The R Project SVN R

/*
 *  R : A Computer Language for Statistical Data Analysis
 *  Copyright (C) 1995, 1996  Robert Gentleman and Ross Ihaka
 *  Copyright (C) 2000        The R Development Core Team
 *  Copyright (C) 2005        The R Foundation
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, a copy is available at
 *  http://www.r-project.org/Licenses/
 */

#include "nmath.h"
#include "dpq.h"

double qlogis(double p, double location, double scale, int lower_tail, int log_p)
{
#ifdef IEEE_754
    if (ISNAN(p) || ISNAN(location) || ISNAN(scale))
    return p + location + scale;
#endif
    R_Q_P01_boundaries(p, ML_NEGINF, ML_POSINF);

    if (scale <  0.) ML_ERR_return_NAN;
    if (scale == 0.) return location;

    /* p := logit(p) = log( p / (1-p) )  : */
    if(log_p) {
    if(lower_tail)
        p = p - log1p(- exp(p));
    else
        p = log1p(- exp(p)) - p;
    }
    else
    p = log(lower_tail ? (p / (1. - p)) : ((1. - p) / p));

    return location + scale * p;
}