Use value of Var as dictionary key

Answered

Huib Meulenbelt

• September 06, 2021 14:08
• Edited

Hi all,

I have the following Var and corresponding constraints:

m = gurobi.Model()

s_vars = m.addVars(4, lb=0, ub=5, vtype=gurobi.GRB.CONTINUOUS, name='s')
m.update()

m.addConstr(s_vars[0] == 2)
m.addConstr(s_vars[1] == 2)
m.addConstr(s_vars[2] == 0)
m.addConstr(s_vars[3] == 3)

m.optimize()

In addition, I have a dictionary:

optimal_values = defaultdict(int)
optimal_values[0] = 0
optimal_values[1] = 10
optimal_values[2] = 100
optimal_values[3] = 1000

If I run 

gurobi.quicksum([optimal_values[s_var] for s_var in s_vars])

the output is 

<gurobi.LinExpr: 11100.0>

I expected this to be 12000

Running 

gurobi.quicksum([optimal_values[s_var.Xn] for s_var in s_vars.values()])

shows 

<gurobi.LinExpr: 12000.0>

Although the second option gives the right output, I am note sure whether this is the appropriate methodology. Maybe I am mistaken, but I haven't seen any other code that uses Xn in quicksum. Please advice

1

• 

  Jaromił Najman

  Gurobi Staff

  • September 06, 2021 15:34
  • Edited

  Hi,

  The addVars function returns a tupledict. As a consequence, the values of \(\texttt{s_var}\) in the

  for s_var in s_vars

  loop return the keys to the variables, i.e., values \(0-3\). Thus, the \(\texttt{quicksum}\)

  gp.quicksum([optimal_values[s_var] for s_var in s_vars])

  returns the sum over

  optimal_values[0], optimal_values[1], optimal_values[2], optimal_values[3]

  However, the second quicksum

  gp.quicksum([optimal_values[s_var.X] for s_var in s_vars.values()])

  actually accesses the variable objects because you loop over the variable objects as you use \(\texttt{s_vars.values()}\). This then sums the values

  optimal_values[s_var[0].X], optimal_values[s_var[1].X], optimal_values[s_var[2].X], optimal_values[s_var[3].X]

  This is the same as

  gp.quicksum([optimal_values[s_vars[s_var].X] for s_var in s_vars])

  This is OK as long as you can ensure that the variable solution values are integer and within the \(0-3\) range.

  Best regards,
  Jaromił

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