Matrix Multiplication in R without using %*% or crossprod

Question

I am trying to multiply two matrix in R without using %*% or crossprod. what i have tried so far

x <- matrix(1:4, ncol = 2)
y <- matrix(5:8, ncol = 2)
MatMul <- function(X,Y)
{

  t(apply(x,1,crossprod,y))

}

MatMul(x,y)

I want to multiply without using crossprod or %*%

I' m totally stuck on this problem since quite some time... Hence any help is extremely welcome.

Answer 1

The only way I could think of solving it, is using embedded for loops (my first ever since 2013...)

The function

MatMult <- function(x, y){
  res <- matrix(NA, dim(x)[1], dim(y)[2])
    for(i in seq_along(y[1, ])){
     for(j in seq_along(x[, 1])){
      res[j, i] <- sum(x[j, ] * y[, i])
     }
    }
  res
 }

Your matrices

x <- matrix(1:4, ncol = 2)
y <- matrix(5:8, ncol = 2)

Testing

MatMult(x, y)
##      [,1] [,2]
## [1,]   23   31
## [2,]   34   46

x%*%y
##      [,1] [,2]
## [1,]   23   31
## [2,]   34   46