# graph.constructors

##### Various methods for creating graphs

These method can create various (mostly regular) graphs: empty graphs, graphs with the given edges, graphs from adjacency matrices, star graphs, lattices, rings, trees.

- Keywords
- graphs

##### Usage

```
graph.empty(n=0, directed=TRUE)
graph(edges, n=max(edges), directed=TRUE)
graph.star(n, mode = c("in", "out", "mutual", "undirected"), center = 1)
graph.lattice(dimvector = NULL, length = NULL, dim = NULL, nei = 1,
directed = FALSE, mutual = FALSE, circular = FALSE, ...)
graph.ring(n, directed = FALSE, mutual = FALSE, circular=TRUE)
graph.tree(n, children = 2, mode=c("out", "in", "undirected"))
graph.full(n, directed = FALSE, loops = FALSE)
graph.full.citation(n, directed = TRUE)
graph.atlas(n)
graph.edgelist(el, directed=TRUE)
graph.extended.chordal.ring(n, w)
```

##### Arguments

- edges
- Numeric vector defining the edges, the first edge points from the first element to the second, the second edge from the third to the fourth, etc.
- directed
- Logical, if TRUE a directed graph will be
created. Note that for while most constructors the default is TRUE,
for
`graph.lattice`

and`graph.ring`

it is FALSE. For`graph.star`

the`mode`

argument sh - n
- The number of vertices in the graph for most functions.
For

`graph`

this parameter is ignored if there is a bigger vertex id in`edges`

. This means that for this function it is safe to supply zero here if the vertex with - mode
- For
`graph.star`

it defines the direction of the edges,`in`

: the edges point*to*the center,`out`

: the edges point*from*the center,`mutual`

: a directed star is created with mutual - center
- For
`graph.star`

the center vertex of the graph, by default the first vertex. - dimvector
- A vector giving the size of the lattice in each
dimension, for
`graph.lattice`

. - nei
- The distance within which (inclusive) the neighbors on the lattice will be connected. This parameter is not used right now.
- mutual
- Logical, if TRUE directed lattices will be mutually connected.
- circular
- Logical, if TRUE the lattice or ring will be circular.
- length
- Integer constant, for regular lattices, the size of the lattice in each dimension.
- dim
- Integer constant, the dimension of the lattice.
- children
- Integer constant, the number of children of a vertex
(except for leafs) for
`graph.tree`

. - loops
- If TRUE also loops edges (self edges) are added.
- graph
- An object.
- el
- An edge list, a two column matrix, character or numeric. See details below.
- w
- A matrix which specifies the extended chordal ring. See details below.
- ...
- Currently ignored.

##### Details

All these functions create graphs in a deterministic way.

`graph.empty`

is the simplest one, this creates an empty graph.

`graph`

creates a graph with the given edges.

`graph.star`

creates a star graph, in this every single vertex is
connected to the center vertex and nobody else.

`graph.lattice`

is a flexible function, it can create lattices of
arbitrary dimensions, periodic or unperiodic ones. It has two
forms. In the first form you only supply `dimvector`

, but not
`length`

and `dim`

. In the second form you omit
`dimvector`

and supply `length`

and `dim`

.

`graph.ring`

is actually a special case of `graph.lattice`

,
it creates a one dimensional circular lattice.

`graph.tree`

creates regular trees.

`graph.full`

simply creates full graphs.

`graph.full.citation`

creates a full citation graph. This is a
directed graph, where every i->j edge is present if and only if j`graph.atlas`

creates graphs from the book An Atlas of Graphs by
Roland C. Read and Robin J. Wilson. The atlas contains all undirected
graphs with up to seven vertices, numbered from 0 up to 1252. The
graphs are listed:

- in increasing order of number of nodes;
- for a fixed number of nodes, in increasing order of the number of edges;
- for fixed numbers of nodes and edges, in increasing order of the degree sequence, for example 111223 < 112222;
- for fixed degree sequence, in increasing number of automorphisms.

`graph.edgelist`

creates a graph from an edge list. Its argument
is a two-column matrix, each row defines one edge. If it is
a numeric matrix then its elements are interpreted as vertex ids. If
it is a character matrix then it is interpreted as symbolic vertex
names and a vertex id will be assigned to each name, and also a
`name`

vertex attribute will be added.
`graph.extended.chordal.ring`

creates an extended chordal ring.
An extended chordal ring is regular graph, each node has the same
degree. It can be obtained from a simple ring by adding some extra
edges specified by a matrix. Let p denote the number of columns in
the `W`

`i`

are added according to column `i mod p`

in
`W`

`W`

`j`

an edge
`i->i+w[ij]`

is added if `i+w[ij]`

is less than the number
of total nodes. See also Kotsis, G: Interconnection Topologies for
Parallel Processing Systems, PARS Mitteilungen 11, 1-6, 1993.

##### Value

- Every function documented here returns a
`graph`

object.

##### concept

- Tree
- Lattice
- Star graph
- Graph Atlas
- Empty graph
- Full graph

##### See Also

`graph.adjacency`

to create graphs from adjacency
matrices, `graph.formula`

for a handy way to create small
graphs, `graph.data.frame`

for an easy way to create
graphs with many edge/vertex attributes.

##### Examples

```
g1 <- graph.empty()
g2 <- graph( c(1,2,2,3,3,4,5,6), directed=FALSE )
g5 <- graph.star(10, mode="out")
g6 <- graph.lattice(c(5,5,5))
g7 <- graph.lattice(length=5, dim=3)
g8 <- graph.ring(10)
g9 <- graph.tree(10, 2)
g10 <- graph.full(5, loops=TRUE)
g11 <- graph.full.citation(10)
g12 <- graph.atlas(sample(0:1252, 1))
el <- matrix( c("foo", "bar", "bar", "foobar"), nc=2, byrow=TRUE)
g13 <- graph.edgelist(el)
g15 <- graph.extended.chordal.ring(15, matrix(c(3,12,4,7,8,11), nr=2))
```

*Documentation reproduced from package igraph, version 0.7.1, License: GPL (>= 2)*