
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. 


main =>
  L = [9,8,7,6,5,0,1,2,3,4],
  Len = L.len,
  println([select(L,1,Len,I) : I in 1..Len]),
  nl.
 
select(List, Left, Right, K) = Select =>
  if Left = Right then
    Select = List[Left]
  else 
    PivotIndex  = partition(List, Left, Right, random(Left,Right)),
    if K == PivotIndex then
      Select = List[K]
    elseif K < PivotIndex then
      Select = select(List, Left, PivotIndex-1, K)
    else
      Select = select(List, PivotIndex+1, Right, K)
    end
  end.
 
partition(List, Left, Right, PivotIndex) = StoreIndex =>
  PivotValue = List[PivotIndex],
  swap(List,PivotIndex,Right),
  StoreIndex = Left,
  foreach(I in Left..Right-1)
    if List[I] @< PivotValue then
      swap(List,StoreIndex,I),
      StoreIndex := StoreIndex+1
    end
  end,
  swap(List,Right,StoreIndex).
 
% swap L[I] <=> L[J]
swap(L,I,J) => 
  T = L[I],
  L[I] := L[J],
  L[J] := T.