
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. 


part := proc(arr, left, right, pivot)
	local val,safe,i:
	val := arr[pivot]:
	arr[pivot], arr[right] := arr[right], arr[pivot]:
	safe := left:
	for i from left to right do
		if arr[i] < val then
			arr[safe], arr[i] := arr[i], arr[safe]:
			safe := safe + 1:
		end if:
	end do:
	arr[right], arr[safe] := arr[safe], arr[right]:
	return safe:
end proc:
 
quickselect := proc(arr,k)
	local pivot,left,right:
	left,right := 1,numelems(arr):
	while(true)do
		if left = right then return arr[left]: end if:
		pivot := trunc((left+right)/2);
		pivot := part(arr, left, right, pivot):
		if k = pivot then
			return arr[k]:
		elif k < pivot then
			right := pivot-1:
		else
			left := pivot+1:
		end if:
	end do:
end proc:
roll := rand(1..20):
demo := Array([seq(roll(), i=1..20)]);
map(x->printf("%d ", x), demo):
print(quickselect(demo,7)):
print(quickselect(demo,14)):