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Java example source code file (GaussIntegrator.java)

This example Java source code file (GaussIntegrator.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

dimensionmismatchexception, gaussintegrator, nonmonotonicsequenceexception

The GaussIntegrator.java Java example source code

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.math3.analysis.integration.gauss;

import org.apache.commons.math3.analysis.UnivariateFunction;
import org.apache.commons.math3.exception.DimensionMismatchException;
import org.apache.commons.math3.exception.NonMonotonicSequenceException;
import org.apache.commons.math3.util.MathArrays;
import org.apache.commons.math3.util.Pair;

/**
 * Class that implements the Gaussian rule for
 * {@link #integrate(UnivariateFunction) integrating} a weighted
 * function.
 *
 * @since 3.1
 */
public class GaussIntegrator {
    /** Nodes. */
    private final double[] points;
    /** Nodes weights. */
    private final double[] weights;

    /**
     * Creates an integrator from the given {@code points} and {@code weights}.
     * The integration interval is defined by the first and last value of
     * {@code points} which must be sorted in increasing order.
     *
     * @param points Integration points.
     * @param weights Weights of the corresponding integration nodes.
     * @throws NonMonotonicSequenceException if the {@code points} are not
     * sorted in increasing order.
     * @throws DimensionMismatchException if points and weights don't have the same length
     */
    public GaussIntegrator(double[] points,
                           double[] weights)
        throws NonMonotonicSequenceException, DimensionMismatchException {
        if (points.length != weights.length) {
            throw new DimensionMismatchException(points.length,
                                                 weights.length);
        }

        MathArrays.checkOrder(points, MathArrays.OrderDirection.INCREASING, true, true);

        this.points = points.clone();
        this.weights = weights.clone();
    }

    /**
     * Creates an integrator from the given pair of points (first element of
     * the pair) and weights (second element of the pair.
     *
     * @param pointsAndWeights Integration points and corresponding weights.
     * @throws NonMonotonicSequenceException if the {@code points} are not
     * sorted in increasing order.
     *
     * @see #GaussIntegrator(double[], double[])
     */
    public GaussIntegrator(Pair<double[], double[]> pointsAndWeights)
        throws NonMonotonicSequenceException {
        this(pointsAndWeights.getFirst(), pointsAndWeights.getSecond());
    }

    /**
     * Returns an estimate of the integral of {@code f(x) * w(x)},
     * where {@code w} is a weight function that depends on the actual
     * flavor of the Gauss integration scheme.
     * The algorithm uses the points and associated weights, as passed
     * to the {@link #GaussIntegrator(double[],double[]) constructor}.
     *
     * @param f Function to integrate.
     * @return the integral of the weighted function.
     */
    public double integrate(UnivariateFunction f) {
        double s = 0;
        double c = 0;
        for (int i = 0; i < points.length; i++) {
            final double x = points[i];
            final double w = weights[i];
            final double y = w * f.value(x) - c;
            final double t = s + y;
            c = (t - s) - y;
            s = t;
        }
        return s;
    }

    /**
     * @return the order of the integration rule (the number of integration
     * points).
     */
    public int getNumberOfPoints() {
        return points.length;
    }

    /**
     * Gets the integration point at the given index.
     * The index must be in the valid range but no check is performed.
     * @param index index of the integration point
     * @return the integration point.
     */
    public double getPoint(int index) {
        return points[index];
    }

    /**
     * Gets the weight of the integration point at the given index.
     * The index must be in the valid range but no check is performed.
     * @param index index of the integration point
     * @return the weight.
     */
    public double getWeight(int index) {
        return weights[index];
    }
}

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