Apache C++ Standard Library Reference Guide

## partial_sort()

Library: Algorithms

    Function

- Local Index
- Summary
- Synopsis
- Description
- Complexity
- Example
- See Also
- Standards Conformance

### Local Index

No Entries

### Summary

A generic algorithm for sorting collections of entities

### Synopsis

    #include <algorithm>

    namespace std {
      template <class RandomAccessIterator>
      void partial_sort(RandomAccessIterator start,
                        RandomAccessIterator mid,
                        RandomAccessIterator finish);

      template <class RandomAccessIterator, class Compare>
      void partial_sort(RandomAccessIterator start,
                        RandomAccessIterator mid,
                        RandomAccessIterator finish,
                        Compare comp);
    }

### Description

The `partial_sort()` algorithm takes the range `[start,finish)` and sorts the first `mid - start` values in ascending order. The remaining elements in the range (those in `[mid, finish)`) are not in any defined order. The first version of the algorithm uses `operator<()` as the comparison operator for the sort. The second version uses the function object `comp`.

### Complexity

`partial_sort()` does approximately`(finish - start) * log(mid-start)` comparisons. This implementation of `partial_sort()` is implemented using heapsort, which is `O(N * log(N))` in the worst case.

### Example

    //
    //  partsort.cpp
    //

    #include <algorithm>   // for partial_sort
    #include <iostream>    // for cout, endl
    #include <iterator>    // for ostream_iterator
    #include <vector>      // for vector

    int main()
    {
        typedef std::vector<int, std::allocator<int> > Vector;

        typedef std::ostream_iterator<int, char,
                                      std::char_traits<char> >
                Iter;

        const Vector::value_type a[] = {
            17, 3,  5,  -4, 1, 12, -10, -1, 14, 7,
            -6, 8, 15, -11, 2, -2,  18,  4, -3, 0
        };

        Vector v1 (a + 0, a + sizeof a / sizeof *a);

        // Output original vector.
        std::cout << "For the vector: ";
        std::copy (v1.begin (), v1.end (), Iter (std::cout, " "));

        // Partial sort the first seven elements.
        std::partial_sort (v1.begin (), v1.begin () + 7,
                           v1.end ());

        // Output result.
        std::cout << "\n\nA partial_sort of seven elements "
                  << "gives: \n     ";
        std::copy (v1.begin (), v1.end (), Iter (std::cout, " "));
        std::cout << std::endl;

        // A vector of ten elements.
        Vector v2 (Vector::size_type (10), 0);

        // Sort the last ten elements in v1 into v2.
        std::partial_sort_copy (v1.begin () + 10, v1.end (),
                                v2.begin (), v2.end ());

        // Output result.
        std::cout
            << "\nA partial_sort_copy of the last ten elements "
            << "gives: \n     ";
        std::copy (v2.begin (), v2.end (), Iter (std::cout, " "));

        std::cout << std::endl;

        return 0;
    }

    Program 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

### See Also

`sort()`, `stable_sort()`, `partial_sort_copy()`

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.1.3