cplex changes the fixed variable after solving the problem in pyomo

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I am trying to solve a MIP, I use pyomo, and Cplex(Interactive Optimizer 20.1.0.0) is solver. The problem is that I want to fix some binary integer variables then solve the problem, and I used:

model.y[1,4].fix(1)
model.y[2,3].fix(0)

, but I have noticed that after solving the problem those fixed variables have changed to another values. How can I say cplex to not change that fixed variables?

pyomo python
2 Answers

let me use the bus example for fixed start with pyomo

import pyomo.environ as pyo
from pyomo.opt import SolverFactory

opt = pyo.SolverFactory("cplex")

model = pyo.ConcreteModel()

model.nbBus = pyo.Var([40,30], domain=pyo.PositiveIntegers)

#fixed start

model.nbBus[40].fix(3)

# end of fixed start

model.OBJ = pyo.Objective(expr = 500*model.nbBus[40] + 400*model.nbBus[30])

model.Constraint1 = pyo.Constraint(expr = 40*model.nbBus[40] + 30*model.nbBus[30] >= 300)

results = opt.solve(model)

print("nbBus40=",int(model.nbBus[40].value))
print("nbBus30=",int(model.nbBus[30].value))

gives

nbBus40= 3
nbBus30= 6

whereas if you remove the fixed start you get

nbBus40= 6
nbBus30= 2
user_37255960

Since fixing variable didn't work for me, I tackle my problem with adding a new constraint to the model, and it works.

def yvar_fix(model, i, j ):
    if (i,j) in y_set_init:
        constraint = (model.y[i,j] == 1)
    else:
        constraint = (model.y[i,j] == 0) 
    return constraint
model.yvar_fix = pe.Constraint(model.edges, rule=yvar_fix)
user_177796510
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