#include <iostream>
 using namespace std;
 const int NO_STEPS = 4;

 void s(int* a, int i);

 int main ( ) {
     int i, a[NO_STEPS] = {2, 4, 1, 3};

     for ( i = 0; i < NO_STEPS; i++ )
         s( a, i );

     for ( i = NO_STEPS - 1; i >= 0; i-- )
         cout << a[i] << ' ';
     cout << "Boom!\n";

     return 0;
 }

 void s(int* a, int i) {
     int* p;
     int  x, n = NO_STEPS / 2;

     if (i != n) {
         p = a + i; // be careful - pointer addition here
         x = *p;
         *p = a[n];
         a[n] = x;
     }
 }