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

Here's one option

b[lower.tri(b, diag=FALSE)] <- a
b <- t(b)
b
#      [,1] [,2] [,3] [,4]
# [1,]    0    1    2    3
# [2,]    0    0    4    5
# [3,]    0    0    0    6
# [4,]    0    0    0    0

Alternatively, reorder a as required and assign that into the upper-right triangle:

ut <- upper.tri(b, diag=FALSE)
b[ut] <- a[order(row(ut)[ut], col(ut)[ut])]
b
     [,1] [,2] [,3] [,4]
[1,]    0    1    2    3
[2,]    0    0    4    5
[3,]    0    0    0    6
[4,]    0    0    0    0

Answer 2

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