ernie.WPI.EDU> cat permute.C

// #define DEBUG // comment out this line when the code is working

// file permute.C, cs2223, D97/98 //

// 24Apr98 NW, First version

// include
#include "headers.h"

// global value
unsigned long int counter;

// prototype
void permute(int, int, int*); // calculate permutations of a vector

// main
int main ()
   {
   
   while (1)
      {
      int n;
      cout << "enter a value of n: ";
      cin >> n;
      if (n <= 0) return 0; // exit criterion
      
      int *vect = new int[n]; // the vector
      for (int i = 0; i < n; i++) vect[i] = i; // fill the vector
      counter = 0;
#ifdef DEBUG
      cout << setw(12) << "number" << "  vect" << endl;
#endif
      permute(0, n, vect);
      cout << "there were " << counter << " permutations" << endl << endl;
      }
   } // end main()

void permute(int k, int n, int *vect) // calculate permutations of a vector
   { // n is the length of the vector, k is the position to begin permutations
   if (k >= n-1)
      { // permutation is complete
      counter++;
#ifdef DEBUG
      cout << setw(12) << counter << "  ";
      for (int i = 0; i < n; i++) cout << setw(3) << vect[i];
      cout << endl;
#endif
      return;
      } // end if(i)
   for (int i = k; i < n; i++)
      {
      int temp = vect[k]; vect[k] = vect[i]; vect[i] = temp; // swap
      permute(k+1, n, vect); // recursive call
      temp = vect[k]; vect[k] = vect[i]; vect[i] = temp; // swap back
      }
   } // end permute()


// Copyright 1998 Norman Wittels

ernie.WPI.EDU> g++ permute.C

ernie.WPI.EDU> a.out

enter a value of n: 3
there were 6 permutations

enter a value of n: 8
there were 40320 permutations

enter a value of n: 10
there were 3628800 permutations

enter a value of n: -3

ernie.WPI.EDU>