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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="igraph-Glossary"></a>Chapter 35. Glossary</h1></div></div></div>
<p>
This glossary defines common terms used throughout the igraph
documentation.
</p>
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem"><p>
<span class="strong"><strong>attribute</strong></span>: A piece of data
associated with a vertex, an edge, or the graph itself. The
igraph C library currently supports numeric, string and Boolean
attribute values, and provides a means for implementing
attribute handlers that support custom types.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>adjacent</strong></span>: Two vertices are
called <span class="strong"><strong>adjacent</strong></span> if there is
an edge connecting them. This term describes a vertex-to-vertex
relation.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>adjacency list</strong></span>: A data
structure that associates a list of neighbours (i.e. adjacent
vertices) to each vertex.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>adjacency matrix</strong></span>: A
representation of a graph as a square matrix.
<code class="literal">A_ij</code> gives the number of edge endpoints
connecting from the <code class="literal">i</code>th vertex to the
<code class="literal">j</code>th vertex. Conventionally, the diagonal of
the adjacency matrix of an undirected graph contains
<span class="emphasis"><em>twice</em></span> the number of self-loops. All igraph
functions follow this convention unless noted otherwise.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>biadjacency matrix</strong></span>: Analogous
to the adjacency matrix, but used for bipartite graphs. Element
<code class="literal">B_ij</code> gives the number of edges from the
<code class="literal">i</code>th vertex of the first group to the
<code class="literal">j</code>th vertex of the second group.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>bipartite graph</strong></span>: A graph
whose vertices can be partitioned into two groups in such a way
that connections are present only between members of different
groups.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>complete graph</strong></span>: Also called
<span class="strong"><strong>full graph</strong></span> within the context
of igraph, a graph in which all pairs of vertices are connected
to each other.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>connected graph</strong></span>: A connected
graph consists of a single component, in which any vertex is
reachable from any other. In igraph, the null graph is not
considered connected, as it has not one, but zero components.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>edge</strong></span>: A
<span class="strong"><strong>connection</strong></span> between two
vertices, also called a <span class="strong"><strong>link</strong></span>.
In igraph, edges are referred to by integer indices called
<span class="strong"><strong>edge IDs</strong></span>.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>finalizer stack</strong></span>: A global
stack used internally by igraph to keep track of currently
allocated objects and their destructors, so that they can be
automatically destroyed in case of an error.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>game</strong></span>: Within igraph, this
term is used for stochastic graph generators, i.e. functions
that sample from random graph models.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>graph</strong></span> or
<span class="strong"><strong>network</strong></span>: A set of vertices
with connections between them. In igraph, graphs may carry
associated data in the form of vertex, edge or graph attributes.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>incident</strong></span>: An edge is called
<span class="strong"><strong>incident</strong></span> to the vertices that
are its endpoints. This term describes a vertex-to-edge
relation.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>incidence list</strong></span>: A data
structure that associates a list of incident edges to each
vertex.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>incidence matrix</strong></span>: A matrix
describing the incidence relation between vertices (rows) and
edges (columns).
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>membership vector</strong></span>: Membership
vectors are a means of encoding a partitioning of items, usually
vertices, into several groups. The <code class="literal">i</code>th
element of the vector gives an integer identifier of the group
the <code class="literal">i</code>th vertex belongs to. Membership vectors
are typically used to describe a vertex clustering obtained
through community detection, or by identifying the connected
components of a graph.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>multi-edges</strong></span> or
<span class="strong"><strong>parallel edges</strong></span>: More than one
edge connecting the same two vertices. In a directed graph,
<code class="literal">a -&gt; b, a -&gt; b</code> are considered parallel
edges, but <code class="literal">a -&gt; b, a &lt;- b</code> are not.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>null graph</strong></span>: A graph with no
vertices (and no edges).
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>self-loop</strong></span>,
<span class="strong"><strong>self-edge</strong></span>, or simply
<span class="strong"><strong>loop</strong></span>: An edge that connects a
vertex to itself.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>simple graph</strong></span>: A graph that
does not have self-loops or multi-edges.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>singleton graph</strong></span>: A graph
having a single vertex. This term usually refers to a single
vertex with no edges, but note that self-loops may in principle
be present.
</p></li>
<li class="listitem"><p>
<span class="strong"><strong>vertex</strong></span>: Graphs consist of
vertices, also called <span class="strong"><strong>nodes</strong></span>,
that are connected to each other. In igraph, vertices are
referred to by integer indices called
<span class="strong"><strong>vertex IDs</strong></span>.
</p></li>
</ul></div>
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