function - Taking a slice out of a string incorrectly C++ -

i attempting write function outputs string of 5 characters string of concatenated primes 1000 characters long starting @ point (n) user specifies. have gotten program compile seem running errors in program , have not yet been taught of tools me see issue in programming. believe issue in getting , displaying slice itself, not concatenated string...but again, may be/probably wrong. advice appreciated.

-aspiring computer crim. major

#include<iostream> #include<string>   std::string get_concatenated_primes() {         unsigned int base, test, cap=1000, prime=0;         std::string plist;         (base=0; base < cap; ++base)         {             (test=2; test < base; ++test)             {                     if (base % test == 0)                             break;                     else                     {                             prime=1;                     }             }              if (prime == 1 &&  plist.length() < 1000)             {                     plist += base;             }     }     return plist; }   std::string get_slice_of_5(const std::string concat_primes, const unsigned int n) {         std::string slice;         slice = concat_primes.substr (n, 5);         return slice; }   int main() {     unsigned int n;      while(std::cin >> n)     {         std::string concat_primes = get_concatenated_primes();         std::cout << get_slice_of_5(concat_primes, n) << std::endl;     }     return 0; } 

terminate called after throwing instance of 'std::out_of_range' what(): basic_string::substr: __pos (which 998) > this->size() (which 997)

i did not want do, should check concat_primes has more characters n.