Extract elements from matrix diagonal saved in multiple lists in R

Question

I´m trying to get different elements from multiple diagonal saved as lists. My data looks something like this:

res <- list()
res[[1]] <- matrix(c(0.04770856,0.02854005,0.02854005,0.03260190), nrow=2, ncol=2)
res[[2]] <- matrix(c(0.05436957,0.04887182,0.04887182, 0.10484454), nrow=2, ncol=2)

> res
[[1]]
           [,1]       [,2]
[1,] 0.04770856 0.02854005
[2,] 0.02854005 0.03260190

[[2]]
           [,1]       [,2]
[1,] 0.05436957 0.04887182
[2,] 0.04887182 0.10484454

> diag(res[[1]])
[1] 0.04770856 0.03260190
> diag(res[[2]])
[1] 0.05436957 0.10484454

I would like to save the first and second elements of each diagonal of a given list into a vector similar to this:

d.1st.el <- c(0.04770856, 0.05436957)
d.2nd.el <- c(0.03260190, 0.10484454)

My issue is to write the function that runs for all given lists and get the diagonals. For some reason, when I use unlist() to extract the values of each matrix for a given level, it doesn't get me the number but the full matrix.

Does anyone have a simple solution?

Answer 1

sapply(res, diag)

           [,1]       [,2]
[1,] 0.04770856 0.05436957
[2,] 0.03260190 0.10484454

# or
lapply(res, diag)
[[1]]
[1] 0.04770856 0.03260190

[[2]]
[1] 0.05436957 0.10484454

If you want the vectors for some reason in your global environment:

alld <- lapply(res, diag)
names(alld) <- sprintf("d.%d.el", 1:length(alld))
list2env(alld, globalenv())

Answer 2

In two steps you can do:

# Step 1 - Get the diagonals
all_diags <- sapply(res, function(x) diag(t(x)))

print(all_diags)

           [,1]       [,2]
[1,] 0.04770856 0.05436957
[2,] 0.03260190 0.10484454

# Step 2 - Append to vectors

d.1st.el <- all_diags[1,]
d.2nd.el <- all_diags[2,]