Returns the hyperbolic tangent of a double. - Java java.lang

Java examples for java.lang:Math Trigonometric Function

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Description

Returns the hyperbolic tangent of a double.

Demo Code

/*//from   ww w  .  j  a  v  a  2s  .c o m
 * ------------------------------------------------------------------------=
 * Copyright (C) 1997 - 1998 by Visual Numerics, Inc. All rights reserved.
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 * granted by Visual Numerics, Inc., provided that the copyright notice
 * above and the following warranty disclaimer are preserved in human
 * readable form.
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 * Because this software is licenses free of charge, it is provided
 * "AS IS", with NO WARRANTY.  TO THE EXTENT PERMITTED BY LAW, VNI
 * DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO ITS PERFORMANCE, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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//package com.java2s;

public class Main {
    private static final double TANH_COEF[] = { -.25828756643634710,
            -.11836106330053497, .009869442648006398, -.000835798662344582,
            .000070904321198943, -.000006016424318120, .000000510524190800,
            -.000000043320729077, .000000003675999055,
            -.000000000311928496, .000000000026468828,
            -.000000000002246023, .000000000000190587,
            -.000000000000016172, .000000000000001372,
            -.000000000000000116, .000000000000000009 };

    /**
     * Returns the hyperbolic tangent of a double.
     * 
     * @param x
     *            A double value.
     * @return The hyperbolic tangent of x.
     */
    static public double tanh(double x) {
        double ans, y;
        y = Math.abs(x);

        if (Double.isNaN(x)) {
            ans = Double.NaN;
        } else if (y < 1.82501e-08) {
            // 1.82501e-08 = Math.sqrt(3.0*EPSILON_SMALL)
            ans = x;
        } else if (y <= 1.0) {
            ans = x * (1.0 + csevl(2.0 * x * x - 1.0, TANH_COEF));
        } else if (y < 7.977294885) {
            // 7.977294885 = -0.5*Math.log(EPSILON_SMALL)
            y = Math.exp(y);
            ans = sign((y - 1.0 / y) / (y + 1.0 / y), x);
        } else {
            ans = sign(1.0, x);
        }
        return ans;
    }

    static double csevl(double x, double coef[]) {
        double b0, b1, b2, twox;
        int i;
        b1 = 0.0;
        b0 = 0.0;
        b2 = 0.0;
        twox = 2.0 * x;
        for (i = coef.length - 1; i >= 0; i--) {
            b2 = b1;
            b1 = b0;
            b0 = twox * b1 - b2 + coef[i];
        }
        return 0.5 * (b0 - b2);
    }

    static private double sign(double x, double y) {
        double abs_x = ((x < 0) ? -x : x);
        return (y < 0.0) ? -abs_x : abs_x;
    }
}

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