a vector to an upper Triangle matrix by row in R

Question

I have a vector say

a = c(1,2,3,4,5,6) 

I would like to organize them into the elements into an upper triangle matrix (without considering

Answer 1

Note that to fill an ASYMMETRIC matrix you could first fill the upper triangle via the code shown above, then the lower with a DIFFERENT vector (no transpose needed).

  c <- c(7,8,9,10,11,12)
  b[lower.tri(b, diag=FALSE)] <- c