Creates a list of connected nodes following the graph representation of a spatial connectivity matrix.

`edges(C, unique_pairs_only = TRUE)`

## Arguments

- C
A connectivity matrix where connection between two nodes is indicated by non-zero entries.

- unique_pairs_only
By default, only unique pairs of nodes (i, j) will be included in the output.

## Value

Returns a `data.frame`

with three columns. The first two columns (`node1`

and `node2`

) contain the indices of connected pairs of nodes; only unique pairs of nodes are included (unless `unique_pairs_only = FALSE`

). The third column (`weight`

) contains the corresponding matrix element, `C[node1, node2]`

.

## Details

This is used internally for `stan_icar`

and it is also helpful for creating the scaling factor for BYM2 models fit with `stan_icar`

.