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<html><head><title>[grape] 3 Functions to inspect graphs, vertices and edges</title></head> <body text="#000000" bgcolor="#ffffff"> [<a href = "chapters.htm">Up</a>] [<a href ="CHAP002.htm">Previous</a>] [<a href ="CHAP004.htm">Nex <h1>3 Functions to inspect graphs, vertices and edges</h1><p> <P> <H3>Sections</H3> <oL> <li> <A HREF="CHAP003.htm#SECT001">IsGraph</a> <li> <A HREF="CHAP003.htm#SECT002">OrderGraph</a> <li> <A HREF="CHAP003.htm#SECT003">IsVertex</a> <li> <A HREF="CHAP003.htm#SECT004">VertexName</a> <li> <A HREF="CHAP003.htm#SECT005">VertexNames</a> <li> <A HREF="CHAP003.htm#SECT006">Vertices</a> <li> <A HREF="CHAP003.htm#SECT007">VertexDegree</a> <li> <A HREF="CHAP003.htm#SECT008">VertexDegrees</a> <li> <A HREF="CHAP003.htm#SECT009">IsLoopy</a> <li> <A HREF="CHAP003.htm#SECT010">IsSimpleGraph</a> <li> <A HREF="CHAP003.htm#SECT011">Adjacency</a> <li> <A HREF="CHAP003.htm#SECT012">IsEdge</a> <li> <A HREF="CHAP003.htm#SECT013">DirectedEdges</a> <li> <A HREF="CHAP003.htm#SECT014">UndirectedEdges</a> <li> <A HREF="CHAP003.htm#SECT015">Distance</a> <li> <A HREF="CHAP003.htm#SECT016">Diameter</a> <li> <A HREF="CHAP003.htm#SECT017">Girth</a> <li> <A HREF="CHAP003.htm#SECT018">IsConnectedGraph</a> <li> <A HREF="CHAP003.htm#SECT019">IsBipartite</a> <li> <A HREF="CHAP003.htm#SECT020">IsNullGraph</a> <li> <A HREF="CHAP003.htm#SECT021">IsCompleteGraph</a> </ol><p> <p> This chapter describes functions to inspect graphs, vertices and edges. <p> <p> <h2><a name="SECT001">3.1 IsGraph</a></h2> <p><p> <a name = "SSEC001.1"></a> <li><code>IsGraph( </code><var>obj</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if <var>obj</var>, which can be an object of arbitrary type, is a graph. <p> <pre> gap> IsGraph( 1 ); false gap> IsGraph( JohnsonGraph( 3, 2 ) ); true </pre> <p> <p> <h2><a name="SECT002">3.2 OrderGraph</a></h2> <p><p> <a name = "SSEC002.1"></a> <li><code>OrderGraph( </code><var>gamma</var><code> )</code> <p> This function returns the number of vertices (the <strong>order</strong>) of the graph <var>gamma</var>. <p> <pre> gap> OrderGraph( JohnsonGraph( 4, 2 ) ); 6 </pre> <p> <p> <h2><a name="SECT003">3.3 IsVertex</a></h2> <p><p> <a name = "SSEC003.1"></a> <li><code>IsVertex( </code><var>gamma</var><code>, </code><var>v</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if <var>v</var> is vertex of the graph <var>gamma</var>. <p> <pre> gap> gamma := JohnsonGraph( 3, 2 );; gap> IsVertex( gamma, 1 ); true gap> IsVertex( gamma, 4 ); false </pre> <p> <p> <h2><a name="SECT004">3.4 VertexName</a></h2> <p><p> <a name = "SSEC004.1"></a> <li><code>VertexName( </code><var>gamma</var><code>, </code><var>v</var><code> )</code> <p> This function returns (an immutable copy of) the name of vertex <var>v</var> in the graph <var>gamma</var>. <p> See also <a href="CHAP003.htm#SSEC005.1">VertexNames</a> and <a href="CHAP002.htm#SSEC011.1 <p> <pre> gap> VertexName( JohnsonGraph(4,2), 6 ); [ 3, 4 ] </pre> <p> <p> <h2><a name="SECT005">3.5 VertexNames</a></h2> <p><p> <a name = "SSEC005.1"></a> <li><code>VertexNames( </code><var>gamma</var><code> )</code> <p> This function returns (an immutable copy of) the list of vertex-names for the graph <var>gamma</var>. The <var>i</var>-th element of this list is the name of vertex <var>i</var>. <p> See also <a href="CHAP003.htm#SSEC004.1">VertexName</a> and <a href="CHAP002.htm#SSEC011.1"> <p> <pre> gap> VertexNames( JohnsonGraph(4,2) ); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ] </pre> <p> <p> <h2><a name="SECT006">3.6 Vertices</a></h2> <p><p> <a name = "SSEC006.1"></a> <li><code>Vertices( </code><var>gamma</var><code> )</code> <p> This function returns the vertex-set <var>{1,..., <code></code><var>gamma</var><code>.order</code>}</ of the graph <var>gamma</var>. <p> <pre> gap> Vertices( JohnsonGraph( 4, 2 ) ); [ 1 .. 