diag(vector)[length(vector):1,]

# create a vector with 5 elements
vector1 = c(1, 2, 3, 4, 5)
 
# display anti-diagonal matrix
print(diag(vector1)[length(vector1):1, ])

     [,1] [,2] [,3] [,4] [,5]
[1,]    0    0    0    0    5
[2,]    0    0    0    4    0
[3,]    0    0    3    0    0
[4,]    0    2    0    0    0
[5,]    1    0    0    0    0

# create a vector with 2 elements
vector1 = c(1, 2.5)
 
# display anti-diagonal matrix
print(diag(vector1)[length(vector1):1, ])

     [,1] [,2]
[1,]    0  2.5
[2,]    1  0.0