Implementing Soft rules on CPLEX

Question

I have a model that I need to add a new constriant to it but I want this rule to be implemented "only" if it is possible. Is there a way to implement this with cplex or docplex ?

Answer 1

You may use slacks to make a constraint soft:

from docplex.mp.model import Model

mdl = Model(name='buses')
nbbus40 = mdl.integer_var(name='nbBus40')
nbbus30 = mdl.integer_var(name='nbBus30')
slack=mdl.integer_var(name='slack')
mdl.add_constraint(nbbus40*40 + nbbus30*30 >= 300, 'kids')
mdl.add_constraint(nbbus40+nbbus30<=7+slack,'softconstraint')

mdl.minimize(nbbus40*500 + nbbus30*400+10000*slack)

mdl.solve()

mdl.export("c:\\temp\\buses.lp")

for v in mdl.iter_integer_vars():
    print(v," = ",v.solution_value)

which gives

nbBus40  =  6.0
nbBus30  =  2.0
slack  =  1.0

why not trying logical constraints if then ?

https://github.com/AlexFleischerParis/zoodocplex/blob/master/zooifthen.py

from docplex.mp.model import Model

mdl = Model(name='buses')
nbbus40 = mdl.integer_var(name='nbBus40')
nbbus30 = mdl.integer_var(name='nbBus30')
mdl.add_constraint(nbbus40*40 + nbbus30*30 >= 300, 'kids')
mdl.minimize(nbbus40*500 + nbbus30*400)

mdl.solve()

for v in mdl.iter_integer_vars():
   print(v," = ",v.solution_value)

print()
print("with if nb buses 40 more than 3  then nbBuses30 more than 7")

#if then constraint
mdl.add(mdl.if_then(nbbus40>=3,nbbus30>=7))
mdl.minimize(nbbus40*500 + nbbus30*400)

mdl.solve()

 

for v in mdl.iter_integer_vars():
    print(v," = ",v.solution_value) 

'''
which gives
nbBus40  =  6.0
nbBus30  =  2.0
with if nb buses 40 more than 3  then nbBuses30 more than 7
nbBus40  =  0
nbBus30  =  10.0
'''