How can I implement a quick sort in Delphi without getting Access violation errors for large numbers of records?

Question

Here is my current code:

function StudentQuickSort(StudentList:TStudentArray;ArrayLength:integer):TStudentArray;
var
  Pivot:TstudentArray;
  LesserList:TStudent         


        

Answer 1

Your code cannot be salvaged. You are going about solving this problem in the wrong way and I advise you to abandon your existing code. Here is how I believe sorting should be done.

Note that I am assuming that you don't have generics available to you. In modern Delphi versions you can use TArray.Sort from Generics.Collections to sort. If you have access to that, you should use it

First of all the key is to separate the sorting from the array being sorted. To achieve that define the following types:

type
  TCompareIndicesFunction = function(Index1, Index2: Integer): Integer of object;
  TExchangeIndicesProcedure = procedure(Index1, Index2: Integer) of object;

The point is that all the common algorithms that can sort an array need only to be able to compare two items, and exchange two items. These procedural types enable separation of the sorting algorithm from the underlying array storage and types.

With these definitions in place, we are ready to write our general purpose sorting algorithms. For quicksort it looks like this:

procedure QuickSort(Count: Integer; Compare: TCompareIndicesFunction; 
  Exchange: TExchangeIndicesProcedure);

  procedure Sort(L, R: Integer);
  var
    I, J, P: Integer;
  begin
    repeat
      I := L;
      J := R;
      P := (L+R) div 2;
      repeat
        while Compare(I, P)<0 do inc(I); 
        while Compare(J, P)>0 do dec(J); 
        if I<=J then 
        begin
          if I<>J then 
          begin
            Exchange(I, J);
            //may have moved the pivot so we must remember which element it is
            if P=I then
              P := J
            else if P=J then
              P := I;
          end;
          inc(I);
          dec(J);
        end;
      until I>J;
      if L=R;
  end;

begin
  if Count>0 then
    Sort(0, Count-1);
end;

In order to use this you need to wrap your array in a class which exposes compare and exchange methods.