Transforming Matrix in R

Question

I currently face issues transforming the matrix to the format below. How do I achieve this in R in the easiest way? Ideally, I would like to use the second matrix as a data frame. Many thanks in advance!

      Estonia Germany Poland
Estonia 0       2       3
Germany 2       0       4
 Poland  3      4       0


Country1      Country2     Weight
Estonia       Estonia        0
Estonia       Germany        2
Estonia       Poland         3
Germany       Estonia        2
...

Answer 1

If the matrix is called mat you can use :

library(tibble)
library(tidyr)

mat %>%
  as.data.frame() %>%
  rownames_to_column('Country1') %>%
  pivot_longer(cols = -Country1, names_to = 'Country2', values_to = 'Weight')

#  Country1 Country2 Weight
#  <chr>    <chr>     <int>
#1 Estonia  Estonia       0
#2 Estonia  Germany       2
#3 Estonia  Poland        3
#4 Germany  Estonia       2
#5 Germany  Germany       0
#6 Germany  Poland        4
#7 Poland   Estonia       3
#8 Poland   Germany       4
#9 Poland   Poland        0

data

mat <- structure(c(0L, 2L, 3L, 2L, 0L, 4L, 3L, 4L, 0L), .Dim = c(3L, 
3L), .Dimnames = list(c("Estonia", "Germany", "Poland"), c("Estonia", 
"Germany", "Poland")))