Author: Rafael Cotta

How to swap two integers without a temporary variable or pointers?

Answer:

Solve 1:

Yes, it´s possible to swap the values of two variables without using a third one or 
swapping pointers. 

How??? The answer is: using xor!!! 

Place a label and a button on a form, and put this on Button´s click event. 
1   
2   procedure TForm1.Button1Click(Sender: TObject);
3   var
4     var1: integer;
5     var2: integer;
6   begin
7   
8     var1 := 19;
9     var2 := 564;
10  
11    var1 := var1 xor var2;
12    var2 := var1 xor var2;
13    var1 := var1 xor var2;
14  
15    // They´re swapped!!!
16  
17    Label1.Caption := 'Var1 = ' + IntToStr(var1) + '; Var2 = ' + IntToStr(var2);
18  end;



Solve 2:
19  
20  // ========================================
21  // This is a FAST swap routine that swaps
22  // the contents of any 2 variables.
23  // The variables may be of any type but
24  // the sizeof the VARS must be passed in Len
25  //
26  // eg.  X1,X2 : integer;
27  //
28  //        SwapMem(X1,X2,SizeOf(Integer));
29  //
30  // ======================================== }
31  
32  procedure SwapMem(var Source, Dest; Len: integer);
33  begin
34    asm
35           push edi
36           push esi
37           mov esi,Source
38           mov edi,Dest
39           mov ecx,Len
40           cld
41       @1:
42           mov al,[edi]
43           xchg [esi],al
44           inc si
45           stosb
46           loop @1
47           pop esi
48           pop edi
49    end;
50  end;