UVa 10789 - Prime Frequency

// UVa 10789 - Prime Frequency
#include <iostream>
#include <string>
#include <string.h>
#include <stdio.h>
using namespace std;

int main() {
	// primes
	bool prime[2001];
	memset(prime, true, sizeof(prime));
	prime[0] = false;
	prime[1] = false;
	for (int i = 2; i <= 44; i++) {
		for (int j = i * i; j < 2001; j += i)
			prime[j] = false;
	}
	//
	int tt;
	cin >> tt;
	string line;
	getline(cin, line);
	for (int t = 1; t <= tt; t++) {
		getline(cin, line);
		int f[256];
		memset(f, 0, sizeof(f));
		for (int i = 0; i < line.length(); i++) {
			f[line[i]]++;
		}
		cout << "Case " << t << ": ";
		bool empty = true;
		for (char c = '0'; c <= '9'; c++)
			if (prime[f[c]]) {
				cout << c;
				empty = false;
			}
		for (char c = 'A'; c <= 'Z'; c++)
			if (prime[f[c]]) {
				cout << c;
				empty = false;
			}
		for (char c = 'a'; c <= 'z'; c++)
			if (prime[f[c]]) {
				cout << c;
				empty = false;
			}
		if (empty)
			cout << "empty";
		cout << endl;
	}
	return 0;
}