Indexing

Answered

Mouhamad Obeid

September 21, 2022 05:21

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m.addConstrs((x[i] + y[i-1] + z[i+1] == demands[i] + y[i] + z[i] for i in range(num_periods)), name="sdsd")

how should I fix my indexing problem in the addConstrs function

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 16, 2025 23:28

  This post is more than three 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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  Jaromił Najman

  • Gurobi Staff

  September 21, 2022 07:34

  What exactly is the issue? Could you please post a minimal reproducible example together with the error message you see?

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  Mouhamad Obeid

  September 21, 2022 14:06

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  So this is my code below I am trying to fix the indexing in the addContrs. How should I go about doing my list to generate such constraints with different indices.

  import gurobipy as gp

  # parameters for the model

  # number of time periods
  num_periods = 10
  # demands for the periods
  demands = [1, 3, 5, 2, 7, 8, 8, 2, 9, 4]
  # unit production costs for the periods
  c_t = [2, 3, 4, 1, 5, 7, 4, 9, 1, 4]
  # unit holding costs at the end of each period
  h_t = [3, 8, 7, 4, 3, 8, 2, 2, 1, 2]
  # backlogging costs
  b_t = [4, 1, 3, 8, 7, 9, 6, 3, 9, 2]

  # define a new model 
  m = gp.Model("production plan")

  # decision variables
  x = m.addVars(num_periods, lb=0, ub=gp.GRB.INFINITY, name="production")
  y = m.addVars(num_periods, lb=0, ub=gp.GRB.INFINITY, name="inventory")
  z = m.addVars(num_periods, lb=0, ub=gp.GRB.INFINITY, name="backlogging")

  # constraints

  m.addConstrs((x[i] + y[i-1] + z[i+1] == demands[i] + y[i] + z[i] for i in range(num_periods)), name="")
   
  # Set objective function to minimize the total cost
  m.setObjective(gp.quicksum(c_t[i]*x[i] + h_t[i]*y[i] + b_t[i]*z[i] for i in range(num_periods)),
                 gp.GRB.MINIMIZE)

  # optimize the model
  m.optimize()

  # Print the optimal objective value
  print("The optimal value is", m.objval)

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  Jaromił Najman

  • Gurobi Staff

  September 21, 2022 15:49

  You get the KeyError

   m.addConstrs((x[i] + y[i-1] + z[i+1] == demands[i] + y[i] + z[i] for i in range(num_periods)), name="")
  KeyError: -1

  because you are trying to access \(\texttt{y[-1]}\)  but you defined your \(\texttt{y}\) variables over \(\{0,\dots,9\}\). You are constructing the constraints over \(i \in \{0,\dots,9\}\) and this results in

  x[0] + y[0-1] + z[0+1] == demands[0] + y[0] + z[0]

  I guess that you have to implement the first and last constraint by hand to avoid key errors with \(\texttt{y[-1]}\) and \(\texttt{z[10]}\).

  Best regards, 
  Jaromił

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