R: Add vertices, edges or another graph to a graph

+.igraph {igraph} | R Documentation

## Add vertices, edges or another graph to a graph

### Description

Add vertices, edges or another graph to a graph

### Usage

    ## S3 method for class 'igraph'
    e1 + e2

### Arguments

`e1` | First argument, probably an igraph graph, but see details below.
`e2` | Second argument, see details below.

### Details

The plus operator can be used to add vertices or edges to graph. The actual operation that is performed depends on the type of the right hand side argument.

- If is is another igraph graph object and they are both named graphs, then the union of the two graphs are calculated, see `union`.

- If it is another igraph graph object, but either of the two are not named, then the disjoint union of the two graphs is calculated, see `disjoint_union`.

- If it is a numeric scalar, then the specified number of vertices are added to the graph.

- If it is a character scalar or vector, then it is interpreted as the names of the vertices to add to the graph.

- If it is an object created with the `vertex` or `vertices` function, then new vertices are added to the graph. This form is appropriate when one wants to add some vertex attributes as well. The operands of the `vertices` function specifies the number of vertices to add and their attributes as well.

  The unnamed arguments of `vertices` are concatenated and used as the ‘`name`’ vertex attribute (i.e. vertex names), the named arguments will be added as additional vertex attributes. Examples:

        g <- g +
              vertex(shape="circle", color= "red")
        g <- g + vertex("foo", color="blue")
        g <- g + vertex("bar", "foobar")
        g <- g + vertices("bar2", "foobar2", color=1:2, shape="rectangle")

  `vertex` is just an alias to `vertices`, and it is provided for readability. The user should use it if a single vertex is added to the graph.

- If it is an object created with the `edge` or `edges` function, then new edges will be added to the graph. The new edges and possibly their attributes can be specified as the arguments of the `edges` function.

  The unnamed arguments of `edges` are concatenated and used as vertex ids of the end points of the new edges. The named arguments will be added as edge attributes.

  Examples:

        g <- make_empty_graph() +
               vertices(letters[1:10]) +
               vertices("foo", "bar", "bar2", "foobar2")
        g <- g + edge("a", "b")
        g <- g + edges("foo", "bar", "bar2", "foobar2")
        g <- g + edges(c("bar", "foo", "foobar2", "bar2"), color="red", weight=1:2)

  See more examples below.

  `edge` is just an alias to `edges` and it is provided for readability. The user should use it if a single edge is added to the graph.

- If it is an object created with the `path` function, then new edges that form a path are added. The edges and possibly their attributes are specified as the arguments to the `path` function. The non-named arguments are concatenated and interpreted as the vertex ids along the path. The remaining arguments are added as edge attributes.

  Examples:

        g <- make_empty_graph() + vertices(letters[1:10])
        g <- g + path("a", "b", "c", "d")
        g <- g + path("e", "f", "g", weight=1:2, color="red")
        g <- g + path(c("f", "c", "j", "d"), width=1:3, color="green")

It is important to note that, although the plus operator is commutative, i.e. is possible to write

      graph <- "foo" + make_empty_graph()

it is not associative, e.g.

      graph <- "foo" + "bar" + make_empty_graph()

results a syntax error, unless parentheses are used:

      graph <- "foo" + ( "bar" + make_empty_graph() )

For clarity, we suggest to always put the graph object on the left hand side of the operator:

      graph <- make_empty_graph() + "foo" + "bar"

### See Also

Other functions for manipulating graph structure: `add_edges()`, `add_vertices()`, `delete_edges()`, `delete_vertices()`, `edge()`, `igraph-minus`, `path()`, `vertex()`

### Examples

    # 10 vertices named a,b,c,... and no edges
    g <- make_empty_graph() + vertices(letters[1:10])

    # Add edges to make it a ring
    g <- g + path(letters[1:10], letters[1], color = "grey")

    # Add some extra random edges
    g <- g + edges(sample(V(g), 10, replace = TRUE), color = "red")
    g$layout <- layout_in_circle
    plot(g)

[Package igraph version 1.2.11 Index]