[PYTHON] Try using platypus, a multipurpose optimization library

Multipurpose optimization

Multi-objective optimization is the simultaneous optimization of multiple objective functions that are in a trade-off relationship. In the case of single-objective optimization, there is one optimal solution, but in the case of multi-objective optimization, there is not always one optimal solution. The optimal solution in multi-objective optimization is called ** Pareto optimal solution **. A diagram of the Pareto optimal solution is shown below.

platypus platypus seems to be one of the libraries for multipurpose optimization. Specifically, it seems that many methods such as NSGA-II, NSGA-III, MOEA / D, IBEA, Epsilon-MOEA, SPEA2, GDE3, OMOPSO, SMPSO, Epsilon-NSGA-II can be used.

Install with pip to use it.

pip install platypus-opt

Test questions

Platypus also provides test questions. This time, let's use a test question called DTLZ2.

from platypus import NSGAII, Problem, Real, nondominated, Integer
import matplotlib.pyplot as plt
from platypus.problems import DTLZ2

def main():
    #problem,Set the algorithm,Search execution
    problem = DTLZ2(2)
    algorithm = NSGAII(problem, population_size=100)
    algorithm.run(10000)

    #Extract non-inferiority
    nondominated_solutions = nondominated(algorithm.result)

    #Draw graph
    plt.scatter([s.objectives[0] for s in nondominated_solutions if s.feasible],
               [s.objectives[1] for s in nondominated_solutions if s.feasible])
    plt.show()

if __name__ == '__main__':
    main()

The result of optimizing as a two-purpose problem as DTLZ2 (2) is as follows. It was confirmed that the Pareto optimal solution was obtained.

I tried to optimize it as a 3 purpose problem as DTLZ2 (3).

Create and optimize problems

Platypus also allows you to optimize your own problems. First, let's set the objective function. This time, we will optimize the two-variable two-purpose minimization problem. The following two are the objective functions.

f(x)=2x_1^2+x_2^2

g(x)=-x_1^2-2x_2^2

#Objective function setting
def objective(vars):
    x1 = int(vars[0])
    x2 = int(vars[1])
    return [2*(x1**2) + x2**2, -x1**2 -2*(x2**2)]

After setting the objective function, let's set the search. You can set the problem with Problem (number of variables, number of objective functions).

#2 variables 2 problem of purpose
problem = Problem(2, 2)

problem.directions [:] = Problem.MINIMIZE sets it as a minimization problem. If you want to make it a maximization problem, you can set it with Problem.MAXIMIZE. If you want to mix minimization and maximization, you can set it for each objective variable. For example, problem.directions [:] = [Problem.MINIMIZE, Problem.MAXIMIZE].

#Set minimize or maximize
problem.directions[:] = Problem.MINIMIZE

Next, let's set the coefficient of determination. This time, both $ x_1 $ and $ x_2 $ are integers, and the range is $ 0 \ leq x_1 \ leq100 $ and $ 0 \ leq x_2 \ leq50 $. This time, the coefficient of determination is an integer, but if you want to handle it as a real number, you can change Integer to Real.

#Set the range of decision variables
int1 = Integer(0, 100)
int2 = Integer(0, 50)
problem.types[:] = [int1, int2]

The above code can be summarized as follows.

def main():
    #2 variables 2 problem of purpose
    problem = Problem(2, 2)
    #Set minimize or maximize
    problem.directions[:] = Problem.MINIMIZE
    #Set the range of decision variables
    int1 = Integer(0, 100)
    int2 = Integer(0, 50)
    problem.types[:] = [int1, int2]
    problem.function = objective
    #Set the algorithm,Search execution
    algorithm = NSGAII(problem, population_size=50)
    algorithm.run(1000)

The following results were obtained. It is not a smooth curve like DTLZ2 (), but I was able to obtain a Pareto optimal solution.

By the way, you can add the information of Pareto optimal solution to DataFrame by adding the following code.

df = pd.DataFrame(columns=("x1", "x2", "f1", "f2"))
for i in range(len(nondominated_solutions)):
    df.loc[i, "x1"] = int1.decode(nondominated_solutions[i].variables[0])
    df.loc[i, "x2"] = int2.decode(nondominated_solutions[i].variables[1])
    df.loc[i, "f1"] = nondominated_solutions[i].objectives[0]
    df.loc[i, "f2"] = nondominated_solutions[i].objectives[1]
df.to_csv("NSGAII.csv")

Reference site

Platypus - Multiobjective Optimization in Python