C :

#include<stdio.h>
int main()
{
	int n,m;
	while((scanf("%d %d",&n,&m))!=EOF)
	{
		int i,j=0,k,t,a[30],s;
		s=n;
		while(n!=0)
		{
			k=n%m;
			t=n/m;
			if(k<0)
			{
				k=k-m;
				t++;
			}
			n=t;
			a[j++]=k;
		}
		printf("%d=",s);
		for(i=j-1;i>=0;i--)
		{
			if(a[i]>=10)
				a[i]=a[i]-10+'A';
			else
				a[i]=a[i]+'0';
			printf("%c",a[i]);
		}
		printf("(base%d)\n",m);
	}
	return 0;
}

C++ :

#include <iostream>
#include <string>
using namespace std;
const char nn[20] = {'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J'};
char a[100];
int main()
{
int m, n, k, t, s,kk;
while(cin >> m >> n)
    {
     s = m;kk=0;
       while(m != 0)
       {
             k = m % n;
             t = m / n;
           if(k < 0)
                 {
                   k -= n;
                    t++;
                 }
           m = t;
           a[kk++]=nn[k];
      }
cout << s << "=";
 for(int i =kk-1 ; i >= 0; i--)
    {
         cout << a[i];
    }
cout << "(base" << n << ")" << endl;
}
return 0;
}