Hi,
I have been struggling for several days to figure out how to use
NonLinearConjugateGradientOptimizer.
My goal is to optimize a multivariate unconstrained function (for example
f(x,y,z) = x^2 + y^2 + z^2) with conjugate gradient method. I have already
managed to optimize my function with SimplexOptimizer and want to try
Gradient Descent now.
For now, my code is (inspired by all the examples I could found):
import java.util.Arrays;
import org.apache.commons.math3.analysis.MultivariateFunction;
import org.apache.commons.math3.analysis.MultivariateVectorFunction;
import
org.apache.commons.math3.analysis.differentiation.DerivativeStructure;
import org.apache.commons.math3.analysis.differentiation.GradientFunction;
import
org.apache.commons.math3.analysis.differentiation.MultivariateDifferentiableFunction;
import org.apache.commons.math3.optim.InitialGuess;
import org.apache.commons.math3.optim.MaxEval;
import org.apache.commons.math3.optim.PointValuePair;
import org.apache.commons.math3.optim.SimpleValueChecker;
import org.apache.commons.math3.optim.nonlinear.scalar.GoalType;
import
org.apache.commons.math3.optim.nonlinear.scalar.GradientMultivariateOptimizer;
import org.apache.commons.math3.optim.nonlinear.scalar.ObjectiveFunction;
import
org.apache.commons.math3.optim.nonlinear.scalar.ObjectiveFunctionGradient;
import
org.apache.commons.math3.optim.nonlinear.scalar.gradient.NonLinearConjugateGradientOptimizer;
import
org.apache.commons.math3.optim.nonlinear.scalar.noderiv.NelderMeadSimplex;
import
org.apache.commons.math3.optim.nonlinear.scalar.noderiv.SimplexOptimizer;
public class ApacheTest {
public static void main(String[] args) {
SimplexOptimizer simplexOptimizer = new SimplexOptimizer(1e-10,
1e-30);
NonLinearConjugateGradientOptimizer gradientOptimizer = new
NonLinearConjugateGradientOptimizer(NonLinearConjugateGradientOptimizer.Formula.POLAK_RIBIERE,
new SimpleValueChecker(1e-13, 1e-13));
final MyFunction f = new MyFunction();
simplexOptimizerMethod(simplexOptimizer, f);
conjugateGradientMethod(gradientOptimizer, f);
}
private static void
conjugateGradientMethod(NonLinearConjugateGradientOptimizer optimizer,
MyFunction f) {
ObjectiveFunction objF = new ObjectiveFunction(f);
final PointValuePair optimum =
optimizer.optimize(new MaxEval(1000), objF,
GoalType.MINIMIZE, new
InitialGuess(new double[]{ 10, 10, 10 }));
System.out.println(Arrays.toString(optimum.getPoint()) + " : "
+ optimum.getSecond());
}
private static void simplexOptimizerMethod(SimplexOptimizer optimizer,
final MyFunction f) {
final PointValuePair optimum =
optimizer.optimize(
new MaxEval(10000),
new ObjectiveFunction(f),
GoalType.MINIMIZE,
new InitialGuess(new double[]{ 100, 100, 60 }),
new NelderMeadSimplex(new double[]{ 0.2, 0.2, 0.2 }));
System.out.println(Arrays.toString(optimum.getPoint()) + " : "
+ optimum.getSecond());
}
private static class MyFunction implements MultivariateFunction {
public double value(double[] variables) {
final double x = variables[0];
final double y = variables[1];
final double z = variables[2];
return x*x + y*y + z*z;
}
}
}
When I execute the code I get the following error:
Exception in thread "main" java.lang.NullPointerException
at
org.apache.commons.math3.optim.nonlinear.scalar.GradientMultivariateOptimizer.computeObjectiveGradient(GradientMultivariateOptimizer.java:53)
at
org.apache.commons.math3.optim.nonlinear.scalar.gradient.NonLinearConjugateGradientOptimizer.doOptimize(NonLinearConjugateGradientOptimizer.java:254)
at
org.apache.commons.math3.optim.nonlinear.scalar.gradient.NonLinearConjugateGradientOptimizer.doOptimize(NonLinearConjugateGradientOptimizer.java:46)
at
org.apache.commons.math3.optim.BaseOptimizer.optimize(BaseOptimizer.java:153)
at
org.apache.commons.math3.optim.BaseMultivariateOptimizer.optimize(BaseMultivariateOptimizer.java:65)
at
org.apache.commons.math3.optim.nonlinear.scalar.MultivariateOptimizer.optimize(MultivariateOptimizer.java:63)
at
org.apache.commons.math3.optim.nonlinear.scalar.GradientMultivariateOptimizer.optimize(GradientMultivariateOptimizer.java:73)
at
org.apache.commons.math3.optim.nonlinear.scalar.gradient.NonLinearConjugateGradientOptimizer.optimize(NonLinearConjugateGradientOptimizer.java:244)
at
com.sri.ai.expresso.apache.ApacheTest.conjugateGradientMethod(ApacheTest.java:96)
at com.sri.ai.expresso.apache.ApacheTest.main(ApacheTest.java:87)
I think that maybe the problem is a missing argument of type
ObjectiveFunctionGradient in the optimize method:
final PointValuePair optimum =
optimizer.optimize(new MaxEval(1000), objF,
GoalType.MINIMIZE, new
InitialGuess(new double[]{ 10, 10, 10 }));
However, I don't know how to build it.
Does anyone has an idea, or an example which looks like what I am looking
for?
Thank you very much
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