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Digraph distance attributes #761

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Digraph distance attributes #761

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126cbb2
Digraph radius, periphery, centre
May 7, 2025
4a3fbbb
fixed attribute
May 7, 2025
12c5b9b
fixed local variable issues
May 7, 2025
561f59d
func changes
May 7, 2025
c66f3b7
fixed fun calling
May 7, 2025
a06da22
-
May 7, 2025
16d09e8
commas
May 7, 2025
10eaefb
typos
May 7, 2025
a4b9f28
added tests
May 7, 2025
0f631e0
fixed lambda functions
May 7, 2025
3519123
temp
May 7, 2025
5d81e18
fix indents
May 7, 2025
4684f3b
temp
May 7, 2025
e11fd25
revrert fail option
May 7, 2025
252e1d8
test update
May 7, 2025
de526a4
fix error where radius would get set to infinity for cycle graphs
May 9, 2025
1cd445f
fix eccentricity error with loops attempt
May 9, 2025
3190ce4
tests and lint
May 9, 2025
612054b
indent fixes
May 9, 2025
06a6e64
temp
May 9, 2025
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5 changes: 5 additions & 0 deletions gap/attr.gd
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Original file line number Diff line number Diff line change
Expand Up @@ -141,3 +141,8 @@ DeclareProperty("IsLowerSemimodularDigraph", IsDigraph);

DeclareAttribute("DigraphJoinTable", IsDigraph);
DeclareAttribute("DigraphMeetTable", IsDigraph);

DeclareAttribute("DigraphDistanceMetrics", IsDigraph);
DeclareAttribute("DigraphRadius", IsDigraph);
DeclareAttribute("DigraphCentre", IsDigraph);
DeclareAttribute("DigraphPeriphery", IsDigraph);
39 changes: 39 additions & 0 deletions gap/attr.gi
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Expand Up @@ -3431,3 +3431,42 @@ D -> DIGRAPHS_IsJoinSemilatticeAndJoinTable(D)[2]);
InstallMethod(DigraphMeetTable, "for a digraph",
[IsDigraph],
D -> DIGRAPHS_IsMeetSemilatticeAndMeetTable(D)[2]);

InstallMethod(DigraphDistanceMetrics, "for a digraph",
[IsDigraph],
function(G)
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@james-d-mitchell james-d-mitchell Aug 25, 2025 •
edited by wilfwilson
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I think G should be changed to D everywhere here, for consistency.

local ecc, c, u, v;
if not IsDigraph(G) then
Error("Input must be a digraph");
elif not IsStronglyConnectedDigraph(G) then
Comment on lines +3439 to +3441
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Suggested change
if not IsDigraph(G) then
Error("Input must be a digraph");
elif not IsStronglyConnectedDigraph(G) then
if not IsStronglyConnectedDigraph(G) then

The first if clause is redundant.

Error("Input digraph is not strongly connected; property undefined");
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@james-d-mitchell james-d-mitchell Aug 25, 2025 •
edited
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Suggested change
Error("Input digraph is not strongly connected; property undefined");
ErrorNoReturn("the argument <G> (a digraph) must be strongly connected");

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And please make a similar change for the error message a few lines above.

fi;
ecc := [];
for u in [1 .. DigraphNrVertices(G)] do
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Suggested change
for u in [1 .. DigraphNrVertices(G)] do
for u in DigraphVertices(G) do

c := 0;
for v in [1 .. DigraphNrVertices(G)] do
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@james-d-mitchell james-d-mitchell Aug 25, 2025 •
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Suggested change
for v in [1 .. DigraphNrVertices(G)] do
for v in DigraphVertices(G) do

if u <> v then
c := Maximum(c, DigraphShortestDistance(G, u, v));
fi;
od;
Add(ecc, c);
od;
return rec(
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I'm not 100% sure but I think the following is not particularly efficient because the computation of Minimum(ecc) and Maximum(ecc) will be repeated for each distinct value of i which is not such a good idea. Better to compute these values prior to this return statement.

Radius := Minimum(ecc),
DigraphCentre := Filtered([1 .. DigraphNrVertices(G)],
i -> ecc[i] = Minimum(ecc)),
Periphery := Filtered([1 .. DigraphNrVertices(G)],
i -> ecc[i] = Maximum(ecc)));
end);

InstallMethod(DigraphRadius, "for a digraph",
[IsDigraph],
D -> DigraphDistanceMetrics(D).Radius);

InstallMethod(DigraphCentre, "for a digraph",
[IsDigraph],
D -> DigraphDistanceMetrics(D).DigraphCentre);

InstallMethod(DigraphPeriphery, "for a digraph",
[IsDigraph],
D -> DigraphDistanceMetrics(D).Periphery);
8 changes: 8 additions & 0 deletions tst/standard/attr.tst
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Expand Up @@ -3173,6 +3173,14 @@ true
gap> B[14, 15] = z;
true

# DigraphDistanceMetrics
gap> DigraphRadius(CycleDigraph(5));
4
gap> DigraphCentre(Digraph([[2], [3], [1]]));
[ 1, 2, 3 ]
gap> DigraphPeriphery(CycleDigraph(13));
[ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 ]

# DigraphAbsorptionProbabilities
gap> gr := Digraph([[2, 3, 4], [3], [2], []]);
<immutable digraph with 4 vertices, 5 edges>
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