Matrix Multiplication in r

Question

I'm trying to multiply each row of a matrix by the column of another matrix. For example:

mat1 <- matrix(rnorm(10), nrow=5, ncol=2)
mat2 <- matrix(rnorm(5), nrow=5)

I want to multiply each row of mat1 by mat2. The desired shape of the output is 5*2.

Answer 1

You could just use apply() to multiply each column of mat1 by mat2. (The "*" will carry out R's typically vectorized element-wise multiplication of the two equal-length vectors).

apply(mat1, 2, "*", mat2)
           [,1]       [,2]
[1,]  0.1785476  0.4175557
[2,]  0.2644247 -0.3745997
[3,] -0.5328542  0.8945527
[4,] -2.7351502 -0.7715341
[5,] -0.9719129 -0.1346929

Or better yet, convert mat1 to a vector to take advantage of R's recycling rules:

mat2 <- matrix(1:10, ncol=2)
mat1 <- matrix(1:5, ncol=1)

as.vector(mat1)*mat2
     [,1] [,2]
[1,]    1    6
[2,]    4   14
[3,]    9   24
[4,]   16   36
[5,]   25   50

Answer 2

Your first matrix has five rows and two columns; your second matrix has five rows and one column. If they have the same number of rows and the second always has one column you can do

mat1 * rep(mat2,ncol(mat1))
           [,1]        [,2]
[1,] -0.2327958  0.76093047
[2,] -0.3636661 -0.18991299
[3,] -0.8729468  0.58214118
[4,]  0.8017349 -0.59781909
[5,] -0.2230380 -0.08296606

If mat1 actually had as many elements in its rows as mat2had in its single column (as your words suggest) you would adjust this slightly

mat1 <- matrix(rnorm(10), nrow=2, ncol=5)
mat2 <- matrix(rnorm(5), nrow=5, ncol=1)
mat1 * rep(mat2,nrow(mat1))
            [,1]        [,2]       [,3]        [,4]       [,5]
[1,] -0.19818805 -0.05938007 -1.7792597  0.06937307 -0.7193403
[2,] -0.05087793  0.10781853  0.2243285 -0.11416273  2.4063926

or in sarah's version

mat1 <- matrix(rnorm(10), nrow=5, ncol=2)
mat2 <- matrix(rnorm(2), nrow=2, ncol=1)
mat1 * rep(mat2,nrow(mat1))
           [,1]        [,2]
[1,]  0.1528393  0.68646359
[2,]  0.2420454  0.22987250
[3,] -0.2592124 -0.07626098
[4,]  0.4431273  0.27320838
[5,] -0.1698307  0.47578667