# isomorphism

Compute isomorphism between two graphs

## Description

computes a graph isomorphism
equivalence relation between graphs `P`

= isomorphism(`G1,G2`

)`G1`

and `G2`

,
if one exists. If no isomorphism exists, then `P`

is an empty
array.

specifies additional options with one or more name-value pair arguments. For
example, you can specify `P`

= isomorphism(___,`Name,Value`

)`'NodeVariables'`

and a list of node
variables to indicate that the isomorphism must preserve these variables to be
valid.

## Examples

### Compute Isomorphism Permutation

Create and plot two directed graphs, and then calculate the isomorphism relation between them.

G1 = digraph([1 1 1 2 3 4],[2 3 4 4 4 1]); G2 = digraph([3 3 3 2 1 4],[1 4 2 3 2 2]); subplot(1,2,1) plot(G1) subplot(1,2,2) plot(G2)

p = isomorphism(G1,G2)

`p = `*4×1*
3
1
4
2

The result indicates that `reordernodes(G2,p)`

has the same structure as `G1`

.

### Compute Isomorphism Between Graphs with Different Labels and Layouts

Create and plot two graphs, `G1`

and `G2`

.

G1 = graph([1 1 1 2 2 3 3 4 5 5 7 7],[2 4 5 3 6 4 7 8 6 8 6 8]); plot(G1,'XData',[1 4 4 1 2 3 3 2],'YData',[4 4 1 1 3 3 2 2])

G2 = graph({'a' 'a' 'a' 'b' 'b' 'b' 'c' 'c' 'c' 'd' 'd' 'd'}, ... {'g' 'h' 'i' 'g' 'h' 'j' 'g' 'i' 'j' 'h' 'i' 'j'}); plot(G2,'XData',[1 2 2 2 1 2 1 1],'YData',[4 4 3 2 3 1 2 1])

Compute the isomorphism relation between the graphs, if one exists. The result indicates that the graph nodes can be permuted to represent the same graph despite their different labels and layouts.

p = isomorphism(G1,G2)

`p = `*8×1*
1
2
5
3
4
7
6
8

### Compute Isomorphism and Preserve Node Properties

Compute two different isomorphism relations between two graphs. One of the relations preserves a node property, while the other ignores it.

Create two similar graphs. Add a node property `Color`

to each of the graphs.

G1 = graph({'d' 'e' 'f'},{'e' 'f' 'd'}); G1.Nodes.Color = {'blue' 'red' 'red'}'; G2 = graph({'a' 'b' 'c'},{'b' 'c' 'a'}); G2.Nodes.Color = {'red' 'red' 'blue'}';

Plot the graphs side-by-side in the same figure. Color the nodes red that have `Color = 'red'`

.

subplot(1,2,1) p1 = plot(G1); highlight(p1,{'e' 'f'},'NodeColor','r') subplot(1,2,2) p2 = plot(G2); highlight(p2,{'a' 'b'},'NodeColor','r')

Compute the isomorphism between the graphs, ignoring the `Color`

property.

p = isomorphism(G1,G2)

`p = `*3×1*
1
2
3

Compute the isomorphism again, but this time preserve the value of the `Color`

property in the comparison. `isomorphism`

returns a different permutation that preserves the `Color`

property.

p = isomorphism(G1,G2,'NodeVariables','Color')

`p = `*3×1*
3
1
2

View the nodes in `G1`

and `G2`

that the isomorphism matches together.

[G1.Nodes.Name, G2.Nodes.Name(p)]

`ans = `*3x2 cell*
{'d'} {'c'}
{'e'} {'a'}
{'f'} {'b'}

## Input Arguments

`G1,G2`

— Input graphs (as separate arguments)

`graph`

objects | `digraph`

objects

### Name-Value Arguments

Specify optional pairs of arguments as
`Name1=Value1,...,NameN=ValueN`

, where `Name`

is
the argument name and `Value`

is the corresponding value.
Name-value arguments must appear after other arguments, but the order of the
pairs does not matter.

*
Before R2021a, use commas to separate each name and value, and enclose*
`Name`

*in quotes.*

**Example: **```
P = isomorphism(G1,G2,'NodeVariables',{'Var1'
'Var2'})
```

`EdgeVariables`

— Edge variables to preserve

character vector | string scalar | cell array of character vectors | string array

Edge variables to preserve, specified as the comma-separated pair
consisting of `'EdgeVariables'`

and a character vector,
string scalar, cell array of character vectors, or string array. Use
this option to specify one or more edge variables that are in both
`G1.Edges`

and `G2.Edges`

. The
isomorphism must preserve the specified edge variables in order to be
valid.

If `G`

is a multigraph, then you can specify the
second output `edgeperms`

to enable reordering edge
variables.

**Data Types: **`char`

| `string`

| `cell`

`NodeVariables`

— Node variables to preserve

character vector | string scalar | cell array of character vectors | string array

Node variables to preserve, specified as the comma-separated pair
consisting of `'NodeVariables'`

and a character vector,
string scalar, cell array of character vectors, or string array. Use
this option to specify one or more node variables that are in both
`G1.Nodes`

and `G2.Nodes`

. The
isomorphism must preserve the specified node variables in order to be
valid.

**Data Types: **`char`

| `string`

| `cell`

## Output Arguments

`P`

— Permutation vector for isomorphism

column vector | `[]`

Permutation vector for isomorphism, returned as a column vector when an
isomorphism exists or as the empty array `[]`

when an
isomorphism does not exist. If `P`

is not empty, then
`reordernodes(G2,P)`

has the same structure as
`G1`

.

`edgeperm`

— Edge permutation

column vector

Edge permutation, returned as a column vector. When working with
multigraphs, the edge permutation vector enables you to preserve edge
variables specified by the `'EdgeVariables'`

name-value
pair. Use these commands to reorder the edge variables of repeated
edges:

[p,edgeperm] = isomorphism(g1,g2,'EdgeVariables',edgevars); g2perm = reordernodes(g2, p); g2perm.Edges(:, 2:end) = g2perm.Edges(edgeperm, 2:end);

## More About

### Graph Isomorphism

Two graphs, `G1`

and `G2`

, are
isomorphic if there exists a permutation of the nodes `P`

such that
`reordernodes(G2,P)`

has the same structure as
`G1`

.

Two graphs that are isomorphic have similar structure. For example, if a graph contains one cycle, then all graphs isomorphic to that graph also contain one cycle.

## Version History

**Introduced in R2016b**

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