Sort a Generic List using Anonymous Comparer Method

How to Sort a Generic TList<T> using an Anonymous Method

Delphi 2009 adds Generics and Anonymous methods to the Delphi language.

When you have objects in some list - one of a commonly required tasks is to sort the list: either ascending, descending or using some list-specific algorithm.

Using anonymous methods you can apply the sorting function directly "inline" with the call to the Sort method.

TList<integer>.Sort(IComparer)

Suppose you have a list of integer numbers. Your task is to sort the numbers in the list.

Here's the code:

uses generics.defaults, generics.collections,...

...

type
  TIntegerComparer = TComparer<integer>;

...

var
  lg : TList<integer>;
  n : integer;
begin
  lg := TList<integer>.Create;
  try
    lg.Add(2) ;
    lg.Add(4) ;
    lg.Add(3) ;
    lg.Add(4) ;
    lg.Add(5) ;
    lg.Add(1) ;

    //will display 2,4,3,4,5,1
    for n in lg do ShowMessage(IntToStr(i)) ;

    //default ascending comparer
    lg.Sort(TComparer<integer>.Default) ;

    //will display 1,2,3,4,4,5
    for n in lg do ShowMessage(IntToStr(i)) ;

    //descending using an anonymous method
    lg.Sort(TIntegerComparer.Construct(
      function (const L, R: integer): integer
      begin
        result := R - L;
      end
      )) ;

    //will display 5,4,4,3,2,1
    for n in lg do ShowMessage(IntToStr(i)) ;

  finally
    lg.Free;
  end;
end;

Note: the TList<T>.Sort methods expects an IComparer<T> object. IComparer<T> is an interface defined in the generics.defaults unit.

TComparer, also defined in the Generics.Defaults, is an abstract base class for IComparer<T> implementations.

The Construct method of the TComparer<T> is a class method defined as:

class function Construct(const Comparison: TComparison<T>): IComparer<T>;

The above returns an IComparer<T> we need for the Sort method AND accepts an anonymous method:

TComparison<T> = reference to function(const Left, Right: T): Integer;

Finally, you have a Sort method with an anonymous method as "parameter":

lg.Sort(TIntegerComparer.Construct(
  function (const L, R: integer): integer
  begin
    result := R - L;
  end
  )) ;

For a non-generic TList object: How to sort items in a TList object.

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