Impementing UnQuickSort in Delphi - How to Use the TListSortCompare function

TListSortCompare - Parameter Function for the Sort method of the TList class

In the How to Randomize / Shuffle Collections and Lists article you can locate a custom Delphi function: Shuffle. Shuffle is a randomizing function that takes a TList type of object and "sorts" the items in the list in a random order - thus making the list randomized.

Rawlyn, a member of the Delphi Programming Forum has a different interesting idea how to randomize the items of a Delphi list (or collection) - by sorting!

Sort, but sort randomly!

The TList Delphi class exposes the Sort method. Sort performs a QuickSort on the list based on the comparison function Compare.

Here's the declaration of the TList.Sort method:

procedure Sort(Compare: TListSortCompare) ;

The TListSortCompare is actually a function type, declared as:

TListSortCompare = function (Item1, Item2: Pointer): Integer;

Thus, the Compare parameter of the Sort method is actually a function - Sort method takes a function as a parameter.

The TListSortCompare is a function you write that indicates how the items are to be ordered. It should return
a positive value if Item1 is less than (<) Item2,
0 if they are equal,
a negative value if Item1 is greater than (>) Item2.

With the above in mind, here's an implementation of the TListSortCompare that will actually result in a randomized list.

function UnSort(Item1, item2: Pointer): integer;
begin
 result := -1 + Random(3);
end;

Random is a RTL function which returns a randomly generated number within a specified range. When MaxValue is specified the random number returned falls between 0 and MaxValue.

Using "-1 + Random(3)" we ensure that -1, 0 or 1 are the only possible values to be returned.

When -1 , 0 or 1 is "applied" as the result of the Sort method, randonmly - we have a randomizer function!

Here's an example of usage:

var
  list: TList;
begin
  list := TList.Create;
  try
    // ** add items to the list here


    Randomize;
    list.Sort(UnSort) ;

    // ** list is randomized now
  finally
    FreeAndNil(List) ;
  end,
end;

Note: if you actually want to sort the items in a list holding, let's say, instances of TDeveloper by a developer name - you can use the CompareText function to compares two strings without case sensitivity.

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