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



x %du% y



Graph objects or lists of graph objects.

x, y

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 (i.e. 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.


Gabor Csardi


## 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]
print_all(g1 %du% g2)
#> IGRAPH 3285bde UN-- 20 19 -- 
#> + attr: name_1 (g/c), name_2 (g/c), mode (g/c), center (g/n), mutual
#> | (g/l), circular (g/l), name (v/c)
#> + edges from 3285bde (vertex names):
#>  [1] a--b a--c a--d a--e a--f a--g a--h a--i a--j k--l l--m m--n n--o o--p p--q
#> [16] q--r r--s s--t k--t