SuperPrime Rib

#include <cstdio>
#include <vector>
#include <fstream>
#include <math.h>
#include <algorithm>

using namespace std;

bool isPrime(int num){
    if(num == 0 || num == 1){ return false; }
    bool k = true;
    for(int f=2; f<sqrt(num)+1; f++){
        if(num % f == 0){
            k = false;
            break;
        }
    }
    return k;
}

void search(int n, vector<int> & res){
    vector<int> primes = {2,3,5,7}; // initialize with one-digit primes
    vector<int> newDigits = {1,3,7,9}; // the digits that we can add
    for(int len = 2; len <= n; len++){
        vector<int> newPrimes;
        for(int prime:primes){
            for(int d:newDigits){
                int newPrime = 10*prime+d;
                if(isPrime(newPrime)){
                    newPrimes.push_back(newPrime);
                }
            }
        }
        primes = newPrimes;
    }
    sort(primes.begin(),primes.end());
    res = primes;
}

int main(){
    // Get the length of primes we want
    ifstream fIn("sprime.in");
    int n;
    fIn >> n;
    fIn.close();

    // Get the superprimes
    vector<int> res;
    search(n,res);

    // Write primes to file
    ofstream fOut("sprime.out");
    for(int p:res){
        fOut << p << endl;
    }
    fOut.close();
}