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  1. #include <type_traits>
  2. #include <vector>
  3. #include <iostream>
  4.  
  5. constexpr auto MAX = 100000000 ; // 10^8
  6. using int_type = std::remove_const< decltype(MAX) >::type ;
  7.  
  8. std::vector<int_type> prime_numbers()
  9. {
  10.     const int_type SZ = MAX / 2 + 1 ;
  11.     std::vector<bool> sieve( SZ, true ) ;
  12.     for( int_type i = 2 ; i < SZ ; ++i ) if( sieve[i] )
  13.        for( int_type j = i+i ; j < SZ ; j += i ) sieve[j] = false ;
  14.  
  15.     std::vector<int_type> result ;
  16.     for( int_type i = 2 ; i < SZ ; ++i ) if( sieve[i] ) result.push_back(i) ;
  17.     return result ;
  18. }
  19.  
  20. int main()
  21. {
  22.     auto primes = prime_numbers() ;
  23.  
  24.     int_type count = 0 ;
  25.  
  26.     for( auto p : primes )
  27.     {
  28.         for( auto p2 : primes )
  29.         {
  30.             if( p*p2 > MAX ) break ;
  31.             if( p2 >= p ) ++count ;
  32.         }
  33.     }
  34.  
  35.     std::cout << count << '\n' ; // 17427258
  36. }
  37.  
Success #stdin #stdout 1.66s 2984KB
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stdout
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17427258
https://ideone.com/I4ibdE
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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