Sort a Generic List using Anonymous Comparer 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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