# U-NSGA-III¶

The algorithm is implemented based on . NSGA-III selects parents randomly for mating. It has been shown that tournament selection performs better than random selection. The U stands for unified and increases the performance of NSGA-III by introducing tournament pressure.

The mating selections works as follows: ## Example¶

:

import numpy as np

from pymoo.algorithms.nsga3 import NSGA3
from pymoo.algorithms.unsga3 import UNSGA3
from pymoo.factory import get_problem
from pymoo.optimize import minimize

problem = get_problem("ackley", n_var=30)

# create the reference directions to be used for the optimization - just a single one here
ref_dirs = np.array([[1.0]])

# create the algorithm object
algorithm = UNSGA3(ref_dirs, pop_size=100)

# execute the optimization
res = minimize(problem,
algorithm,
termination=('n_gen', 150),
save_history=True,
seed=1)

print("UNSGA3: Best solution found: \nX = %s\nF = %s" % (res.X, res.F))

UNSGA3: Best solution found:
X = [-0.03361763 -0.0765763   0.08319404 -0.01884934 -0.08956898  0.02424948
-0.0742758  -0.03324263  0.01375688  0.07366408 -0.07184055 -0.03413214
0.05850567  0.00359355  0.05727249  0.08145104  0.02723058 -0.00022916
0.01453122 -0.07837368 -0.03632389  0.02878503 -0.01358104  0.00727682
-0.06518616 -0.02728898  0.01839204  0.01375858 -0.06653483  0.05340933]
F = [0.33397188]


U-NSGA-III has for single- and bi-objective problems a tournament pressure which is known to be useful. In the following we provide a quick comparison (here just one run, so not a valid experiment), to see the difference in convergence.

:

_res = minimize(problem,
NSGA3(ref_dirs, pop_size=100),
termination=('n_gen', 150),
save_history=True,
seed=1)
print("NSGA3: Best solution found: \nX = %s\nF = %s" % (res.X, res.F))

NSGA3: Best solution found:
X = [-0.03361763 -0.0765763   0.08319404 -0.01884934 -0.08956898  0.02424948
-0.0742758  -0.03324263  0.01375688  0.07366408 -0.07184055 -0.03413214
0.05850567  0.00359355  0.05727249  0.08145104  0.02723058 -0.00022916
0.01453122 -0.07837368 -0.03632389  0.02878503 -0.01358104  0.00727682
-0.06518616 -0.02728898  0.01839204  0.01375858 -0.06653483  0.05340933]
F = [0.33397188]

:

import numpy as np
import matplotlib.pyplot as plt

ret = [np.min(e.pop.get("F")) for e in res.history]
_ret = [np.min(e.pop.get("F")) for e in _res.history]

plt.plot(np.arange(len(ret)), ret, label="unsga3")
plt.plot(np.arange(len(_ret)), _ret, label="nsga3")
plt.title("Convergence")
plt.xlabel("Generation")
plt.ylabel("F")
plt.legend()
plt.show() ## API¶

class pymoo.algorithms.unsga3.UNSGA3(ref_dirs, selection=<pymoo.operators.selection.tournament_selection.TournamentSelection object>, **kwargs)