reimplement induced_subgraph for edge induced subgraphs, fix code style

Author: pgrepds

src/overrides.jl

@@ -148,24 +148,29 @@ end
 function induced_subgraph(g::T, elist::AbstractVector{U}) where T <: AbstractSimpleWeightedGraph where U <: AbstractEdge
     allunique(elist) || throw(ArgumentError("Edges in subgraph list must be unique"))
     E = eltype(g)
-    W = edgetype(g)
-    vertexSet = Set{E}()
-    for e in elist
+    vertex_set = Set{E}()
+    @inbounds for e in elist
         if has_edge(g, e)
-            push!(vertexSet, src(e), dst(e))
+            push!(vertex_set, src(e), dst(e))
         else 
             @warn "Skipping the edge $(e), since it does not exist in the graph!"
         end
     end
-    vertexList = collect(vertexSet)
-    new_weights = g.weights[vertexList, vertexList]
-
-    newg = zero(g)
-    newg.weights = new_weights
-    for e in edges(newg)
-        if e ∉ elist
-            newg.weights[dst(e), src(e)] = 0 
+    vertex_list = collect(vertex_set)
+    sort!(vertex_list)
+    index_map = Dict(vertex_list[i] => i for i=1:length(vertex_list))
+    n = length(vertex_list)
+    new_weights = spzeros(weighttype(g), E, E(n), E(n))
+    @inbounds for e in elist
+        if has_edge(g, e)
+            weights = g.weights[dst(e), src(e)]
+            new_weights[index_map[dst(e)], index_map[src(e)]] = weights
+            if !is_directed(g)
+                new_weights[index_map[src(e)], index_map[dst(e)]] = weights
+            end
         end
     end
-    return newg, Vector{W}(collect(edges(newg)))
+    newg = zero(g)
+    newg.weights = new_weights
+    return newg, Vector{edgetype(g)}(collect(edges(newg)))
 end

test/simpleweightedgraph.jl

@@ -334,14 +334,25 @@ using SparseArrays
     # this testset was implemented for https://github.com/JuliaGraphs/SimpleWeightedGraphs.jl/issues/32
     @testset "induced_subgraph should preserve weights for edge lists" begin
         g = SimpleWeightedGraph([0 2; 2 0])
-        graphWeights = weights(g)
-        expectedGraphWeights = sparse([0 2; 2 0])
-        @test graphWeights == expectedGraphWeights
+        expected_graph_weights = sparse([0 2; 2 0])
         # vertex induced subgraph
-        vertexInducedSubgraphWeights = weights(first(induced_subgraph(g, [1, 2])))
-        @test vertexInducedSubgraphWeights == expectedGraphWeights
+        vertex_induced_subgraph_weights = weights(first(induced_subgraph(g, [1, 2])))
+        @test vertex_induced_subgraph_weights == expected_graph_weights
         # edge induced subgraph
-        edgeInducedSubgraphWeights = weights(first(induced_subgraph(g, [Edge(1, 2)])))
-        @test edgeInducedSubgraphWeights == expectedGraphWeights
+        edge_induced_subgraph_weights = weights(first(induced_subgraph(g, [Edge(1, 2)])))
+        @test edge_induced_subgraph_weights == expected_graph_weights
+
+        # test edge induced graph with one edge removed
+        # graph isomorphic to C_5
+        g = SimpleWeightedGraph([0 2 0 0 2; 2 0 2 0 0; 0 2 0 2 0; 0 0 2 0 2; 2 0 0 2 0])
+        expected_graph_weights = sparse([0 2 0 0 0; 2 0 2 0 0; 0 2 0 2 0; 0 0 2 0 2; 0 0 0 2 0]);
+        # create edge induced subgraph isomorphic to P_5. The edge (1, 5) is missing and test if weights are correct.
+        edge_induced_subgraph_weights = weights(first(induced_subgraph(g, [Edge(1, 2), Edge(2, 3), Edge(3, 4), Edge(4, 5)])))
+        @test edge_induced_subgraph_weights == expected_graph_weights
+
+        # test edge induced subgraph which does not contain the whole vertex set, especially remove the first column (vertex 1)
+        edge_induced_subgraph_weights = weights(first(induced_subgraph(g, [Edge(2, 3)])))
+        expected_graph_weights = sparse([0 2; 2 0])
+        @test edge_induced_subgraph_weights == expected_graph_weights
     end
 end