The diag() function in R

Question

Is there a way to use the diag() function in a Matrix without using the built-in function or iteration?

   M<-matrix(1:9, ncol=3) # make a m         


        

Answer 1

You can use the functions row and col to find the indices where the column number is identical to the row number:

row(M) == col(M)
#       [,1]  [,2]  [,3]
# [1,]  TRUE FALSE FALSE
# [2,] FALSE  TRUE FALSE
# [3,] FALSE FALSE  TRUE

M[row(M) == col(M)]
# [1] 1 5 9