Partial Sort

The generic algorithm partial_sort() sorts only a portion of a sequence. In the first version of the algorithm, three iterators are used to describe the beginning, middle, and end of a sequence. If n represents the number of elements between the start and middle, then the smallest n elements will be moved into this range in order. The remaining elements are moved into the second region. The order of the elements in this second region is undefined.

void partial_sort (RandomAccessIterator first, 
    RandomAccessIterator middle, 
    RandomAccessIterator last [ , Compare ]);
 

A second version of the algorithm leaves the input unchanged. The output area is described by a pair of random access iterators. If n represents the size of this area, then the smallest n elements in the input are moved into the output in order. If n is larger than the input, then the entire input is sorted and placed in the first n locations in the output. In either case the end of the output sequence is returned as the result of the operation.

RandomAccessIterator partial_sort_copy 
    (InputIterator first, InputIterator last, 
    RandomAccessIterator result_first, 
    RandomAccessIterator result_last [, Compare ] );
 

Because the input to this version of the algorithm is specified only as a pair of input iterators, the partial_sort_copy() algorithm can be used with any of the containers in the standard library. In the example program it is used with a list.

void partial_sort_example () 
       // illustrate the use of the partial sort algorithm
 {
       // make a vector of 15 random integers
    vector<int> aVec(15);
    generate (aVec.begin(), aVec.end(), randomValue);
 
       // partial sort the first seven positions
    partial_sort (aVec.begin(), aVec.begin() + 7, aVec.end());
 
       // make a list of random integers
    list<int> aList(15, 0);
    generate (aList.begin(), aList.end(), randomValue);
    
       // sort only the first seven elements
    vector<int> start(7);
    partial_sort_copy (aList.begin(), aList.end(),
       start.begin(), start.end(), greater<int>());
 }