Change objective in GEKKO optimisation suite python

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I am using GEKKO as a MINP-solver. To establish more or less lower and upper bounds, I would like to minimise the sum of variables once, and then maximise it. GEKKO adds up the objectives on the second optimisation, however. How can I delete the old objective and add a new one? And how can I reset the variables to their default values after the first optimisation?

Thank you for any help

solver = GEKKO()
solver.Minimize(sum(x))
solver.solve(disp=False)
print(x)
solver.Maximize(sum(x))
solver.solve(disp=False)
print(x)
gekko python
1 Answers

Thanks for proposing the solution to your own question. You can create an answer to your own question and mark it as the accepted answer. The comments section is also a good place to put the solution but then StackOverflow thinks the questions is still unanswered.

Approach 1

Here are two ways to switch from minimize to maximimize. If it is just Minimize(sum(x)) to Maximize(sum(x)) then just included an extra Maximize(sum(x)) to cancel out the prior Minimize(sum(x)). Here is a simple script that shows this first approach:

from gekko import GEKKO
m = GEKKO(remote=False)
x = m.Array(m.Var,3,lb=0,ub=1)
m.Minimize(m.sum(x))
m.solve(disp=False)
print('-'*20)
print('x: ', x)
print('obj: ', m.options.OBJFCNVAL)

m.Maximize(2*m.sum(x))
m.solve(disp=False)
print('-'*20)
print('x: ', x)
print('obj: ', m.options.OBJFCNVAL)

Approach 2

A second approach (as you already found) is to clear the _objectives list with m._objectives.clear(). Here is a sample script with that second approach:

from gekko import GEKKO
m = GEKKO(remote=False)
x = m.Array(m.Var,3,lb=0,ub=1)
m.Minimize(m.sum(x))
m.solve(disp=False)
print('-'*20)
print('x: ', x)
print('obj: ', m.options.OBJFCNVAL)

m._objectives.clear()
m.Maximize(m.sum(x))
m.solve(disp=False)
print('-'*20)
print('x: ', x)
print('obj: ', m.options.OBJFCNVAL)
user_23669410
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