Give a default value to `traj_prop`

Preparing to process phantom patterns.

Trajectories/trajectories.py

@@ -49,9 +49,10 @@ else:
     # This happens if there are only isolated segments.
     print("No cost values in the input file.")
 
-traj_prop = g.new_vertex_property('int')
-# Trajectory index to which a segment belongs. The trajectory index is
-# also the index in lists traj_vert_ind, traj_segm and expanded_traj.
+traj_prop = g.new_vertex_property('int', val = - 1)
+# Trajectory index to which a segment belongs.  - 1 means the segment
+# does not belong yet to any trajectory. The trajectory index is also
+# the index in lists traj_vert_ind, traj_segm and expanded_traj.
 
 ind_traj = - 1 # initialize index of trajectory
 
@@ -74,7 +75,7 @@ for n in topology.topological_sort(g):
             # Assign to n the trajectory of closest_pred. This means
             # updating ind_traj, traj_prop and
             # traj_vert_ind. closest_pred is already in a trajectory,
-            # traj_prop[closest_pred]] is defined. We extend the
+            # traj_prop[closest_pred]] != - 1. We extend the
             # trajectory of closest_pred forward.
             traj_prop[n] = traj_prop[closest_pred]
             traj_vert_ind[traj_prop[n]].append(n)
@@ -83,7 +84,7 @@ for n in topology.topological_sort(g):
     else:
         ind_traj = new_traj(ind_traj, traj_prop, n, traj_vert_ind)
 
-    # traj_prop[n] is defined
+    # traj_prop[n] != - 1
 
 print("Number of trajectories: ", len(traj_vert_ind))