Subgraph

doc/operators.md

@@ -23,3 +23,10 @@ Produces a graph with edges from graph *g* that do not exist in graph *h*, and v
 
 `compose(g, h)`  
 Merges graphs *g* and *h* by taking the set union of all vertices and edges.
+
+`function inducedsubgraph(g::AbstractGraph, iter)`
+Filter *g* to include only the vertices present in iter which should not have duplicates.
+Returns the subgraph of *g* induced by set(iter) along with the mapping from the old vertex names to the new vertex names.
+
+`inducedsubgraph!(h, g, newvid)`
+Inplace filtering for preallocated output, edge iterable, vertex mapping.

src/LightGraphs.jl

@@ -20,6 +20,7 @@ module LightGraphs
     # operators
     complement, reverse, reverse!, union, intersect,
     difference, symmetric_difference, compose,
+    inducedsubgraph,
 
     # graph visit
     AbstractGraphVisitor, TrivialGraphVisitor, LogGraphVisitor,

src/graph.jl

@@ -107,3 +107,5 @@ degree(g::Graph, v::Int) = indegree(g,v)
 #         union(neighbors(g,v), [e.dst for e in g.binclist[v]])
 #     )
 density(g::Graph) = (2*ne(g)) / (nv(g) * (nv(g)-1))
+
+

src/operators.jl

@@ -114,3 +114,29 @@ function compose{T<:AbstractGraph}(g::T, h::T)
     end
     return r
 end
+
+#@doc "filter g to include only the vertices present in iter which should not have duplicates
+#returns the subgraph of g induced by set(iter) along with the mapping from the old vertex names to the new vertex names" ->
+function inducedsubgraph(g::AbstractGraph, iter)
+    n = length(iter)
+    h = Graph(n)
+    newvid = Dict()
+    i=1
+    for v in iter
+        newvid[v] = i
+        i +=1
+    end
+    inducedsubgraph!(h,edges(g),newvid)
+    return h, newvid
+end
+
+#@doc "inplace filtering for preallocated output, edge iterable, vertex mapping" ->
+function inducedsubgraph!(h::AbstractGraph, edges, newvid)
+    for edg in edges
+        newsrc = get(newvid, src(edg), 0)
+        newdst = get(newvid, dst(edg), 0)
+        if newsrc > 0 && newdst > 0
+            add_edge!(h, newsrc, newdst)
+        end
+    end
+end

test/runtests.jl

@@ -53,7 +53,8 @@ tests = [
     "maxadjvisit",
     "centrality/betweenness",
     "centrality/closeness",
-    "centrality/degree"
+    "centrality/degree",
+    "subgraphs"
     ]
 
 

test/subgraphs.jl

@@ -0,0 +1,17 @@
+using LightGraphs
+import LightGraphs: inducedsubgraph
+using Base.Test
+
+g = BullGraph()
+n = 3
+h, vmap = inducedsubgraph(g, [1:n])
+@test nv(h) == n
+@test ne(h) == 3
+
+h, vmap = inducedsubgraph(g, [1,2,4])
+@test nv(h) == n
+@test ne(h) == 2
+
+h, vmap = inducedsubgraph(g, [1,5])
+@test nv(h) == 2
+@test ne(h) == 0