The R Project SVN R

/*
 *  R : A Computer Language for Statistical Data Analysis
 *  Copyright (C) 1995, 1996  Robert Gentleman and Ross Ihaka
 *
 *  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 "Mathlib.h"

double pt(double x, double n)
{
/* return  P[ T <= x ]  where
 * T ~ t_{n}  (t distrib. with n degrees of freedom).

 *  --> ./pnt.c for NON-central
 */
    double val;
#ifdef IEEE_754
    if (ISNAN(x) || ISNAN(n))
    return x + n;
#endif
    if (n <= 0.0) {
    ML_ERROR(ME_DOMAIN);
    return ML_NAN;
    }
#ifdef IEEE_754
    if(!R_FINITE(x))
    return (x < 0) ? 0 : 1;
    if(!R_FINITE(n))
    return pnorm(x, 0.0, 1.0);
#endif
    if (n > 4e5) { /*-- Fixme(?): test should depend on `n' AND `x' ! */
    /* Approx. from  Abramowitz & Stegun 26.7.8 (p.949) */
    val = 1./(4.*n);
    return pnorm(x*(1. - val)/sqrt(1. + x*x*2.*val), 0.0, 1.0);
    }
    val = 0.5 * pbeta(n / (n + x * x), n / 2.0, 0.5);
    return (x > 0.0) ? 1 - val : val;
}