How to add matrix across the diagonal ("folding it in half") in R?

Question

I need to add one half of my matrix to the other half across the diagonal. In my matrix (shown below), I need the "1" in 63,25 to be added to the "2" in 25,63, and so on for all values in the matrix.

Then I need a way to clear out half of the matrix, either above or below the diagonal.

I tried:

sum(diag(lakes_matrix))

but this did not work.

 25 63 1567 40 50 60 70 80 25 0 2 0 0 0 0 0 0 63 1 0 0 0 0 0 0 0 1567 0 1 0 0 0 0 0 0 40 0 0 1 0 0 0 0 0 50 0 0 0 2 0 0 0 0 60 0 0 0 0 0 0 0 0 70 0 0 0 0 0 1 0 0 80 0 0 0 0 0 0 0 0

Russ Hyde · Accepted Answer · 2019-01-16 19:50:36Z

m <- matrix(1:9, nrow = 3) m [,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9

n <- m + t(m) # add transpose to original n [,1] [,2] [,3] [1,] 2 6 10 [2,] 6 10 14 [3,] 10 14 18

n * upper.tri(n) # clear out the lower diagonal

 [,1] [,2] [,3] [1,] 0 6 10 [2,] 0 0 14 [3,] 0 0 0

So you can make a function

my_func <- function(m) { # do some assertions: m is matrix, square and numeric etc (m + t(m)) * upper.tri(m) }

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