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106 lines (92 loc) · 2.34 KB
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/*
* @BEGIN LICENSE
*
* Psi4: an open-source quantum chemistry software package
*
* Copyright (c) 2007-2016 The Psi4 Developers.
*
* The copyrights for code used from other parties are included in
* the corresponding files.
*
* 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.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*
* @END LICENSE
*/
/*!
\file
\brief Contains some probability functions
\ingroup QT
*/
namespace psi {
/*!
** combinations() : Calculates the number of ways to choose k objects
** from n objects, or "n choose k"
**
** Parameters:
** \param n = number of objects in total
** \param k = number of objects taken at a time
**
** Returns:
** number of combinations of n objects taken k at a time ("n choose k")
** (returned as a double).
**
** \ingroup QT
*/
double combinations(int n, int k)
{
double factorial(int) ;
double comb ;
if (n == k) return (1.0) ;
else if (k > n) return(0.0) ;
else if (k == 0) return(1.0) ;
comb = factorial(n) / (factorial(k) * factorial(n-k)) ;
return(comb) ;
}
/*!
** factorial(): Returns n!
**
** Parameters:
** \param n = number to take factorial of
**
** Returns:
** n factorial, as a double word (since n! can get very large).
** \ingroup QT
*/
double factorial(int n)
{
if (n == 0 || n == 1) return(1.0);
if (n < 0) return(0.0) ;
else {
return ((double) n * factorial(n-1)) ;
}
}
/*
** test combinations routines
**
#include <cstdio>
main()
{
int i, j ;
double factorial() ;
double combinations() ;
printf("Enter two numbers: ") ;
scanf("%d %d", &i, &j) ;
printf("i! = %.2f\n", factorial(i)) ;
printf("j! = %.2f\n", factorial(j)) ;
printf("i choose j = %.2f\n", combinations(i,j)) ;
}
**
*/
}