**graph6** and **sparse6** are formats for storing undirected
graphs in a compact manner, using only printable ASCII characters.
Files in these formats have text type and contain one line per graph.

**graph6** is suitable for small graphs, or large dense graphs.
**sparse6** is more space-efficient for large sparse graphs.

If you really want the formal definition of these formats, the details are available here.

The **gtools** package that comes with
nauty contains many programs for manipulating
graphs in these formats. However, there is also a stand-alone reader
**showg** which can convert them into many other formats,
including some good for human consumption.

**showg** is available as a program written in a portable subset of
C: showg.c.

We also provide some ready-made
executables. Note that a command line interface is required for
**showg** to be used properly.

Linux X86, libc6

Sun SPARC, Solaris 2

Alpha, OSF

*If you can help to make more of these, please
contact me.*

Brief instructions for using **showg** can be obtained by
entering `showg -help`:

Usage: showg [-p#:#l#o#Ftq] [-a|-A|-c|-d|-e] [infile [outfile]] Write graphs in human-readable format. infile is the input file in graph6 or sparse6 format outfile is the output file Defaults are standard input and standard output. -p#, -p#:#, -p#-# : only display one graph or a sequence of graphs. The first graph is number 1. A second number which is empty or zero means infinity. -a : write the adjacency matrix -A : same as -a with a space between entries -d : write output to satisfy dreadnaut -c : write compact dreadnaut form with minimal line-breaks -e : write a list of edges, preceded by the order and the number of edges -o# : specify number of first vertex (default is 0) -t : write upper triangle only (affects -a, -A, -d and default) -F : write a form-feed after each graph except the last -l# : specify screen width limit (default 78, 0 means no limit) This is not currently implemented with -a or -A. -q : suppress auxiliary output -a, -A, -c, -d and -e are incompatible.

We will give an example. The regular graphs or order 8 and degree
2 look like this in **graph6** format:

G?r@`_ G?qa`_ GCQR@O

Suppose you have them in a file called reg82.g6.
Here are some of your options:

List of neighbours: `showg reg82.g6`
output

List of edges: `showg -e reg82.g6`
output

The same starting the numbering at 1: `showg -eo1 reg82.g6`
output

Adjacency matrix: `showg -a reg82.g6`
output

Only the upper triangle: `showg -ta reg82.g6`
output

The same, less verbosely: `showg -taq reg82.g6`
output

The same, just the first two graphs: `showg -tap1:2 reg82.g6`
output

The options can be concatenated as in the examples, or given
separately.

"`-tap1:2`" is the same as "`-t -a -p1:2`".

Page Master: Brendan McKay,
bdm@cs.anu.edu.au and
http://cs.anu.edu.au/~bdm.

Thanks to Gordon Royle for ideas on the design of this page.