**Network Simplex Solver: NetworkX**

We will solve the instance of a Minimum cost flow problem described in (1) with NetworkX. Node 1 is the source node, nodes 2 and 3 are the transshipment nodes and node 4 is the sink node.

Solving a Minimum Cost Flow Problem with NetworkX is pretty straight forward.

# import networkx import networkx as nx # create directed graph G = nx.DiGraph() # add node to graph with negative (!) supply for each supply node G.add_node(1, demand = -4) # you can ignore transshipment nodes with zero supply when you are working with the mcfp-solver # of networkx # add node to graph with positive (!) demand for each demand node G.add_node(4, demand = 4) # add arcs to the graph: fromNode,toNode,capacity,cost (=weight) G.add_edge(1, 2, capacity = 4, weight = 2) G.add_edge(1, 3, capacity = 2, weight = 2) G.add_edge(2, 3, capacity = 2, weight = 1) G.add_edge(2, 4, capacity = 3, weight = 3) G.add_edge(3, 4, capacity = 5, weight = 1) # solve the min cost flow problem # flowDict contains the optimized flow # flowCost contains the total minimized optimum flowCost, flowDict = nx.network_simplex(G) print "Optimum: %s" %flowCost #should be 14

As you can see the code is very readable and easy to understand. The only thing you could get trouble with in the formulation of a min cost flow problem with networkx is the fact that the supply nodes get a negative supply value (because the python attribute is called „demand“) while the demand nodes require positive demand values.

This differs from the usual mathematical notation of supply and demand values in the network flow problems where supply values are positive and demand values of negative sign.

Also note that you only need to add supply and demand nodes to the graph. You don’t have to care of the transshipment nodes.

Dear Prof.Johannes

My name is Nhu Mai Luan, I am a master student. Currently, I am using the „networkx“ package to solve my transportation problem. But an error named „networkx.exception.NetworkXUnfeasible: no flow satisfies all node demands“ was raised. I dont know the meaning of it. If you dont mind, would you please share me about this error meaning. Thank you very much!

Best regards!

Nhu Mai Luan