Best way to allocate matrix in R, NULL vs NA?

Question

I am writing R code to create a square matrix. So my approach is:

1. Allocate a matrix of the correct size
2. Loop through each element of my matrix and fil

Answer 1

rows<-3
cols<-3    
x<-rep(NA, rows*cols)
x1 <- matrix(x,nrow=rows,ncol=cols)