std::stable_partition is an STL algorithm to partition elements in a container by a certain predicate, preserving the order of the elements. For example, in the code shown below, a std::vector is partitioned into primes and non-primes.


#include <algorithm>

#include <cassert>

#include <cmath>

#include <iostream>

#include <iterator>

#include <vector>



//From http://www.richelbilderbeek.nl/CppIsPrime.htm

bool IsPrime(const int x)

{

  const int max

  = static_cast<int>(

      std::sqrt(static_cast<double>(x))

    ) + 1;



  for (int i=2; i!=max; ++i)

  {

    if (x % i == 0) return false;

  }



  return true;

}



int main()

{

  std::vector<int> v;

  for (int i=1; i!=10; ++i) v.push_back(i);



  //Partition the std::vector in primes and non-primes

  typedef std::vector<int>::iterator Iterator;

  const Iterator p_end = std::stable_partition(v.begin(),v.end(),IsPrime);



  std::cout << "Primes: ";

  std::copy(v.begin(),p_end,std::ostream_iterator<int>(std::cout," "));

  std::cout << "\nNon-primes: ";

  std::copy(p_end,v.end(),std::ostream_iterator<int>(std::cout," "));

}

Screen output:


Primes: 1 2 3 5 7

Non-primes: 4 6 8 9

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(C++) std::stable_partition