Warning

This documents an unmaintained version of NetworkX. Please upgrade to a maintained version and see the current NetworkX documentation.

networkx.algorithms.flow.max_flow_min_cost

max_flow_min_cost(G, s, t, capacity='capacity', weight='weight')[source]

Returns a maximum (s, t)-flow of minimum cost.

G is a digraph with edge costs and capacities. There is a source node s and a sink node t. This function finds a maximum flow from s to t whose total cost is minimized.

Parameters

  • G (NetworkX graph) – DiGraph on which a minimum cost flow satisfying all demands is to be found.

  • s (node label) – Source of the flow.

  • t (node label) – Destination of the flow.

  • capacity (string) – Edges of the graph G are expected to have an attribute capacity that indicates how much flow the edge can support. If this attribute is not present, the edge is considered to have infinite capacity. Default value: ‘capacity’.

  • weight (string) – Edges of the graph G are expected to have an attribute weight that indicates the cost incurred by sending one unit of flow on that edge. If not present, the weight is considered to be 0. Default value: ‘weight’.

Returns

flowDict – Dictionary of dictionaries keyed by nodes such that flowDict[u][v] is the flow edge (u, v).

Return type

dictionary

Raises

  • NetworkXError – This exception is raised if the input graph is not directed or not connected.

  • NetworkXUnbounded – This exception is raised if there is an infinite capacity path from s to t in G. In this case there is no maximum flow. This exception is also raised if the digraph G has a cycle of negative cost and infinite capacity. Then, the cost of a flow is unbounded below.

See also

cost_of_flow(), min_cost_flow(), min_cost_flow_cost(), network_simplex()

Notes

This algorithm is not guaranteed to work if edge weights or demands are floating point numbers (overflows and roundoff errors can cause problems). As a workaround you can use integer numbers by multiplying the relevant edge attributes by a convenient constant factor (eg 100).

Examples

>>> G = nx.DiGraph()
>>> G.add_edges_from([(1, 2, {'capacity': 12, 'weight': 4}),
...                   (1, 3, {'capacity': 20, 'weight': 6}),
...                   (2, 3, {'capacity': 6, 'weight': -3}),
...                   (2, 6, {'capacity': 14, 'weight': 1}),
...                   (3, 4, {'weight': 9}),
...                   (3, 5, {'capacity': 10, 'weight': 5}),
...                   (4, 2, {'capacity': 19, 'weight': 13}),
...                   (4, 5, {'capacity': 4, 'weight': 0}),
...                   (5, 7, {'capacity': 28, 'weight': 2}),
...                   (6, 5, {'capacity': 11, 'weight': 1}),
...                   (6, 7, {'weight': 8}),
...                   (7, 4, {'capacity': 6, 'weight': 6})])
>>> mincostFlow = nx.max_flow_min_cost(G, 1, 7)
>>> mincost = nx.cost_of_flow(G, mincostFlow)
>>> mincost
373
>>> from networkx.algorithms.flow import maximum_flow
>>> maxFlow = maximum_flow(G, 1, 7)[1]
>>> nx.cost_of_flow(G, maxFlow) >= mincost
True
>>> mincostFlowValue = (sum((mincostFlow[u][7] for u in G.predecessors(7)))
...                     - sum((mincostFlow[7][v] for v in G.successors(7))))
>>> mincostFlowValue == nx.maximum_flow_value(G, 1, 7)
True