C++ examples for File Stream:stream
Reading and writing the primes file
#include <fstream> #include <iostream> #include <iomanip> #include <cmath> #include <string> using std::ios;/*from w ww. j a v a2 s . c om*/ using std::string; using ulong = unsigned long long; ulong nextprime(ulong aprime, const string filename); // Find the prime following aprime int main() { string filename {"D:\\primes.txt"}; int nprimes {5}; int count {}; ulong lastprime {}; int perline {5}; std::ifstream inFile; // Create input file stream inFile.open(filename); // Open the filename file for input std::ifstream::pos_type here {inFile.tellg()}; if (!inFile.fail()) { while (true) { inFile >> lastprime; if (inFile.eof()) break; std::cout << std::setw(10) << lastprime << (++count % perline == 0 ? "\n" : ""); if (count == nprimes) { inFile.close(); std::cout << std::endl << count << "primes found in file." << std::endl; return 0; } } } // If we get to here, we need to find more primes inFile.clear(); inFile.close(); try { int oldCount {count}; std::ofstream outFile; if (oldCount == 0) { outFile.open(filename); if (!outFile.is_open()) throw ios::failure {string {"Error opening output file "} + filename + " in main()"}; outFile << "2\n3\n5\n"; // Write 1st three primes to file outFile.close(); std::cout << std::setw(10) << 2 << std::setw(10) << 3 << std::setw(10) << 5; lastprime = 5ULL; count = 3; } while (count < nprimes){ lastprime = nextprime(lastprime, filename); outFile.open(filename, ios::out | ios::app); // Open file to append data if (!outFile.is_open()) throw ios::failure {string {"Error opening output file "} + filename + " in main()"}; outFile << lastprime << '\n'; outFile.close(); std::cout << std::setw(10) << lastprime << (++count % perline == 0 ? "\n" : ""); } std::cout << std::endl << nprimes << " primes found. " << nprimes-oldCount << " added to file." << std::endl; } catch (std::exception& ex) { std::cout << typeid(ex).name() << ": " << ex.what() << std::endl; return 1; } } // Find the next prime after last ulong nextprime(ulong last, const string filename) { bool isprime {false}; // true when we have a prime ulong aprime {}; // Stores a prime from the file std::ifstream inFile; // Local file input stream object // Find the next prime ulong limit {}; while (true) { last += 2ULL; // Next value for checking limit = static_cast<ulong>(std::sqrt(last)); // Try dividing the candidate by all the primes up to limit inFile.open(filename); // Open the primes file if (!inFile.is_open()) throw ios::failure {string {"Error opening input file "} +filename + " in nextprime()"}; do { inFile >> aprime; } while (aprime <= limit && !inFile.eof() && (isprime = last % aprime > 0)); inFile.close(); if (isprime) // We got one... return last; // ...so return it } }