Delphi Generics: TArray.Sort

Question

I am just starting to get my feet wet with this .

PNode = ^TNode;

TNode = record
  Obstacle : boolean;
  Visited : boolean;
  GCost : double; // Distance from Starting Node
  HCost : double; // Distance from Target Node
  x : integer;
  y : integer;
  vecNeighBour : array of PNode;
  Parent : PNode;
end;

OnFormShow I fill the array :

 SetLength(Node,4,4);

 Image1.Height:=Length(Node)*50;
 Image1.Width:=Length(Node)*50;

 for x := Low(Node) to High(Node) do
   for y := Low(Node) to High(Node) do
      begin
        Node[x,y].Obstacle:=false;
        Node[x,y].Visited:=false;
        Node[x,y].GCost:=infinity;
        Node[x,y].HCost:=infinity;
        Node[x,y].x:=x;
        Node[x,y].y:=y;
      end;

Now I would like to Sort this array by HCost , so I tried this .

  TArray.Sort<TNode>(Node , TComparer<TNode>.Construct(
  function(const Left, Right: TNode): double
  begin
    if Left.HCost > Right.HCost then
      Result:=Left.HCost
    else
      Result:=Right.HCost;
  end));

My knowledge is seriously lacking in this thing . I get a error from Delphi

"Incompatible types: 'System.Gerenics.Defaults.TComparison and 'Procedure'

What am I doing wrong here?

Answer 1

A comparison function has to return an Integer value. It is defined like so:

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

Your function returns the wrong type.

function CompareDoubleInc(Item1, Item2: Double): Integer;
begin
  if Item1=Item2 then begin
    Result := 0;
  end else if Item1<Item2 then begin
    Result := -1
  end else begin
    Result := 1;
  end;
end;

....

TArray.Sort<TNode>(
  Node, 
  TComparer<TNode>.Construct(
    function(const Left, Right: TNode): Integer
    begin
      Result := CompareDoubleInc(Left.HCost, Right.HCost);
    end
  )
);

Or if you want to make it even simpler you can delegate to the default double comparer:

TArray.Sort<TNode>(
  Node, 
  TComparer<TNode>.Construct(
    function(const Left, Right: TNode): Integer
    begin
      Result := TComparer<Double>.Default.Compare(Left.HCost, Right.HCost);
    end
  )
);