6 ] </pre> <p> <p> <h2><a name="SECT007">3.7 VertexDegree</a></h2> <p><p> <a name = "SSEC007.1"></a> <li><code>VertexDegree( </code><var>gamma</var><code>, </code><var>v</var><code> )</code> <p> This function returns the (out)degree of the vertex <var>v</var> of the graph <var>gamma</var>. <p> <pre> gap> VertexDegree( JohnsonGraph( 3, 2 ), 1 ); 2 </pre> <p> <p> <h2><a name="SECT008">3.8 VertexDegrees</a></h2> <p><p> <a name = "SSEC008.1"></a> <li><code>VertexDegrees( </code><var>gamma</var><code> )</code> <p> This function returns the set of vertex (out)degrees for the graph <var>gamma</var>. <p> <pre> gap> VertexDegrees( JohnsonGraph( 4, 2 ) ); [ 4 ] </pre> <p> <p> <h2><a name="SECT009">3.9 IsLoopy</a></h2> <p><p> <a name = "SSEC009.1"></a> <li><code>IsLoopy( </code><var>gamma</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var> has a <strong>loop</strong>, i.e. an edge of the form <var>[x,x]</var>. <p> <pre> gap> IsLoopy( JohnsonGraph( 4, 2 ) ); false gap> IsLoopy( CompleteGraph( Group( (1,2,3), (1,2) ), 3 ) ); false gap> IsLoopy( CompleteGraph( Group( (1,2,3), (1,2) ), 3, true ) ); true </pre> <p> <p> <h2><a name="SECT010">3.10 IsSimpleGraph</a></h2> <p><p> <a name = "SSEC010.1"></a> <li><code>IsSimpleGraph( </code><var>gamma</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var> is <strong>simple</strong>, i.e. has no loops and whenever <var>[x,y]</var> is an edge then so is <var>[y,x]</var>. <p> <pre> gap> IsSimpleGraph( CompleteGraph( Group( (1,2,3) ), 3 ) ); true gap> IsSimpleGraph( CompleteGraph( Group( (1,2,3) ), 3, true ) ); false </pre> <p> <p> <h2><a name="SECT011">3.11 Adjacency</a></h2> <p><p> <a name = "SSEC011.1"></a> <li><code>Adjacency( </code><var>gamma</var><code>, </code><var>v</var><code> )</code> <p> This function returns (a copy of) the set of vertices of the graph <var>gamma</var> adjacent to the vertex <var>v</var> of <var>gamma</var>. A vertex <var>w</var> is <strong>adjacent</strong> to <var>v</var> if and only if <var>[v,w]</var> is an edge. <p> <pre> gap> Adjacency( JohnsonGraph( 4, 2 ), 1 ); [ 2, 3, 4, 5 ] gap> Adjacency( JohnsonGraph( 4, 2 ), 6 ); [ 2, 3, 4, 5 ] </pre> <p> <p> <h2><a name="SECT012">3.12 IsEdge</a></h2> <p><p> <a name = "SSEC012.1"></a> <li><code>IsEdge( </code><var>gamma</var><code>, </code><var>e</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if <var>e</var> is an edge of the graph <var>gamma</var>. <p> <pre> gap> IsEdge( JohnsonGraph( 4, 2 ), [ 1, 2 ] ); true gap> IsEdge( JohnsonGraph( 4, 2 ), [ 1, 6 ] ); false </pre> <p> <p> <h2><a name="SECT013">3.13 DirectedEdges</a></h2> <p><p> <a name = "SSEC013.1"></a> <li><code>DirectedEdges( </code><var>gamma</var><code> )</code> <p> This function returns the set of directed (ordered) edges of the graph <var>gamma</var>. <p> See also <a href="CHAP003.htm#SSEC014.1">UndirectedEdges</a>. <p> <pre> gap> gamma := JohnsonGraph( 4, 3 ); rec( isGraph := true, order := 4, group := Group([ (1,4,3,2), (3,4) ]), schreierVector := [ -1, 1, 1, 1 ], adjacencies := [ [ 2, 3, 4 ] ], representatives := [ 1 ], names := [ [ 1, 2, 3 ], [ 1, 2, 4 ], [ 1, 3, 4 ], [ 2, 3, 