C :

#include<stdio.h>
#include<string.h>
char base[21]="0123456789ABCDEFGHIJ";
int main(){
	int N,b,r,i;
	char ans[30]={0};
	while(scanf("%d%d",&N,&b)==2){
		printf("%d=",N);
		i=0;
		while(N!=0){
			r=N%b;
			N=N/b;
			if(r<0){
				r-=b;
				N++;
			}
			ans[i++]=base[r];
		}
		for(i=strlen(ans)-1;i>=0;i--){
			printf("%c",ans[i]);
		}
		printf("(base%d)\n",b);
	}
	return 0;
}

C++ :

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