I'm modeling a flow problem on graph using sage.
Once the graph was ready I've modeled a MILP problem with constraints.
I've problems having unique variables.
What I do is to use v = p.new_variable() and subsequently use the v
variable in a loop iterating the edges of the graph in such as:
for origin, destination, attributes in G.edges():
attributes['v_component_for_that_edgs'] = v[('origin', 'destination')]
But at the end some variable are duplicate!
The code producing that strange behavior is attached to this mail.
Is an error of mine or a Sage problem?
Thanks.
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# -*- coding: utf8 -*-
vertices = {
'A': {'flow_balance_af': -7, 'flow_balance_fa': 12},
'B': {'flow_balance_af': 0, 'flow_balance_fa': 0},
'C': {'flow_balance_af': 0, 'flow_balance_fa': 0},
'D': {'flow_balance_af': 0, 'flow_balance_fa': 0},
'E': {'flow_balance_af': 0, 'flow_balance_fa': 0},
'F': {'flow_balance_af': 7, 'flow_balance_fa': -12}
}
edges = [
('A', 'B', {'fixed_cost': 10000, 'variable_cost': 300}),
('A', 'E', {'fixed_cost': 20000, 'variable_cost': 150}),
('B', 'C', {'fixed_cost': 10000, 'variable_cost': 300}),
('B', 'F', {'fixed_cost': 20000, 'variable_cost': 300}),
('C', 'D', {'fixed_cost': 10000, 'variable_cost': 500}),
('C', 'A', {'fixed_cost': 20000, 'variable_cost': 350}),
('D', 'E', {'fixed_cost': 10000, 'variable_cost': 200}),
('D', 'B', {'fixed_cost': 20000, 'variable_cost': 300}),
('E', 'C', {'fixed_cost': 10000, 'variable_cost': 400}),
('F', 'A', {'fixed_cost': 5000, 'variable_cost': 400}),
('F', 'D', {'fixed_cost': 5000, 'variable_cost': 250}),
('F', 'E', {'fixed_cost': 5000, 'variable_cost': 300})
]
# Creating the graph
G = DiGraph()
G.add_vertices(vertices)
G.add_edges(edges)
p = MixedIntegerLinearProgram(maximization=False)
# Posta ad uno se l'arco viene scelto
choice_variable = p.new_variable()
p.set_binary(choice_variable)
# Flusso che viene instradato da A ad F attraverso un nodo se scelto
used_resource_variable_af = p.new_variable()
p.set_real(used_resource_variable_af)
# Flusso che viene instradato da F ad A attraverso un nodo se scelto
used_resource_variable_fa = p.new_variable()
p.set_real(used_resource_variable_fa)
# Assigning variables components to edges attributes
for o, d, a in G.edges():
# Associamo le variabili sugli archi in modo da facilitare il lavoro dopo
a['choice_var'] = choice_variable[(o, d)]
a['used_res_var_af'] = used_resource_variable_af[(o, d)]
a['used_res_var_fa'] = used_resource_variable_fa[(o, d)]
for a in G.edges():
print a