4 ] ], isSimple := true ) gap> DirectedEdges( gamma ); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 1 ], [ 2, 3 ], [ 2, 4 ], [ 3, 1 ], [ 3, 2 ], [ 3, 4 ], [ 4, 1 ], [ 4, 2 ], [ 4, 3 ] ] gap> UndirectedEdges( gamma ); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ] </pre> <p> <p> <h2><a name="SECT014">3.14 UndirectedEdges</a></h2> <p><p> <a name = "SSEC014.1"></a> <li><code>UndirectedEdges( </code><var>gamma</var><code> )</code> <p> This function returns the set of undirected (unordered) edges of <var>gamma</var>, which must be a simple graph. <p> See also <a href="CHAP003.htm#SSEC013.1">DirectedEdges</a>. <p> <pre> gap> gamma := JohnsonGraph( 4, 3 ); rec( isGraph := true, order := 4, group := Group([ (1,4,3,2), (3,4) ]), schreierVector := [ -1, 1, 1, 1 ], adjacencies := [ [ 2, 3, 4 ] ], representatives := [ 1 ], names := [ [ 1, 2, 3 ], [ 1, 2, 4 ], [ 1, 3, 4 ], [ 2, 3, 4 ] ], isSimple := true ) gap> DirectedEdges( gamma ); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 1 ], [ 2, 3 ], [ 2, 4 ], [ 3, 1 ], [ 3, 2 ], [ 3, 4 ], [ 4, 1 ], [ 4, 2 ], [ 4, 3 ] ] gap> UndirectedEdges( gamma ); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ] </pre> <p> <p> <h2><a name="SECT015">3.15 Distance</a></h2> <p><p> <a name = "SSEC015.1"></a> <li><code>Distance( </code><var>gamma</var><code>, </code><var>X</var><code>, </code><var>Y</var><code> )</code> <li><code>Distance( </code><var>gamma</var><code>, </code><var>X</var><code>, </code><var>Y</var><code>, </code><va <p> This function returns the distance from <var>X</var> to <var>Y</var> in <var>gamma</var>. The parameters <var>X</var> and <var>Y</var> may be vertices or nonempty lists of vertices. We define the <strong>distance</strong> <var>d(<var>X</var>,<var>Y</var>)</var> from <var>X</var> to <var>Y</va length of a (directed) path joining a vertex of <var>X</var> to a vertex of <var>Y</var> if such a path exists, and <var>-1</var> otherwise. <p> The optional parameter <var>G</var>, if present, is assumed to be a subgroup of <var>Aut(<var>gamma</var>)</var> fixing <var>X</var> setwise. Including such a <var>G</var> can speed up the function. <p> See also <a href="CHAP003.htm#SSEC016.1">Diameter</a>. <p> <pre> gap> Distance( JohnsonGraph(4,2), 1, 6 ); 2 gap> Distance( JohnsonGraph(4,2), 1, 5 ); 1 gap> Distance( JohnsonGraph(4,2), [1], [5,6] ); 1 </pre> <p> <p> <h2><a name="SECT016">3.16 Diameter</a></h2> <p><p> <a name = "SSEC016.1"></a> <li><code>Diameter( </code><var>gamma</var><code> )</code> <p> This function returns the diameter of <var>gamma</var>. A diameter of <var>-1</var> is returned if <var>gamma</var> is not (strongly) connected. Otherwise, the <strong>diameter</strong> of <var>gamma</var> is equal to the maximum (directed) distance <var>d(x,y)</var> in <var>gamma</var> (as <var>x</var> and <var>y</var> range over all the vertices of <var>gamma</var>). <p> See also <a href="CHAP003.htm#SSEC015.1">Distance</a>. <p> <pre> gap> Diameter( JohnsonGraph( 5, 3 ) ); 2 gap> Diameter( JohnsonGraph( 5, 4 ) ); 1 </pre> <p> <p> <h2><a name="SECT017">3.17 Girth</a></h2> <p><p> <a name = "SSEC017.1"></a> <li><code>Girth( </code><var>gamma</var><code> )</code> <p> This function returns the girth of <var>gamma</var>, which must be a simple graph. A girth of <var>-1</var> is returned if <var>gamma</var> is a forest. Otherwise the <strong>girth</stron is the length of a shortest cycle in <var>gamma</var>. <p> <pre> gap> Girth( JohnsonGraph( 4, 2 ) ); 3 </pre> <p> <p> <h2><a