问题
May be this is too simple of a question but I couldn't find a functional answer. How can we extract the opposite diagonal elements of any square matrix in R? In the example below that would be: 7, 2, 8.
r <- matrix(c(1, 5, 8, 1:3, 7:9), 3)
回答1:
An approach could be
r[(n<-nrow(r))^2-(1:n)*(n-1)]
# [1] 7 2 8
## microbenchmark (matrix(1:1e6,1000))
# Unit: microseconds
# expr min lq mean median uq max neval
# r[(n<-nr... 26.897 39.0075 65.36835 47.309 85.9345 316.97 100
# diag(r[,... 18070.388 18905.3475 20237.09599 19956.615 20423.4695 27798.88 100
# rev(r[ro... 14220.609 21206.7220 21238.59515 22036.275 22599.4490 33252.58 100
回答2:
diag(r[,rev(sequence(NCOL(r)))])
#OR
rev(r[row(r) == NCOL(r) - col(r) + 1])
#OR
rev(r[(row(r) + col(r)) == (nrow(r) + 1)])
#[1] 7 2 8
回答3:
We can also generate an index to extract those elements
n <- nrow(r)
r[seq(n, by = n-1, length = n)]
#[1] 8 2 7
If the order is important we can reverse the extracted elements.
rev(r[seq(n, by = n-1, length = n)])
#[1] 7 2 8
回答4:
You can use matrix subsetting like:
r[matrix(c(1:nrow(r), ncol(r):1), ncol=2)]
#[1] 7 2 8
or vector subsetting like
r[1:nrow(r) + (ncol(r):1-1)*nrow(r)]
#[1] 7 2 8
来源：https://stackoverflow.com/questions/58277528/extract-opposite-diagonal-not-off-diagonal-elements-of-a-matrix