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nth_element
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nth_element
Algorithm
- Summary
- Data Type and Member Function Indexes
- Synopsis
- Description
- Complexity
- Example
- Warning
- See Also
Summary
Rearranges a collection so that all elements lower in sorted order than the nth element come before it and all elements higher in sorter order than the nth element come after it.
Data Type and Member Function Indexes
(exclusive of constructors and destructors)
None
Synopsis
#include <algorithm> template <class RandomAccessIterator> void nth_element (RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template <class RandomAccessIterator, class Compare> void nth_element (RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);
Description
The nth_element algorithm rearranges a collection according to either the default comparison operator (>) or the provided comparison operator. After the algorithm applies, three things are true:
The element that would be in the nth position if the collection were completely sorted is in the nth position
All elements prior to the nth position would precede that position in an ordered collection
All elements following the nth position would follow that position in an ordered collection
That is, for any iterator i in the range [first, nth) and any iterator j in the range [nth, last) it holds that !(*i > *j) or comp(*i, *j) == false.
Note that the elements that precede or follow the nth position are not necessarily sorted relative to each other. The nth_element algorithm does not sort the entire collection.
Complexity
The algorithm is linear, on average, where N is the size of the range [first, last).
Example
// // nthelem.cpp // #include <algorithm> #include <vector> #include <iostream.h> template<class RandomAccessIterator> void quik_sort(RandomAccessIterator start, RandomAccessIterator end) { size_t dist = 0; distance(start, end, dist); //Stop condition for recursion if(dist > 2) { //Use nth_element to do all the work for quik_sort nth_element(start, start+(dist/2), end); //Recursive calls to each remaining unsorted portion quik_sort(start, start+(dist/2-1)); quik_sort(start+(dist/2+1), end); } if(dist == 2 && *end < *start) swap(start, end); } int main() { //Initialize a vector using an array of ints int arr[10] = {37, 12, 2, -5, 14, 1, 0, -1, 14, 32}; vector<int> v(arr, arr+10); //Print the initial vector cout << "The unsorted values are: " << endl << " "; vector<int>::iterator i; for(i = v.begin(); i != v.end(); i++) cout << *i << ", "; cout << endl << endl; //Use the new sort algorithm quik_sort(v.begin(), v.end()); //Output the sorted vector cout << "The sorted values are: " << endl << " "; for(i = v.begin(); i != v.end(); i++) cout << *i << ", "; cout << endl << endl; return 0; } Output : The unsorted values are: 37, 12, 2, -5, 14, 1, 0, -1, 14, 32, The sorted values are: -5, -1, 0, 1, 2, 12, 14, 14, 32, 37,
Warning
If your compiler does not support default template parameters, then you need to always supply the Allocator template argument. For instance, you will need to write :
vector<int, allocator<int> >
instead of :
vector<int>
See Also
©Copyright 1996, Rogue Wave Software, Inc.