GUROBI实战案例（三）——交通运输（网络流模型）

问题介绍见：网络流问题：交通调度、仓储运输 (aliyun.com)

import gurobipy as gb
from gurobipy import *
# 参数

start= ["a"]
mid_point = ["b","c","d","e","f"]
end = ["g"]

arcs, capacity = gb.multidict({
    ("a","b"):50,
    ("a","c"):100,
    ("b","d"):40,
    ("b","e"):20,
    ("c","d"):60,
    ("c","f"):20,
    ("d","e"):50,
    ("d","f"):60,
    ("e","g"):70,
    ("f","g"):70
})

# model
model = gb.Model()

# decision var
flow = model.addVars(arcs, lb = 0, vtype = GRB.INTEGER, name = "flow")
# constraints
# mid_points constraints
model.addConstrs((flow.sum("*", mid) == flow.sum(mid, "*") for mid in mid_point), name = "mid_point constrains")
# capacity constraints
model.addConstrs((flow[arc] <= capacity[arc] for arc in arcs), name = "capacity constraints")
# model.addConsts((flow.select(arc) <= capacity.select(arc) for arc in arcs), name = "capacity constraints")

# objective
obj = flow.sum("*", "g")
model.setObjective(obj, GRB.MAXIMIZE) #默认是求最小值
model.optimize()
# print(flow)
print(model.objVal)  # 打印目标函数值
for v in model.getVars():
  # 打印最终解中的决策变量
    print(v.x)