The union of two or more graphs are created. The graphs are assumed to have disjoint vertex sets.

`disjoint_union(...)`x %du% y

x,y

Graph objects.

…

Graph objects or lists of graph objects.

A new graph object.

`disjoint_union`

creates a union of two or more disjoint graphs.
Thus first the vertices in the second, third, etc. graphs are relabeled to
have completely disjoint graphs. Then a simple union is created. This
function can also be used via the %du% operator.

`graph.disjont.union`

handles graph, vertex and edge attributes. In
particular, it merges vertex and edge attributes using the basic `c()`

function. For graphs that lack some vertex/edge attribute, the corresponding
values in the new graph are set to `NA`

. Graph attributes are simply
copied to the result. If this would result a name clash, then they are
renamed by adding suffixes: _1, _2, etc.

Note that if both graphs have vertex names (ie. a `name`

vertex
attribute), then the concatenated vertex names might be non-unique in the
result. A warning is given if this happens.

An error is generated if some input graphs are directed and others are undirected.

# NOT RUN { ## A star and a ring g1 <- make_star(10, mode="undirected") V(g1)$name <- letters[1:10] g2 <- make_ring(10) V(g2)$name <- letters[11:20] str(g1 %du% g2) # }