USACO preface

/*
ID:goharsh1
TASK:preface
LANG:C++
*/
#include <fstream>
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
using namespace std;

ifstream fin("preface.in");
ofstream fout("preface.out");

#define cin fin
#define cout fout


/*
 * Note: I didn't write the function "roman" , since it was a stupid repeating thing to do, I just googled it :D
 */
void roman(char *s, unsigned n)
/* Writes the Roman numeral representing n into the buffer s.
Handles up to n = 3999. Since C doesn't have exceptions, n = 0
causes the whole program to exit unsuccessfully. s should be
have room for at least 16 characters, including the trailing
null. */
 {if (n == 0)
     {puts("Roman numeral for zero requested.");
      exit(EXIT_FAILURE);}

  #define digit(loop, num, c) \
      loop (n >= num)         \
         {*(s++) = c;         \
          n -= num;}
  #define digits(loop, num, c1, c2) \
      loop (n >= num)               \
         {*(s++) = c1;              \
          *(s++) = c2;              \
          n -= num;}

  digit  ( while, 1000, 'M'      )
  digits ( if,     900, 'C', 'M' )
  digit  ( if,     500, 'D'      )
  digits ( if,     400, 'C', 'D' )
  digit  ( while,  100, 'C'      )
  digits ( if,      90, 'X', 'C' )
  digit  ( if,      50, 'L'      )
  digits ( if,      40, 'X', 'L' )
  digit  ( while,   10, 'X'      )
  digits ( if,       9, 'I', 'X' )
  digit  ( if,       5, 'V'      )
  digits ( if,       4, 'I', 'V' )
  digit  ( while,    1, 'I'      )

  #undef digit
  #undef digits

  *s = 0;}

int main(void)
  {
    char buffer[16];
    int n;
    cin>>n;
    int i;
    int all[7]={};
    for(i=1;i<=n;i++)
    {
        roman(buffer,i);
        int j;
        for(j=0;j<16&&buffer[j]!='\0';j++)
        {
            switch(buffer[j])
            {
                case 'I':
                    all[0]++;
                    break;
                case 'V':
                    all[1]++;
                    break;
                case 'X':
                    all[2]++;
                    break;
                case 'L':
                    all[3]++;
                    break;
                case 'C':
                    all[4]++;
                    break;
                case 'D':
                    all[5]++;
                    break;
                case 'M':
                    all[6]++;
                    break;

            }
        }
    }

    if(all[0]!=0)
        cout<<"I "<<all[0]<<endl;
    if(all[1]!=0)
        cout<<"V "<<all[1]<<endl;
    if(all[2]!=0)
        cout<<"X "<<all[2]<<endl;
    if(all[3]!=0)
        cout<<"L "<<all[3]<<endl;
    if(all[4]!=0)
        cout<<"C "<<all[4]<<endl;
    if(all[5]!=0)
        cout<<"D "<<all[5]<<endl;
    if(all[6]!=0)
        cout<<"M "<<all[6]<<endl;

    return 0;
   }