Can't fix the value of a variable

[PastelBelem8]

It is not possible to fix the value of a variable.

Integer(0,0)

It would be useful in the context of multiple iterations in a optimization process where I keep narrowing the solution space, focusing on the best solutions.

[dhadka]

One option is to define a custom type for constants:

class Constant(Type):
    def __init__(self, value):
        super(Constant, self).__init__()
        self.value = value
        
    def rand(self):
        return self.value
        
    def __str__(self):
        return "Constant(%s)" % (self.value)

The only issue with this is you must explicitly tell Platypus what variation operators to use. It does not have default operators for mixed types.

from platypus import *

def example(x):
    return [x[0], x[1]]

problem = Problem(2, 2)
problem.types[0] = Real(-10, 10)
problem.types[1] = Constant(5)
problem.function = example

algorithm = NSGAII(problem, variator=CompoundOperator(SBX(), PM()))
algorithm.run(10000)
print(algorithm.result)

The mutation and crossover operators will only be applied to the Real variable(s) and will skip the Constant variable.

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