Constraints in Gurobi

Answered

Shahrzad Valizadeh

May 27, 2024 03:55

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I have defined days of week at the start of my code as:

days_of_week = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]

I also have set of:

constraint_days_Loc = ["Tuesday", "Wednesday", "Friday", "Saturday", "Sunday"]

and variables as:

        y = {}
        for r in blood_types:
            for i in num_hospitals:
                y[r, i, "42"] = Issuing_model.addVar(vtype=GRB.INTEGER, name=f'y_{r}_{i}_{"42"}')
        yy = {}
        for r in blood_types:
            for i in num_hospitals:
                for demand_index in range(100):
                    yy[r, i, "42", demand_index] = Issuing_model.addVar(vtype=GRB.INTEGER, name=f'yy_{r}_{i}_{"42"}_{demand_index}')

in which r = ["O+", "O-"], and num_hospitals = ["1", "2"]

in a loop over days of the week as:

for day in days_of_week:

I want to check if the current day is in set constraint_days_Loc, for all "r":

y[r, "2", "42"] == 0
else:
y[r, "2", "42"] >= 0

if day in ["Tuesday", "Friday", "Saturday"]:

for all demand_index, yy[r, "2", "42", demand_index] == 0

else:
yy[r, "2", "42", demand_index] >= 0

I did it by the following constraints, but unfortunately gives wrong result:

        if day.strip() in constraint_days_Loc:
            for r in blood_types:
                Issuing_model.addConstr(y[r, "2", "42"] == 0, name=f"non_order_days_Loc_{day}")
        else:
            for r in blood_types:
                Issuing_model.addConstr(y[r, "2", "42"] >= 0, name = f"order_days_Loc_{day}")
# Checking whether tomorrow is an orde day or not in Local
        if day.strip() in ["Tuesday", "Friday", "Saturday"]:
            for demand_index in range(100):
                Issuing_model.addConstr(yy[r, "2", "42", demand_index] == 0)
        else:
            for demand_index in range(100):
                Issuing_model.addConstr(yy[r, "2", "42", demand_index] >= 0)

0

• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 13, 2026 01:24

  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.
• 

  Gurobot (AI-generated response)

  • Gurobi Staff

  June 03, 2024 05:00

  Your approach is generally correct, but there are a few issues with how the constraints are being added and iterated. Here's a revised version of your code to ensure that constraints are applied correctly based on the day of the week:

  1. Ensure all blood types and hospitals are included in the loop when adding constraints.
  2. Correctly handle the indexing of yy variables within the inner loop.

  Note that the default lower bound for variables in Gurobi is zero. Assuming y and yy variables cannot take negative values, there is no need to add constraints for the conditions.

  y[r, "2", "42"] >= 0
  yy[r, "2", "42", demand_index] >= 0 

  Here is the revised version:

  from gurobipy import Model, GRB
  
  # Define sets
  days_of_week = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
  constraint_days_Loc = ["Tuesday", "Wednesday", "Friday", "Saturday", "Sunday"]
  blood_types = ["O+", "O-"]
  num_hospitals = ["1", "2"]
  
  # Initialize model
  Issuing_model = Model()
  
  # Define variables
  y = {}
  yy = {}
  for r in blood_types:
      for i in num_hospitals:
          y[r, i, "42"] = Issuing_model.addVar(vtype=GRB.INTEGER, name=f'y_{r}_{i}_42')
          for demand_index in range(100):
              yy[r, i, "42", demand_index] = Issuing_model.addVar(vtype=GRB.INTEGER, name=f'yy_{r}_{i}_42_{demand_index}')
  
  # Add constraints
  for day in days_of_week:
      if day in constraint_days_Loc:
          for r in blood_types:
              Issuing_model.addConstr(y[r, "2", "42"] == 0, name=f"non_order_days_Loc_{day}_{r}")
     
      if day in ["Tuesday", "Friday", "Saturday"]:
          for r in blood_types:
              for demand_index in range(100):
                  Issuing_model.addConstr(yy[r, "2", "42", demand_index] == 0, name=f"non_order_days_yy_{day}_{r}_{demand_index}")
     
  # Optimize model
  Issuing_model.optimize()
  

  This code:

  1. Iterates over days_of_week.
  2. Checks if the current day is in constraint_days_Loc to set constraints for y and yy variables accordingly.
  3. Adds constraints based on the conditions specified, ensuring all variables are iterated correctly.

  Ensure that you replace num_hospitals and blood_types with the correct sets if they are different in your actual implementation. This script ensures the constraints are applied correctly for each combination of blood_types and num_hospitals on specified days. 

  0

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