In R creating vector from specific matrix (dataframe) elements

Question

I want to form a vector from some specific elements of matrix (or dataframe).

I would like to create vector from for example minor diagonal elements from this matrix (e.g elements matrix[3,1], matrix[2,2] and matrix [1,3]).

How could I do it without looping? My task is quite big and I would like skip looping. Following command:

matrix[c(3, 2, 1), c(1, 2, 3)]

instead of vector c(3, 5, 7) gives me another matrix.

Answer 1

Either of these will do what you want:

x <- matrix(1:9, 3, 3)
x
#      [,1] [,2] [,3]
# [1,]    1    4    7
# [2,]    2    5    8
# [3,]    3    6    9
rc <- seq(ncol(x))
diag(x[rev(rc),])
# [1] 3 5 7
x[cbind(rev(rc), rc)]
# [1] 3 5 7

Answer 2

try this

mat <- matrix(1:9, nrow = 3, byrow = T)
diag(apply(mat,2,rev))

Answer 3

Your matrix:

mymatrix <- matrix(data= c(1,2,3,4,5,6,7,8,9), nrow = 3, byrow = T)

#     [,1] [,2] [,3]
#[1,]    1    2    3
#[2,]    4    5    6
#[3,]    7    8    9

you can get the diagonal like this:

diag(mymatrix)
# [1] 1 5 9