# subgraph

Extract subgraph

## Syntax

``H = subgraph(G,nodeIDs)``
``H = subgraph(G,idx)``

## Description

example

````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

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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')``` 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

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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])`

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.

FormSingle NodeMultiple 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'})`

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

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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.

## Version History

Introduced in R2015b