subgraph

Extract subgraph

Syntax

H = subgraph(G,nodeIDs)

H = subgraph(G,idx)

Description

H = subgraph(G,nodeIDs) returns a subgraph of G that contains only the nodes specified by nodeIDs.

H = subgraph(G,idx) specifies the subgraph nodes using a logical vector.

Examples

Extract Subgraph

Create and plot a graph.

s = [1 1 1 1 2 2 2 2 2 2 2 2 2 2 15 15 15 15 15];
t = [3 5 4 2 14 6 11 12 13 10 7 9 8 15 16 17 19 18 20];
G = graph(s,t);
plot(G,'Layout','force')

Extract a subgraph from G by specifying which nodes to include. The node numbering in the subgraph is reset.

idx = [2 15 16 17 18 19 20 1 3 4 5];
H = subgraph(G,idx);
plot(H,'Layout','force')

Subgraph with Node and Edge Properties

Create and plot a weighted graph with named nodes.

s = [1 1 1 2 2 2 8 8 8 8];
t = [2 3 4 5 6 7 9 10 11 12];
weights = [10 30 40 80 60 60 20 30 90 80];
names = {'A' 'B' 'C' 'D' 'E' 'F' 'G' 'H' 'I' 'J' 'K' 'L'};
G = graph(s,t,weights,names);
plot(G,'EdgeLabel',G.Edges.Weight)

Extract a subgraph that contains node 'B' and all of its neighbors. subgraph preserves the node names and edge weights. However, the numeric node IDs in H are renumbered compared to G.

N = neighbors(G,'B');
H = subgraph(G, ['B'; N]);
plot(H,'EdgeLabel',H.Edges.Weight)

Input Arguments

G — Input graph

graph object | digraph object

Input graph, specified as either a graph or digraph object. Use graph to create an undirected graph or digraph to create a directed graph.

Example: G = graph(1,2)

Example: G = digraph([1 2],[2 3])

nodeIDs — Node identifiers

node indices | node names

Node identifiers, specified as one or more node indices or node names. nodeIDs selects a subset of the nodes in G to generate the subgraph, H.

This table shows the different ways to refer to one or more nodes either by their numeric node indices or by their node names.

Form	Single Node	Multiple Nodes
Node index	Scalar Example: 1	Vector Example: [1 2 3]
Node name	Character vector Example: 'A'	Cell array of character vectors Example: {'A' 'B' 'C'}
	String scalar Example: "A"	String array Example: ["A" "B" "C"]

Example: H = subgraph(G,[1 2 5])

Example: H = subgraph(G,{'A' 'B' 'E'})

idx — Node selection vector

vector

Node selection vector, specified as a logical vector. The subgraph contains only the nodes J for which idx(J) is logical 1 (true). The index of node J in H is I(J), where I = find(idx).

Example: subgraph(G,degree(G)>2)

Data Types: logical

Output Arguments

H — Subgraph

graph object | digraph object

Subgraph, returned as a graph or digraph object. H contains only the nodes that were selected with nodeIDs or idx. Other nodes in G (and the edges connecting to those nodes) are discarded. The node properties and edge properties of the selected nodes and edges are carried over from G into H.

See graph or digraph for more information about graph objects.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The node identifiers nodeIDs must be node indices of the input graph G. Node names are not supported.

Thread-Based Environment
Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

Version History

Introduced in R2015b