我对 Gurobi 有点熟悉,但转向 Gekko,因为后者似乎有一些优势。不过,我遇到了一个问题,我将用我想象的苹果园来说明这一问题。 5周的收获期(#horizon: T=5
)就在我们身上,我的——非常微薄的——产出将是:[3.0, 7.0, 9.0, 5.0, 4.0]
我自己留一些苹果[2.0, 4.0, 2.0, 4.0, 2.0]
,剩下的农产品我会在农贸市场按以下价格出售:[0.8, 0.9, 0.5, 1.2, 1.5]
。我有可容纳 6 个苹果的存储空间,因此我可以提前计划并在最佳时刻出售苹果,从而最大化我的收入。我尝试使用以下模型确定最佳时间表:
m = GEKKO()
m.time = np.linspace(0,4,5)
orchard = m.Param([3.0, 7.0, 9.0, 5.0, 4.0])
demand = m.Param([2.0, 4.0, 2.0, 4.0, 2.0])
price = m.Param([0.8, 0.9, 0.5, 1.2, 1.5])
### manipulated variables
# selling on the market
sell = m.MV(lb=0)
sell.DCOST = 0
sell.STATUS = 1
# saving apples
storage_out = m.MV(value=0, lb=0)
storage_out.DCOST = 0
storage_out.STATUS = 1
storage_in = m.MV(lb=0)
storage_in.DCOST = 0
storage_in.STATUS = 1
### storage space
storage = m.Var(lb=0, ub=6)
### constraints
# storage change
m.Equation(storage.dt() == storage_in - storage_out)
# balance equation
m.Equation(sell + storage_in + demand == storage_out + orchard)
# Objective: argmax sum(sell[t]*price[t]) for t in [0,4]
m.Maximize(sell*price)
m.options.IMODE=6
m.options.NODES=3
m.options.SOLVER=3
m.options.MAX_ITER=1000
m.solve()
由于某种原因这是不可行的(错误代码 = 2)。有趣的是,如果设置demand[0] to 3.0, instead of 2.0
(即等于orchard[0]
,该模型确实产生了成功的解决方案。
- 为什么会这样呢?
- 即使“成功”的输出值也有点奇怪:存储空间没有被使用一次,并且
storage_out
在最后一个时间步中没有受到适当的约束。显然,我没有正确地制定约束条件。我应该怎么做才能获得与 gurobi 输出相当的实际结果(参见下面的代码)?
output = {'sell' : list(sell.VALUE),
's_out' : list(storage_out.VALUE),
's_in' : list(storage_in.VALUE),
'storage' : list(storage.VALUE)}
df_gekko = pd.DataFrame(output)
df_gekko.head()
> sell s_out s_in storage
0 0.0 0.000000 0.000000 0.0
1 3.0 0.719311 0.719311 0.0
2 7.0 0.859239 0.859239 0.0
3 1.0 1.095572 1.095572 0.0
4 26.0 24.124924 0.124923 0.0
Gurobi 模型解决了demand = [3.0, 4.0, 2.0, 4.0, 2.0]
。请注意,gurobi 还产生了一个解决方案demand = [2.0, 4.0, 2.0, 4.0, 2.0]
。这对结果只有很小的影响:nt=0 时出售的苹果变为1
.
T = 5
m = gp.Model()
### horizon (five weeks)
### supply, demand and price data
orchard = [3.0, 7.0, 9.0, 5.0, 4.0]
demand = [3.0, 4.0, 2.0, 4.0, 2.0]
price = [0.8, 0.9, 0.5, 1.2, 1.5]
### manipulated variables
# selling on the market
sell = m.addVars(T)
# saving apples
storage_out = m.addVars(T)
m.addConstr(storage_out[0] == 0)
storage_in = m.addVars(T)
# storage space
storage = m.addVars(T)
m.addConstrs((storage[t]<=6) for t in range(T))
m.addConstrs((storage[t]>=0) for t in range(T))
m.addConstr(storage[0] == 0)
# storage change
#m.addConstr(storage[0] == (0 - storage_out[0]*delta_t + storage_in[0]*delta_t))
m.addConstrs(storage[t] == (storage[t-1] - storage_out[t] + storage_in[t]) for t in range(1, T))
# balance equation
m.addConstrs(sell[t] + demand[t] + storage_in[t] == (storage_out[t] + orchard[t]) for t in range(T))
# Objective: argmax sum(a_sell[t]*a_price[t] - b_buy[t]*b_price[t])
obj = gp.quicksum((price[t]*sell[t]) for t in range(T))
m.setObjective(obj, gp.GRB.MAXIMIZE)
m.optimize()
output:
sell storage_out storage_in storage
0 0.0 0.0 0.0 0.0
1 3.0 0.0 0.0 0.0
2 1.0 0.0 6.0 6.0
3 1.0 0.0 0.0 6.0
4 8.0 6.0 0.0 0.0