Convert a set of rankings to an adjacency matrix summarising wins and losses between pairs of items.

## Arguments

- object
a

`rankings`

object, or an object that can be coerced by`as.rankings`

.- weights
an optional vector of weights for the rankings.

- ...
further arguments passed to/from methods.

## Details

For a `"rankings"`

object based on N items, the adjacency matrix is an
N by N matrix, with element (i, j) being the number of times item i wins over
item j. For example, in the ranking \1\ > \3, 4\ > \2\,
item 1 wins over items 2, 3, and 4, and items 3 and 4 win over item 2.

If `weights`

is specified, the values in the adjacency matrix are the
weighted counts.

## Examples

```
X <- matrix(c(2, 1, 2, 1, 2,
3, 2, 0, 0, 1,
1, 0, 2, 2, 3), nrow = 3, byrow = TRUE)
X <- as.rankings(X)
adjacency(X)
#> 1 2 3 4 5
#> 1 0 0 1 1 1
#> 2 2 0 1 0 1
#> 3 0 0 0 0 1
#> 4 1 0 1 0 2
#> 5 1 1 0 0 0
#> attr(,"class")
#> [1] "adjacency" "matrix"
adjacency(X, weights = c(1, 1, 2))
#> 1 2 3 4 5
#> 1 0 0 2 2 2
#> 2 2 0 1 0 1
#> 3 0 0 0 0 2
#> 4 1 0 1 0 3
#> 5 1 1 0 0 0
#> attr(,"class")
#> [1] "adjacency" "matrix"
```