partial_sort

Algorithm

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• Synopsis
• Description
• Complexity
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Templated algorithm for sorting collections of entities.

None

#include <algorithm>

template <class RandomAccessIterator>
 void partial_sort (RandomAccessIterator first,
                    RandomAccessIterator middle,
                    RandomAccessIterator last);

template <class RandomAccessIterator, class Compare>
 void partial_sort (RandomAccessIterator first,
                    RandomAccessIterator middle, 
                    RandomAccessIterator last, Compare comp);

The partial_sort algorithm takes the range [first,last) and places the first middle - first values into sorted order. The result is that the range [first, middle)is sorted like it would be if the entire range [first,last) were sorted. The remaining elements in the range (those in [middle, last)) are not in any defined order. The first version of the algorithm uses less than (operator<) as the comparison operator for the sort. The second version uses the comparison function comp.

partial_sort does approximately (last - first) * log(middle-first) comparisons.

//
// partsort.cpp
//
#include <vector>
#include <algorithm>
#include <iostream.h>

int main()
 {
   int d1[20] = {17, 3,  5,  -4, 1, 12, -10, -1, 14, 7,
                 -6, 8, 15, -11, 2, -2,  18,  4, -3, 0};
   //
   // Set up a vector.
   //
   vector<int> v1(d1+0, d1+20);
   //
   // Output original vector.
   //
   cout << "For the vector: ";
   copy(v1.begin(), v1.end(), 
        ostream_iterator<int,char>(cout," "));
   //
   // Partial sort the first seven elements.
   //
   partial_sort(v1.begin(), v1.begin()+7, v1.end());
   //
   // Output result.
   //
   cout << endl << endl << "A partial_sort of seven elements   
                            gives: "
        << endl << "     ";
   copy(v1.begin(), v1.end(), 
        ostream_iterator<int,char>(cout," "));
   cout << endl;
   //
   // A vector of ten elements.
   //
   vector<int> v2(10, 0);
   //
   // Sort the last ten elements in v1 into v2.
   //
   partial_sort_copy(v1.begin()+10, v1.end(), v2.begin(),  
                     v2.end());
   //
   // Output result.
   //
   cout << endl << "A partial_sort_copy of the last ten elements 
                    gives: "
        << endl << "     ";
   copy(v2.begin(), v2.end(), 
        ostream_iterator<int,char>(cout," "));
   cout << endl;

   return 0;
 }

Output :
For the vector: 17 3 5 -4 1 12 -10 -1 14 7 -6 8 15 -11 2 -2 18 4 -3 0
A partial_sort of seven elements gives:
     -11 -10 -6 -4 -3 -2 -1 17 14 12 7 8 15 5 3 2 18 4 1 0
A partial_sort_copy of the last ten elements gives:
     0 1 2 3 4 5 7 8 15 18

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

vector<int, allocator<int> >

instead of :

vector<int>

sort, stable_sort, partial_sort_copy

©Copyright 1996, Rogue Wave Software, Inc.