How to compare a expression with 0, Invalid data in vars array

回答済み

伟星 郭

2023年03月12日 11:47

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This is my obj : to minimize $\sum^{-1}_{t=-T_H}(a_t*q_t*p*K + h*y_t+b*z_t)$

A constraint is ​$y_t = max(0, y_{t-1} + \sum_{i∈{j:j+l_j=t}}q_i * p - d_t)$

And all I want to do is to get max(expression, 0). What should I do?

np.random.seed(4)

T_H = [i for i in range(10)]
T = len(T_H)
# transport_cost
K = 50
# holding cost
h = 1
# stock-out cost
b = 100
# The number of boxes contained in a tray
p = 40
# VLT
l = [2 for i in range(10)]
# demand
d = [np.random.randint(100) for i in range(10)]
# initial inventory level
I = 109

# var
q = model.addVars(10, vtype=gp.GRB.INTEGER, name='q')
y = model.addVars(10, vtype=gp.GRB.INTEGER, name='y')
z = model.addVars(10, vtype=gp.GRB.INTEGER, name='z')
a = model.addVars(10, vtype=gp.GRB.BINARY, name='a')

# obj
obj = gp.quicksum((a[t] * q[t] * p * K + h * y[t] + b * z[t]) for t in range(T))
model.setObjective(obj, gp.GRB.MINIMIZE)

# cons
for t in range(T):
    if t == 0:
        model.addConstr(y[0] == I)
        model.addConstr(z[0] == 0)
    else:
        # model.addConstr(temp == y[t - 1] - d[t] + gp.quicksum(q[i] * p for i in range(t) if i + l[i] == t))
        # model.addConstr(temp1 == 0)
        model.addConstr(y[t] == gp.max_(y[t - 1] - d[t] + gp.quicksum(q[i] * p for i in range(t) if i + l[i] == t), 0))
        # model.addGenConstrMin(y[t], [temp, temp1])

        # model.addConstr(z[t] == (z[t-1] - d[t] + gp.quicksum(q[i] * p for i in range(t) if i + l[i] == t)))
        model.addConstr(y[t] >= 0)
        model.addConstr(z[t] >= 0)
        model.addConstr(q[t] >= 0)

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関連

• Python
• General Constraints

1

• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:47

  This post is more than two years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  伟星 郭

  2023年03月12日 11:57

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  In another word, Max of value and if negative, return 0. 

  

  These are my two main constraints, y_t means the inventory level while z_t means the stock-out inventory level in time t.

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  伟星 郭

  2023年03月12日 12:16

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  Oh,sorry, z_t shoule be like this,

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  Mario Ruthmair

  • Gurobi Staff

  2023年03月13日 10:37

  Hi,

  The issue here is that gp.max_() requires a list of variables (and an optional constant).
  You need to introduce additional helper variables that are set equal to the term in your max-function. Then you can use these variables in the max_() function.

  Alternatively, you do not necessarily need the max_() function here. You could just set a lower bound of 0 for the y- and z-variables, and add

  y_t >= y_{t-1} + sum ...

  to push y_t up if the right-hand side is positive (similar for z_t).
  Due to the minimization objective, the y_t and z_t variables should then always be equal to the max of 0 and the term in an optimal solution.

  Best regards,
  Mario

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