name="SECT018">3.18 IsConnectedGraph</a></h2> <p><p> <a name = "SSEC018.1"></a> <li><code>IsConnectedGraph( </code><var>gamma</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var> is (strongly) <strong>connected</strong>, i.e. there is a (directed) path from <var>x</var> to <var>y</var> for every pair of vertices <var>x,y</var> of <var>gamma</var>. <p> <pre> gap> IsConnectedGraph( JohnsonGraph(4,2) ); true gap> IsConnectedGraph( NullGraph(SymmetricGroup(4)) ); false </pre> <p> <p> <h2><a name="SECT019">3.19 IsBipartite</a></h2> <p><p> <a name = "SSEC019.1"></a> <li><code>IsBipartite( </code><var>gamma</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var>, which must be simple, is <strong>bipartite</strong>, i.e. if the vertex-set can be expressed as the disjoint union of two sets, on each of which <var>gamma</var> induces a null graph (these two sets are called the <strong>bicomponents</strong> or <strong>parts</strong> of <var>gamma</var>). <p> See also <a href="CHAP005.htm#SSEC003.1">Bicomponents</a> and <a href="CHAP006.htm#SSEC010. <p> <pre> gap> gamma := JohnsonGraph(4,2); rec( isGraph := true, order := 6, group := Group( [ (1,4,6,3)(2,5), (2,4)(3,5) ] ), schreierVector := [ -1, 2, 1, 1, 1, 1 ], adjacencies := [ [ 2, 3, 4, 5 ] ], representatives := [ 1 ], names := [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ], isSimple := true ) gap> IsBipartite(gamma); false gap> delta := BipartiteDouble(gamma); rec( isGraph := true, order := 12, group := Group( [ ( 1, 4, 6, 3)( 2, 5)( 7,10,12, 9)( 8,11), ( 2, 4)( 3, 5)( 8,10)( 9,11), ( 1, 7)( 2, 8)( 3, 9)( 4,10)( 5,11) ( 6,12) ] ), schreierVector := [ -1, 2, 1, 1, 1, 1, 3, 3, 3, 3, 3, 3 ], adjacencies := [ [ 8, 9, 10, 11 ] ], representatives := [ 1 ], isSimple := true, names := [ [ [ 1, 2 ], "+" ], [ [ 1, 3 ], "+" ], [ [ 1, 4 ], "+" ], [ [ 2, 3 ], "+" ], [ [ 2, 4 ], "+" ], [ [ 3, 4 ], "+" ], [ [ 1, 2 ], "-" ], [ [ 1, 3 ], "-" ], [ [ 1, 4 ], "-" ], [ [ 2, 3 ], "-" ], [ [ 2, 4 ], "-" ], [ [ 3, 4 ], "-" ] ] ) gap> IsBipartite(delta); true </pre> <p> <p> <h2><a name="SECT020">3.20 IsNullGraph</a></h2> <p><p> <a name = "SSEC020.1"></a> <li><code>IsNullGraph( </code><var>gamma</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var> has no edges. <p> See also <a href="CHAP002.htm#SSEC003.1">NullGraph</a>. <p> <pre> gap> IsNullGraph( CompleteGraph( Group(()), 3 ) ); false gap> IsNullGraph( CompleteGraph( Group(()), 1 ) ); true </pre> <p> <p> <h2><a name="SECT021">3.21 IsCompleteGraph</a></h2> <p><p> <a name = "SSEC021.1"></a> <li><code>IsCompleteGraph( </code><var>gamma</var><code> )</code> <li><code>IsCompleteGraph( </code><var>gamma</var><code>, </code><var>mustloops</var><code> )</code> <p> This boolean function returns <code>true</code> if and only if the graph <var>gamma</var> is a complete graph. The optional boolean parameter <var>mustloops</var> determines whether all loops must be present for <var>gamma</var> to be considered a complete graph (default: <code>false</code> (loops are ignored)). <p> See also <a href="CHAP002.htm#SSEC004.1">CompleteGraph</a>. <p> <pre> gap> IsCompleteGraph( NullGraph( Group(()), 3 ) ); false gap> IsCompleteGraph( NullGraph( Group(()), 1 ) ); true gap> IsCompleteGraph( CompleteGraph(SymmetricGroup(3)), true ); false </pre> <p> [<a href = "chapters.htm">Up</a>] [<a href ="CHAP002.htm">Previous</a>] [<a href ="CHAP004.htm">Nex <P> <address>grape manual<br>September 2025 </address></body></html>
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