
Sorting Algorithm

This is a sorting algorithm.   It may be applied to a set of data in order to sort it.    
For comparing various sorts, see compare sorts.  
For other sorting algorithms,   see sorting algorithms,   or:

O(n logn) sorts
Heap sort |
Merge sort |
Patience sort |
Quick sort
O(n log2n) sorts
Shell Sort
O(n2) sorts
Bubble sort |
Cocktail sort |
Cocktail sort with shifting bounds |
Comb sort |
Cycle sort |
Gnome sort |
Insertion sort |
Selection sort |
Strand sort
other sorts
Bead sort |
Bogo sort |
Common sorted list |
Composite structures sort |
Custom comparator sort |
Counting sort |
Disjoint sublist sort |
External sort |
Jort sort |
Lexicographical sort |
Natural sorting |
Order by pair comparisons |
Order disjoint list items |
Order two numerical lists |
Object identifier (OID) sort |
Pancake sort |
Quickselect |
Permutation sort |
Radix sort |
Ranking methods |
Remove duplicate elements |
Sleep sort |
Stooge sort |
[Sort letters of a string] |
Three variable sort |
Topological sort |
Tree sort


Use the quickselect algorithm on the vector

 [9, 8, 7, 6, 5, 0, 1, 2, 3, 4]
To show the first, second, third, ... up to the tenth largest member of the vector, in order, here on this page.

 Note: Quicksort has a separate task. 


#include <stdio.h>
#include <string.h>
 
int qselect(int *v, int len, int k)
{
#	define SWAP(a, b) { tmp = v[a]; v[a] = v[b]; v[b] = tmp; }
	int i, st, tmp;
 
	for (st = i = 0; i < len - 1; i++) {
		if (v[i] > v[len-1]) continue;
		SWAP(i, st);
		st++;
	}
 
	SWAP(len-1, st);
 
	return k == st	?v[st]
			:st > k	? qselect(v, st, k)
				: qselect(v + st, len - st, k - st);
}
 
int main(void)
{
#	define N (sizeof(x)/sizeof(x[0]))
	int x[] = {9, 8, 7, 6, 5, 0, 1, 2, 3, 4};
	int y[N];
 
	int i;
	for (i = 0; i < 10; i++) {
		memcpy(y, x, sizeof(x)); // qselect modifies array
		printf("%d: %d\n", i, qselect(y, 10, i));
	}
 
	return 0;
}