In weighted graphs, a real number is assigned to each (directed or undirected) edge.
is_weighted(graph)A logical scalar.
In igraph edge weights are represented via an edge attribute, called
‘weight’. The is_weighted() function only checks that such an
attribute exists. (It does not even checks that it is a numeric edge
attribute.)
Edge weights are used for different purposes by the different functions. E.g. shortest path functions use it as the cost of the path; community finding methods use it as the strength of the relationship between two vertices, etc. Check the manual pages of the functions working with weighted graphs for details.
g <- make_ring(10)
shortest_paths(g, 8, 2)
#> $vpath
#> $vpath[[1]]
#> + 5/10 vertices, from a486f9c:
#> [1] 8 9 10 1 2
#>
#>
#> $epath
#> NULL
#>
#> $predecessors
#> NULL
#>
#> $inbound_edges
#> NULL
#>
E(g)$weight <- seq_len(ecount(g))
shortest_paths(g, 8, 2)
#> $vpath
#> $vpath[[1]]
#> + 7/10 vertices, from a486f9c:
#> [1] 8 7 6 5 4 3 2
#>
#>
#> $epath
#> NULL
#>
#> $predecessors
#> NULL
#>
#> $inbound_edges
#> NULL
#>