How to replace lower/upper triangular elements of a matrix?

Question

My question:

Amat <- diag(4)

I would like to replace all the lower triangular values of Amat (i.e. Amat[2,1],

Answer 1

This is pretty well documented in the docs for upper.tri.

Amat[upper.tri(Amat)] <- NA
Amat
#      [,1] [,2] [,3] [,4]
# [1,]    1   NA   NA   NA
# [2,]    0    1   NA   NA
# [3,]    0    0    1   NA
# [4,]    0    0    0    1

Of course, Amat[lower.tri(Amat)] <- NA would do the same for converting the lower triangle to NAs.