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Implement edges function for IGraph #26

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Implement edges function for IGraph #26

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15 changes: 15 additions & 0 deletions src/graph_api.jl
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Expand Up @@ -34,6 +34,21 @@ end
Base.eltype(::IGraph) = LibIGraph.igraph_int_t
Base.zero(::Type{IGraph}) = IGraph(0)
# Graphs.edges # TODO
function Graphs.edges(g::IGraph)
edge_list = Vector{Graphs.edgetype(g)}()
adjlist = IGAdjList()
LibIGraph.adjlist_init(g,adjlist,LibIGraph.IGRAPH_ALL,LibIGraph.IGRAPH_LOOPS_TWICE,LibIGraph.IGRAPH_MULTIPLE)
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@Krastanov Krastanov Oct 24, 2025

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Good find! This seems like a pretty expensive allocating operation. I would expect there to be a way to iterate over edges without having to create the adjlist.

I suspect this will be a better way: https://igraph.org/c/html/latest/igraph-Iterators.html#edge-iterators

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@Krastanov Krastanov Oct 24, 2025

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and we also need correctness tests

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@subbarayudu8660 subbarayudu8660 Nov 10, 2025

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what do you mean by correctness tests?

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@Krastanov Krastanov Nov 10, 2025

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A new feature usually comes with additions to the test suite, so that one can attest that the feature is correctly implemented. More importantly, such tests will be executed for any change in the future, ensuring that the feature is not accidentally broken.

For instance, here you are implementing an edges function. A convenient correctness test would be some self-consistency check. E.g. create a graph by specifically adding some random set of edges. Then call the edges function on that graph and verify that it returns the same set as the initial set you started with. This type of "round trip" consistency checks are a powerful way to detect issues.

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@Krastanov Krastanov Nov 11, 2025

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commenting on the edge iterators: you will probably also need this to convert edge id to pair of vertex ids https://igraph.org/c/html/latest/igraph-Basic.html#igraph_edge

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@subbarayudu8660 subbarayudu8660 Nov 11, 2025

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igraph_edge — Returns the head and tail vertices of an edge.
igraph_error_t igraph_edge(
const igraph_t *graph, igraph_int_t eid,
igraph_int_t *from, igraph_int_t *to
); for using this function, I have to pass in some pointer for from and to as I will be using a raw binding so how do i make a pointer in julia?

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@Krastanov Krastanov Nov 11, 2025

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This is a idiom in the C language. A C function can not return sophisticated objects like tuples, so instead you give the C function pointers to the locations of each object that you want to "return". So in C even outputs need to be provided as arguments to a function.

In the Julia wrapper for this library, some of these are made a bit nicer. Check the description in the library. There should be a julia function LibIGraph.igraph_edge that is a barebones wrapper of the C function and requires these pointer arguments, but there should also be an autogenerated nice function LibIGraph.edge that directly returns what would have been written in those pointers (and thus does not require pointer arguments). At least that should be the case if the automatic wrapper worked out for this function.

for i in Graphs.vertices(g)
neigh = IGVectorInt(Ref(unsafe_load(adjlist.objref[].adjs,i)))
for k in 1:LibIGraph.vector_int_size(neigh)
j = LibIGraph.vector_int_get(neigh,k-1)+1
if i < j
push!(edge_list,Graphs.SimpleGraphs.SimpleEdge(i,j))
end
end
end
return edge_list
end
Graphs.edgetype(g::IGraph) = Graphs.SimpleGraphs.SimpleEdge{eltype(g)} # TODO maybe expose the edge id information from IGraph
Graphs.has_edge(g::IGraph,s,d) = LibIGraph.get_eid(g,s,d,false,false)[1]!=-1
Graphs.has_vertex(g::IGraph,n::Integer) = 1≤n≤Graphs.nv(g)
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