What??? Swapping two variables without pointers or a third one? (Rafael Cotta) an article on delphi3000.com ||

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What??? Swapping two variables without pointers or a third one?

Swapping two values without using pointers or a third variable.
Product: Delphi all versions	Category: Algorithm	Skill Level:	Scoring:	Last Update: 07/30/2001
Search Keys: delphi delphi3000 article borland vcl code-snippet swap temporary integer curiosity			Times Scored: 10	Visits: 5180
Uploader: Rafael Cotta Company:		Reference: N/A

Question/Problem/Abstract: How to swap two integers without a temporary variable or pointers?
Answer: 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. //------------------------------ procedure TForm1.Button1Click(Sender: TObject); var var1 : integer; var2 : integer; begin var1 := 19; var2 := 564; var1 := var1 xor var2; var2 := var1 xor var2; var1 := var1 xor var2; // They´re swapped!!! Label1.Caption := 'Var1 = ' + IntToStr(var1) + '; Var2 = ' + IntToStr(var2); end; //------------------------------ Try this!

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Hmmm. John Mollll (Mar 25 2002 4:20PM)
Oh wow. I've never seen this before! (Do a search on Delphi3000 itself to find this exact same code.) Respond

not (always) worth it....
anonymus (Jul 30 2001 9:03PM)

I've used the xor trick in the good ol' days when you always ran out of CPU registers and memory. How ever....

Today it's better to leave this to the compiler and write readable code instead...Why? Check this..
a := a xor b
b = a xor b
a = a xor b

is translated into something like this by the compiler:

mov eax,[b]
xor  [a],eax
mov eax,[a]
xor [b],eax
mov eax,[b]
xor [a],eax

and
  tmp = a
  a = b
  b = tmp

is translated into
  mov eax,[a]
  mov [c],eax
  mov eax,[b]
  mov [a],eax
  mov eax,[tmp]
  mov [b],eax

So...Absolutely nothing saved.....
The assembler codes is taken directly from Delphi 5 using global vars - Using local var for the tmp var, Delphi should optimize this...

So, even though it is cute, you don't really save anything in all cases. The asm version showed in another comment is great for swapping large buffers etc, but for standard types... leave it to delphi. Even though I haven't checked it out, you might run into trouble swapping to objects this way - not sure, but there might be cases where it could spoil Delphi's reference counting....

And don't forget... it's always nice to understand your own code in 3 months.... :-)

Respond

RE: not (always) worth it....
Rafael Cotta (Jul 31 2001 3:34AM)

I´ll never use this on my programs. It´s just an algorithmic curiosity.

Rafael
Respond

RE: a shorter assembler form
Ernesto De Spirito (Aug 20 2001 1:12AM)

  asm
    mov eax, var1;
    xchg var2, eax;
    mov var1, eax;
  end;

Respond

Assembler Version Mike Heydon (Jul 30 2001 9:29AM)
// ======================================== // This is a FAST swap routine that swaps // the contents of any 2 variables. // The variables may be of any type but // the sizeof the VARS must be passed in Len // // eg. X1,X2 : integer; // // SwapMem(X1,X2,SizeOf(Integer)); // // ======================================== } procedure SwapMem(var Source,Dest; Len : integer); begin asm push edi push esi mov esi,Source mov edi,Dest mov ecx,Len cld @1: mov al,[edi] xchg [esi],al inc si stosb loop @1 pop esi pop edi end; end; Respond

another one Jiang Hong (Jul 30 2001 8:44AM)
var1:=var1+var2; var2:=var1-var2; var1:=var1-var2; Respond RE: another one Mat Hobbs (Jul 31 2001 11:07PM) ...but watch out for out of range... Respond

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