Version: 0.5.0

# Problem¶

There exist a couple of different ways for defining an optimization problem in pymoo. In contrast to other optimization frameworks in Python, the preferred way is to define an object. However, a problem can also be defined by functions as shown here. Most algorithms in pymoo are population-based, which implies in each generation, not a single but multiple solutions are evaluated. Thus, the problem implementation retrieves the set of solutions to provide the most flexibility to the end-user. This flexibility allows you to implement a custom parallelization and thus to use your hardware most efficiently. Three different ways of defining a problem are shown below:

Overview

• Problem: Object-oriented definition Problem which implements a method evaluating a set of solutions.

• ElementwiseProblem: Object-oriented definition ElementwiseProblem which implements a function evaluating a single solution at a time.

• FunctionalProblem: Define a problem FunctionalProblem by using a function for each objective and constraint.

Next, we define an unconstrained optimization problem with two variables and two objectives. Because the lower and upper bounds are identical for both variables, only a float value is passed to the Problem constructor. Assuming the Algorithm has a population size N, the input variable x is a two-dimensional matrix with the dimensions (N,2). The input has two columns because the optimization problem has n_var=2. Thus, to evaluated the problem makes use of the vectorized calculations [:, 0] and [:, 1] to select the first and second variables for each row in the input matrix x.

[1]:

import numpy as np
from pymoo.core.problem import Problem

class MyProblem(Problem):

def __init__(self):
super().__init__(n_var=2,
n_obj=2,
xl=-2.0,
xu=2.0)

def _evaluate(self, x, out, *args, **kwargs):
f1 = 100 * (x[:, 0]**2 + x[:, 1]**2)
f2 = (x[:, 0]-1)**2 + x[:, 1]**2
out["F"] = np.column_stack([f1, f2])


Below we define a constrained optimization problem with two variables and two objectives. Here, the problem is defined element-wise. The lower and upper bounds, xl and xu, are defined using a vector with a length equal to the number of variables. The input x is a one-dimensional array of length two and is called N times in each iteration for the algorithm discussed above.

[2]:

import numpy as np
from pymoo.core.problem import ElementwiseProblem

class MyProblem(ElementwiseProblem):

def __init__(self):
super().__init__(n_var=2,
n_obj=2,
n_constr=2,
xl=np.array([-2,-2]),
xu=np.array([2,2]))

def _evaluate(self, x, out, *args, **kwargs):
f1 = 100 * (x[0]**2 + x[1]**2)
f2 = (x[0]-1)**2 + x[1]**2

g1 = 2*(x[0]-0.1) * (x[0]-0.9) / 0.18
g2 = - 20*(x[0]-0.4) * (x[0]-0.6) / 4.8

out["F"] = [f1, f2]
out["G"] = [g1, g2]

problem = MyProblem()


Tip

For more information, please look at the problem tutorial. Moreover, a number of test problems frequently being use for benchmarking the performance of an algorithm are listed here.