igraph (version 0.7.1)

graph.formula: Creating (small) graphs via a simple interface

Description

This function is useful if you want to create a small (named) graph quickly, it works for both directed and undirected graphs.

Usage

graph.formula(..., simplify = TRUE)

Arguments

...

The formulae giving the structure of the graph, see details below.

simplify

Logical scalar, whether to call simplify on the created graph. By default the graph is simplified, loop and multiple edges are removed.

Value

• A new graph object.

Details

graph.formula is very handy for creating small graphs quickly. You need to supply one or more R expressions giving the structure of the graph. The expressions consist of vertex names and edge operators. An edge operator is a sequence of - and + characters, the former is for the edges and the latter is used for arrow heads. The edges can be arbitrarily long, ie. you may use as many - characters to draw them as you like.

If all edge operators consist of only - characters then the graph will be undirected, whereas a single + character implies a directed graph.

Let us see some simple examples. Without arguments the function creates an empty graph: graph.formula()

A simple undirected graph with two vertices called A and B and one edge only: graph.formula(A-B)

Remember that the length of the edges does not matter, so we could have written the following, this creates the same graph: graph.formula( A-----B )

If you have many disconnected components in the graph, separate them with commas. You can also give isolate vertices. graph.formula( A--B, C--D, E--F, G--H, I, J, K )

The : operator can be used to define vertex sets. If an edge operator connects two vertex sets then every vertex from the first set will be connected to every vertex in the second set. The following form creates a full graph, including loop edges: graph.formula( A:B:C:D -- A:B:C:D )

In directed graphs, edges will be created only if the edge operator includes a arrow head (+) at the end of the edge: graph.formula( A -+ B -+ C ) graph.formula( A +- B -+ C ) graph.formula( A +- B -- C ) Thus in the third example no edge is created between vertices B and C.

Mutual edges can be also created with a simple edge operator: graph.formula( A +-+ B +---+ C ++ D + E) Note again that the length of the edge operators is arbitrary, +, ++ and +-----+ have exactly the same meaning.

If the vertex names include spaces or other special characters then you need to quote them: graph.formula( "this is" +- "a silly" -+ "graph here" ) You can include any character in the vertex names this way, even + and - characters.

See more examples below.

See Also

graph for more general graph creation methods.

Examples

# A simple undirected graph
g <- graph.formula( Alice-Bob-Cecil-Alice, Daniel-Cecil-Eugene, Cecil-Gordon )
g

# Another undirected graph, ":" notation
g2 <- graph.formula( Alice-Bob:Cecil:Daniel, Cecil:Daniel-Eugene:Gordon )
g2

# A directed graph
g3 <- graph.formula( Alice +-+ Bob --+ Cecil +-- Daniel,
                     Eugene --+ Gordon:Helen )
g3

# A graph with isolate vertices
g4 <- graph.formula( Alice -- Bob -- Daniel, Cecil:Gordon, Helen )
g4
V(g4)$name

# "Arrows" can be arbitrarily long
g5 <- graph.formula( Alice +---------+ Bob )
g5

# Special vertex names
g6 <- graph.formula( "+" -- "-", "*" -- "/", "%%" -- "%/%" )
g6