in R, it's in matlab. It's hard to tell if the questioner wants to multiple a vector, 1 row matrix, or 1 column matrix given the mixed notation.

An alternate answer to this question is simply switch the order of the multiplication.

v1 <- c(1,2,3)
v2 <- matrix(c(3,1,2,2,1,3,3,2,1), ncol = 3, byrow = TRUE)
v2 %*% v1

This yields an answer that's a single column rather than a single row matrix.