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
 *
 *  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, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA.
 */
#include "nmath.h"
#include "dpq.h"

double plogis(double x, double location, double scale,
          int lower_tail, int log_p)
{
#ifdef IEEE_754
    if (ISNAN(x) || ISNAN(location) || ISNAN(scale))
    return x + location + scale;
#endif
    if (scale <= 0.0)   ML_ERR_return_NAN;

    x = (x - location) / scale;
    if (ISNAN(x))   ML_ERR_return_NAN;
    if(!R_FINITE(x)) {
    if (x > 0) return R_DT_1;
    /* x < 0 */return R_DT_0;
    }
    x = exp(lower_tail ? -x : x);
    return (log_p ? -log1p(x) : 1 / (1 + x));
}