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partition

Changes the order of the elements so that elements that match a criterion are at the front
+ View version details

Key Facts

Gyroscopic Couple: The rate of change of angular momentum (\tau) = I\omega\Omega (In the limit).
  • I = Moment of Inertia.
  • \omega = Angular velocity
  • \Omega = Angular velocity of precession.


Blaise Pascal (1623-1662) was a French mathematician, physicist, inventor, writer and Catholic philosopher.

Leonhard Euler (1707-1783) was a pioneering Swiss mathematician and physicist.

Definition

The partition() algorithm is defined in the standard header <algorithm> and in the nonstandard backward-compatibility header <algo.h>.

Interface

#include <algorithm>
template < class BidirectionalIterator, class Predicate >
   BidirectionalIterator partition(
      BidirectionalIterator first, 
      BidirectionalIterator last, 
      Predicate comp
   );

Parameters:
Parameter Description
first A bidirectional iterator addressing the position of the first element in the range to be partitioned
last A bidirectional iterator addressing the position one past the final element in the range to be partitioned
comp User-defined predicate function object that defines the condition to be satisfied if an element is to be classified. A predicate takes a single argument and returns true or false

Description

Partition function cuts a range in two parts : all elements in the first part satisfy a predicate and elements in the second part doesn't.

After a call to partition, relative order between elements may be changed.

Return Value

Returns the fi rst element that doesn't satisfy the predicate.

Complexity

The complexity is linear; performs last - first applications of pred and at most (last - first)/2 swaps.
Example:

Example - partition algorithm
Problem
This program illustrates the use of the STL partition() algorithm to partition the integer values in a vector of integers into two groups: those that are divisible by 3, and those that are not.
Workings
#include <iostream>
#include <vector>
#include <algorithm>
 
using namespace std;
 
/* Determines if an integer is divisible by 3.
   n contains an integer
   Returns true if n is divisible by 3, and otherwise false. */
bool isDivisibleByThree
(
  int n //in 
)
{
  return (n%3 == 0);
}
 
int main()
{
  int a[] ={11, 7, 9, 4, 8, 12, 2, 5, 3, 10, 1, 6};
  vector<int> v(a, a+12);
  cout <<"\nHere are the contents of the vector:\n";
  for (vector<int>::size_type i=0; i<v.size(); i++)
    cout <<v.at(i)<<" ";
 
  vector<int>::iterator p = partition(v.begin(), v.end(), 
                                      isDivisibleByThree);
  cout <<"\nAnd here are the contents of the partitioned vector:\n";
  for (vector<int>::size_type i=0; i<v.size(); i++)
    cout <<v.at(i)<<" ";
 
  cout <<"\nThe iterator p returned by the algorithm points to ";
  if (p == v.end())
    cout <<"\nthe end of the ouput container.";
  else
    cout <<"the value "<<*p<<".";
 
  return 0;
}
Solution
Output:

Here are the contents of the vector:
11 7 9 4 8 12 2 5 3 10 1 6

And here are the contents of the partitioned vector:
6 3 9 12 8 4 2 5 7 10 1 11

The iterator p returned by the algorithm points to the value 8.
References