C++ examples for File Stream:stream
Writing primes to a file
#include <cmath> #include <fstream> #include <iomanip> #include <iostream> #include <string> #include <vector> using ulong = unsigned long long; int main(){//from w w w . j a v a 2s . co m int max {5}; std::vector<ulong> primes {2ULL, 3ULL, 5ULL}; // First three primes defined ulong trial {5ULL}; // Candidate prime bool isprime {false}; // true when a prime is found ulong limit {}; // Maximum divisor while (primes.size() < max) { trial += 2; // Next value for checking limit = static_cast<ulong>(std::sqrt(trial)); for (auto prime : primes) { if (prime > limit) break; // Only check divisors < square root isprime = trial % prime > 0; // false for exact division... if (!isprime) break; // ...if so it's not a prime } if (isprime) // If we found one... primes.push_back(trial); // ...save it } std::string filename {"d:\\Example_Data\\primes.txt"}; std::ofstream outFile {filename}; // Define file stream object // Output primes to file int perline {5}; // Prime values per line int count {}; for (auto prime : primes) { outFile << std::setw(10) << prime; if (++count % perline == 0) // New line after every perline primes outFile << std::endl; } outFile << std::endl; std::cout << max << " primes written to " << filename << std::endl; }