prog19_03.cpp

一本语言类编程书籍

// Program 19.3 Reading and writing the primes file  File: prog19_03.cpp
#include <fstream>
#include <iostream>
#include <iomanip>
#include <cmath>
#include <string>
using std::ios;
using std::cout;
using std::cin;
using std::endl;
using std::string;

long nextprime(long lastprime, const char* filename);  // Find the prime after lastprime

int main() {
  const char* filename = "C:\\JunkData\\primes.txt";
  int nprimes = 0;                          // Number of primes required
  int count = 0;                            // Count of primes found
  long lastprime = 0;                       // Last prime found

  // Get number of primes required
  int tries = 0;                            // Number of input tries
  cout << "How many primes would you like (at least 3)?: ";
  do {
    if(tries)
      cout << endl << " You must request at least 3, try again: ";
    cin >> nprimes;

    if(++tries == 5) {                      // Five tries is generous
      cout << endl << " I give up!" << endl;
      return 1;
    }
  } while(nprimes < 3);

  std::ifstream inFile;                     // Create input file stream object
  inFile.open(filename);                    // Open the file as an input stream

  cout << endl;
  if(!inFile.fail()) {
    do {
      inFile >> lastprime;
      cout << (count++ % 5 == 0 ? "\n" : "") << std::setw(10) << lastprime;
    } while(count < nprimes && !inFile.eof());
    inFile.close();
  }
  inFile.clear();                           // Clear any errors

  try {
    std::ofstream outFile;
    if(count == 0) {
      outFile.open(filename);               // Open file to create it
      if(!outFile.is_open())
        throw ios::failure(string("Error opening output file ") +
                           string(filename) +
                           string(" in main()"));
      outFile << " 2 3 5";
      outFile.close();
      cout << std::setw(10) << 2 << std::setw(10) << 3 << std::setw(10) << 5;
      lastprime = 5;
      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 ") +
                                string(filename) +
                                string(" in main()"));
      outFile << " " << lastprime;
      outFile.close();
      cout << (count++ % 5 == 0 ? "\n" : "") << std::setw(10) << lastprime;
    }
    cout << endl;
    return 0;
  }
  catch(std::exception& ex) {
    cout << endl << typeid(ex).name() << ": " << ex.what();
    return 1;
  }
}

// Find the next prime after the argument
long nextprime(long lastprime, const char* filename) {
  bool isprime = false;                     // Indicator that we have a prime
  long aprime = 0;                          // Stores primes from the file
  std::ifstream inFile;                     // Local input stream object

  // Find the next prime
  for( ; ; ) {
    lastprime += 2;                         // Next value for checking
    long limit = static_cast<long>(std::sqrt(static_cast<double>(lastprime)));

    // 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 ") +
                         string(filename) +
                         string(" in nextprime()"));
    do {
      inFile >> aprime;
    } while(aprime <= limit && !inFile.eof() && (isprime = lastprime % aprime > 0));

    inFile.close();
    if(isprime)                                        // We got one...
      return lastprime;                                // ...so return it
  }
}