sieve.cpp

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//: C17:Sieve.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 1999
// Copyright notice in Copyright.txt
#include <string>
#include <iostream>
using namespace std;

int main() {
  // Create a 50 char string and set each 
  // element to 'P' for Prime
  string sieveChars(50, 'P');
  // By definition neither 0 nor 1 is prime.
  // Change these elements to "N" for Not Prime
  sieveChars.replace(0, 2, "NN");
  // Walk through the array:
  for(int i = 2;  
    i <= (sieveChars.size() / 2) - 1; i++)
    // Find all the factors:
    for(int factor = 2;
      factor * i < sieveChars.size();factor++)
      sieveChars[factor * i] = 'N';
     
  cout << "Prime:" << endl;
  // Return the index of the first 'P' element:
  int j = sieveChars.find('P');
  // While not at the end of the string:
  while(j != sieveChars.npos) {
    // If the element is P, the index is a prime
    cout << j << " ";
    // Move past the last prime
    j++;
    // Find the next prime
    j = sieveChars.find('P', j);
  }
  cout << "\n Not prime:" << endl;
  // Find the first element value not equal P:
  j = sieveChars.find_first_not_of('P');
  while(j != sieveChars.npos) {
    cout << j << " ";
    j++;
    j = sieveChars.find_first_not_of('P', j);
  }
} ///:~