add file for getting v2emat from new connectivity info

chunkie/@chunkgraph/build_v2emat.m

@@ -0,0 +1,42 @@
+function A = build_v2emat(obj)
+%V2EMAT get a "vertex to edge" matrix for the chunkgraph object
+% 
+% input: obj - a chunkgraph object 
+% 
+% output: A - a nedges x nverts sparse matrix 
+%  A(i,j) = 1 if edge i starts at vertex j
+%  A(i,j) = -1 if edge i ends at vertex j
+%  A(i,j) = 2 if edge i starts and ends at vertex j
+%
+
+nverts = size(obj.verts,2);
+nedges = size(obj.edgesendverts,2);
+
+
+ii = zeros(2*nedges,1);
+jj = zeros(2*nedges,1);
+vv = zeros(2*nedges,1);
+nf = 0;
+
+for i = 1:nedges
+    j1 = obj.edgesendverts(1,i);
+    j2 = obj.edgesendverts(2,i);
+    if j1 == j2
+        nf = nf + 1;
+        ii(nf) = i;
+        jj(nf) = j1;
+        vv(nf) = 2;
+    else
+        nf = nf + 1;
+        ii(nf) = i;
+        jj(nf) = j1;
+        vv(nf) = -1;
+        nf = nf + 1;
+        ii(nf) = i;
+        jj(nf) = j2;
+        vv(nf) = 1;
+    end
+end
+
+A = sparse(ii(1:nf),jj(1:nf),vv(1:nf),nedges,nverts);
+