Implementing QuickSort Sorting Algorithm in Delphi

One of the common problems in programming is to sort an array of values in some order (ascending or descending).

While there are many "standard" sorting algorithms, QuickSort is one of the fastest. Quicksort sorts by employing a divide and conquer strategy to divide a list into two sub-lists.

QuickSort Algorithm

The basic concept is to pick one of the elements in the array, called a pivot. Around the pivot, other elements will be rearranged. Everything less than the pivot is moved left of the pivot - into the left partition. Everything greater than the pivot goes into the right partition. At this point, each partition is recursive "quick sorted".

Here's QuickSort algorithm implemented in Delphi:

 procedure QuickSort(var A: array of Integer; iLo, iHi: Integer) ;
 var
   Lo, Hi, Pivot, T: Integer;
 begin
   Lo := iLo;
   Hi := iHi;
   Pivot := A[(Lo + Hi) div 2];
   repeat
     while A[Lo] < Pivot do Inc(Lo) ;
     while A[Hi] > Pivot do Dec(Hi) ;
     if Lo <= Hi then
     begin
       T := A[Lo];
       A[Lo] := A[Hi];
       A[Hi] := T;
       Inc(Lo) ;
       Dec(Hi) ;
     end;
   until Lo > Hi;
   if Hi > iLo then QuickSort(A, iLo, Hi) ;
   if Lo < iHi then QuickSort(A, Lo, iHi) ;
 end;

Usage:

 var
   intArray : array of integer;
 begin
   SetLength(intArray,10) ;
 
   //Add values to intArray
   intArray[0] := 2007;
   ...
   intArray[9] := 1973;
 
   //sort
   QuickSort(intArray, Low(intArray), High(intArray)) ;

Note: in practice, the QuickSort becomes very slow when the array passed to it is already close to being sorted.

There's a demo program that ships with Delphi, called "thrddemo" in the "Threads" folder which shows additional two sorting algorithms: Bubble sort and Selection Sort.