Please correct my code

Answered

Zatul Sahwee

March 31, 2023 11:02

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I have this code below. However when I'm running it says my mod.update() & my mod.optimal() are invalid syntax
Could you help me to debug this?

def paper_production(demand, costs, inventory_level, production_level):
    ##Declare model
    mod = gp.Model('paper_production')
    mod.setParam('OutputFlag', 0)
    mod.setParam('LogToConsole', 0)

    obj = None
    production = None
    emergency = None
    
    ## Declare variables
    x = [mod.addVar(lb=0, vtype=GRB.CONTINUOUS, name='x_{i}')for i in range (5)]
    y = [mod.addVar(lb=0, vtype=GRB.CONTINUOUS, name='y_{i}')for i in range (5)]
    s = [mod.addVar(lb=0, vtype=GRB.CONTINUOUS, name='s_{i}')for i in range (5)]
    x[0] = 4000
    
    ## Constraints
    mod.addConstr(x[0] == production_level_ex['start']) 
    mod.addConstr(500 + x[1] + y[1] - s[1]  == demand[1])
    mod.addConstr(500 + x[1] + y[1] - demand[1]  == s[1])
    mod.addConstr(s[1] + x[2] + y[2] - s[2]  == demand[2])
    mod.addConstr(s[2] + x[3] + y[3] - s[3]  == demand[3])
    mod.addConstr(s[3] + x[3] + y[4] - s[4]  == demand[4])
    mod.addConstr(x[4] == production_level_ex['end'])
    mod.addConstr(s[4] >= inventory_level_ex['end'])                                          
    
    ## Objective
    mod.setObjective(gp.quicksum(x[i]+(costs_ex['emergency']*y[i])+(costs_ex['increase']* max(0,x[i]-x[i-1]))+(costs_ex['decrease']*max(0,x[i-1]-x[i])) + (costs_ex['storage']*s[i]) + (costs_ex['penalty']*max(0,s[4]-inventory_level_ex['start'])), GRB.MINIMIZE
      
   ## Update model and optimize
    mod.update()
    mod.optimize()
                        
    return mod.objVal

    print(paper_production(demand_ex, costs_ex, inventory_level_ex, production_level_ex))

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 13, 2026 01:45

  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.
• 

  Marika Karbstein

  • Gurobi Staff

  March 31, 2023 12:48

  Your code is not reproducible but there are some issues with your objective definition

  • some brackets ')' are missing
  • 'i' is not defined 
  • you need to model max via general constraints, see Model.addGenConstrMax()
    please also have a look into the following article which gives a general idea of How do I model logical and general expressions? – Gurobi Help Center

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