This function calculates the line graph of another graph.
Details
The line graph L(G) of a G undirected graph is defined as
follows. L(G) has one vertex for each edge in G and two
vertices in L(G) are connected by an edge if their corresponding
edges share an end point.
The line graph L(G) of a G directed graph is slightly
different, L(G) has one vertex for each edge in G and two
vertices in L(G) are connected by a directed edge if the target of
the first vertex's corresponding edge is the same as the source of the
second vertex's corresponding edge.
Author
Gabor Csardi csardi.gabor@gmail.com, the first version of the C code was written by Vincent Matossian.
Examples
# generate the first De-Bruijn graphs
g <- make_full_graph(2, directed = TRUE, loops = TRUE)
make_line_graph(g)
#> IGRAPH ce316a9 D--- 4 8 -- Line graph
#> + attr: name (g/c)
#> + edges from ce316a9:
#> [1] 1->1 3->1 1->2 3->2 2->3 4->3 2->4 4->4
make_line_graph(make_line_graph(g))
#> IGRAPH 9f0837b D--- 8 16 -- Line graph
#> + attr: name (g/c)
#> + edges from 9f0837b:
#> [1] 1->1 2->1 5->2 6->2 1->3 2->3 5->4 6->4 3->5 4->5 7->6 8->6 3->7 4->7 7->8
#> [16] 8->8
make_line_graph(make_line_graph(make_line_graph(g)))
#> IGRAPH 23e5ec8 D--- 16 32 -- Line graph
#> + attr: name (g/c)
#> + edges from 23e5ec8:
#> [1] 1-> 1 2-> 1 3-> 2 4-> 2 9-> 3 10-> 3 11-> 4 12-> 4 1-> 5 2-> 5
#> [11] 3-> 6 4-> 6 9-> 7 10-> 7 11-> 8 12-> 8 5-> 9 6-> 9 7->10 8->10
#> [21] 13->11 14->11 15->12 16->12 5->13 6->13 7->14 8->14 13->15 14->15
#> [31] 15->16 16->16