How to do R multiplication with Nx1 1xM for Matrix NxM?

Question

I want to do a simple column (Nx1) times row (1xM) multiplication, resulting in (NxM) matrix. Code where I create a row by sequence, and column by transposing a similar seq

Answer 1

An easy way to look at this is to transform your vectors to a matrix

row1.mat = matrix(row1)
col1.mat = matrix(col1)

and then use dim to see the dimension of the matrices:

dim(row1.mat)
dim(col1.mat)

If you want the product to work for this you need a 6*1 matrix, multiplied by a 1*6 matrix. so you need to transpose the col1.mat using t(col1.mat).

And as you might know the matrix product is %*%

row1.mat %*% t(col1.mat)

Comparison of this method to others

library("microbenchmark")
x <- runif(1000)
y <- runif(1000)
xx = matrix(x)
yy = matrix(y)
microbenchmark(tcrossprod(x,y),x%*%t(y),outer(x,y), xx %*% t(yy), times=2000)

Unit: milliseconds
             expr      min       lq     mean   median       uq      max neval
 tcrossprod(x, y) 2.829099 3.243785 6.015880 4.801640 5.040636 77.87932  2000
       x %*% t(y) 2.847175 3.251414 5.942841 4.810261 5.049474 86.53374  2000
      outer(x, y) 2.886059 3.277811 5.983455 4.788054 5.074997 96.12442  2000
     xx %*% t(yy) 2.868185 3.255833 6.126183 4.699884 5.056234 87.80024  2000