value the variable
AnsweredHello All!
I have a MIP model in gurobi and I want to value the variables.
in other words I have a binary variable called x(r,i,j). it is 1 when vehicle r goes from i to j.
I want to value this variable like this in loop and compare the objectives:
for j in N:
x[(1,0,j)].ub=1
x[(1,0,j)].lb=1
it is important to me to know what is difference when vehicle 1 after node 0 go to node 1 or 2 or 3 or ...

Hi Niloofar,
You can access the value of an optimization variable after at least one feasible solution has been found via the X attribute. Example mip1.py shows how to print variable values after a successful optimization.
Best regards,
Jaromił0 
Actually I do not want to print variable value, I want to give them value by myself and then compare each solution.
0 
If you want to fix a variable to a particular value you can either add an equality constraint fixing it to a constant value or setting a variable's lower and upper bound to the same value.
# fix via equality constraint
x = model.addVar(vtype=GRB.BINARY)
model.addConstr(x==1)
# fix via bounds
x.lb = 1
x.ub = 1
# [...]
# optimize model with fixed values to get new solution and objective value
model.optimize()Best regards,
Jaromił0 
Dear,
Actually I want to give value 1 to all of x variable but in order.
for example: at first x(1,0,2)=1 and get model objective, then x(1,0,3)=1 and get model objective by this criteria again.
this would be loop for value this variable.
please give me hand to get the solution.
0 
You can use the code from my previous comment and put it in a \(\texttt{for}\)loop to achieve what you need.
for j in J:
# fix variable to desired value
x[(1,0,j)].lb = 1
x[(1,0,j)].ub = 1
# optimize model
model.optimize()
# if at least one solution point has been found,
# set obj to objective value and infinity otherwise
if (model.SolCount > 0)
obj = model.ObjVal
else
obj = GRB.INFINITY
# do something with the objective value for this run
# [...]
# reset bound of the variable
x[(1,0,j)].lb = 0
x[(1,0,j)].ub = 1You can find more information about model and variable attributes in the attributes documentation.
Best regards,
Jaromił0 
Thank a lot. It works.
0 
Sorry, I have another question.
how can I bring the objective value in a dictionary or table with their variable?
0 
Note that this is no longer Gurobi related. You can use a standard Python dictionary for this
d = {}
for j in J:
#[..]
d[(1,0,j)] = obj
# access objective value for fixed variable (1,0,0)
print(d[(1,0,0)])0 
Thanks again.
0
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