partition

     Algorithm

Places all of the entities that satisfy the given predicate before all of the entities that do not.

Contents

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

#include <algorithm>

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

The partition algorithm places all the elements in the range [first, last) that satisfy pred before all the elements that do not satisfy pred. It returns an iterator that is one past the end of the group of elements that satisfy pred. In other words, 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.

Note that partition does not necessarily maintain the relative order of the elements that match and elements that do not match the predicate. Use the algorithm stable_partition if relative order is important.

The partition algorithm does at most (last - first)/2 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 integers.
   //
   int init[10] = { 1,2,3,4,5,6,7,8,9,10 };
   deque<int> d1(init+0, init+10);
   deque<int> d2(init+0, init+10);
   //
   // Print out the original values.
   //
   cout << "Unpartitioned values: " << "\t\t";
   copy(d1.begin(), d1.end(), ostream_iterator<int>(cout," "));
   cout << endl;
   //
   // A partition of the deque according to even/oddness.
   //
   partition(d2.begin(), d2.end(), is_even<int>());
   //
   // Output result of partition.
   //
   cout << "Partitioned values: " << "\t\t";
   copy(d2.begin(), d2.end(), ostream_iterator<int>(cout," "));
   cout << endl;
   //
   // A stable partition of the deque according to even/oddness.
   //
   stable_partition(d1.begin(), d1.end(), is_even<int>());
   //
   // Output result of partition.
   //
   cout << "Stable partitioned values: " << "\t";
   copy(d1.begin(), d1.end(), ostream_iterator<int>(cout," "));
   cout << endl;
   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 need to write :

deque<int, allocator>

instead of :

deque<int>

stable_partition