add induced_subgraph function for creating induced subgraph from edge list

Author: pgrepds

src/overrides.jl

@@ -144,3 +144,28 @@ function induced_subgraph(g::T, vlist::AbstractVector{U}) where T <: AbstractSim
     newg.weights = new_weights
     return newg, Vector{E}(vlist)
 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
+        if has_edge(g, e)
+            push!(vertexSet, 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 
+        end
+    end
+    return newg, Vector{W}(collect(edges(newg)))
+end

test/simpleweightedgraph.jl

@@ -1,4 +1,5 @@
 using SimpleWeightedGraphs
+using SparseArrays
 
 @testset verbose = true "SimpleWeightedGraphs" begin
     @info("Ignore warnings relating to adding and removing vertices and edges")
@@ -329,4 +330,18 @@ using SimpleWeightedGraphs
         @test g[1, 3, Val{:weight}()] ≈ 0
         @test g[2, 3, Val{:weight}()] ≈ 0.5
     end
+
+    # 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
+        # vertex induced subgraph
+        vertexInducedSubgraphWeights = weights(first(induced_subgraph(g, [1, 2])))
+        @test vertexInducedSubgraphWeights == expectedGraphWeights
+        # edge induced subgraph
+        edgeInducedSubgraphWeights = weights(first(induced_subgraph(g, [Edge(1, 2)])))
+        @test edgeInducedSubgraphWeights == expectedGraphWeights
+    end
 end