stable_partition

     Algorithm

Places all of the entities that satisfy the given predicate before all of the entities that do not, while maintaining the relative order of elements in each group.

Contents

• Synopsis
• Description
• Complexity
• Example
• Warning
• See Also

#include <algorithm>

template <class BidirectionalIterator, class Predicate>
BidirectionalIterator
stable_partition (BidirectionalIterator first,
                  BidirectionalIterator last,
                  Predicate pred);

The stable_partition algorithm places all the elements in the range [first, last) that satisfy pred before all the elements that do not satisfy it. It returns an iterator i that is one past the end of the group of elements that satisfy pred. In other words stable_partition returns i such that for any iterator j in the range [first, i), pred(*j) == true, and for any iterator k in the range [i, last), pred(*j) == false. The relative order of the elements in both groups is preserved.

The partition algorithm can be used when it is not necessary to maintain the relative order of elements within the groups that do and do not match the predicate.

The stable_partition algorithm does at most (last - first) * log(last - first) swaps. and applies the predicate exactly last - first times.

//
// prtition.cpp
//
 #include <functional>
 #include <deque>
 #include <algorithm>
 #include <iostream.h>
 //Create a new predicate from unary_function
 template<class Arg>
 class is_even : public unary_function<Arg, bool>
 {
   public:
   bool operator()(const Arg& arg1)
   {
      return (arg1 % 2) == 0;
   } 
 };
 int main()
 {
   //Initialize a deque with an array of ints
   int init[10] = {1,2,3,4,5,6,7,8,9,10};
   deque<int> d(init, init+10);
   //Print out the original values
   cout << "Unpartitioned values: " << endl << "     ";
   copy(d.begin(),d.end(),ostream_iterator<int>(cout," "));
   cout << endl << endl;
   //Partition the deque according to even/oddness
   stable_partition(d.begin(), d.end(), is_even<int>());
   //Output result of partition
   cout << "Partitioned values: " << endl << "     ";
   copy(d.begin(),d.end(),ostream_iterator<int>(cout," "));
   return 0;
 }
Output :
Unpartitioned values:           1 2 3 4 5 6 7 8 9 10
Partitioned values:             10 2 8 4 6 5 7 3 9 1
Stable partitioned values:      2 4 6 8 10 1 3 5 7 9

If your compiler does not support default template parameters, then you need to always supply the Allocator template argument. For instance, you will need to write :

deque<int, allocator>

instead of :

deque<int>

partition