de_xtr de_xtr - 1 month ago 6
R Question

Want to know what this mathematical operator does in R - %*%

I am new to R programming and while reading a book I came across this mathematical operation involving a matrix in R - "%*%". I could not understand what the following operation does. Can someone help me please?

> y
[,1] [,2]
[1,] 1 3
[2,] 2 4
> y%*%y
[,1] [,2]
[1,] 7 15
[2,] 10 22
>

Answer

%*% multiplies two matrices.

In your example:

        [,1]        [,2]
 [1,]   1*1+3*2   1*3+3*4
 [2,]   2*1+4*2   2*3+4*4 

I.e. [1,1] is a result of sum-product of first row elements (1 and 3) with first column elements (1 and 2): 1*1+3*2.