swapping chars/numbers

[maverix]

Hi All,

To swap chars/numbers between two variables, I've used the below code successfully on some implementations.

char a='a', b='b';
a ^= b ^= a ^= b;

I recently tried it on a different machine and it doesn't seem to work.
Could someone explain as to what might be the issue with this code.

regards
maverix

[laserlight]

I had the impression that the XOR swap is pretty much guaranteed to work for integer types. Does it work if you change it to:

char a = 'a', b = 'b';
a ^= b;
b ^= a;
a ^= b;

One possibility why your original code did not work is that is it in the realm of undefined behaviour since you are reading and modifying a twice within consecutive sequence points.

[maverix]

char a = 'a', b = 'b';
a ^= b;
b ^= a;
a ^= b;

i'll check this and let you know once i reach home (coz this behavior is in my home PC)

[Dino]

There are several threads about this code on this forum,

a ^= b ^= a ^= b;

and the consensus is, the behavior is undefined.

Todd

[matsp]

Also bear in mind that modern processors probably do better on creating a temporary variable and shuffling the data back and forth, rather than "confusing" the processor with an XOR operation.

On older processors (particularly small embedded processors), XOR is perhaps faster because it saves a temporary location.

--
Mats

[cpjust]

Could someone give me some examples of what you would use that for?
I'm guessing it has something to do with encryption...?

[matsp]

No, it's just doing "char t = a; a = b; b = t;", so swapping a's with b's content, in a way that if you are not familiar with the concept, you won't see what it does. That in itself is a good reason NOT to do this in "professional code".

--
Mats

[laserlight]

Could someone give me some examples of what you would use that for?

These days, for educational purposes. In the past, or maybe when doing embedded programming, it would be to avoid a temporary when performing a swap. I note that matsp pretty much pointed this out in the post before your own

[maverix]

Thank you all for your valuable inputs,

laserlight...I checked it in my home PC and it works when the statements are separated.

I was of the impression that this code would be efficient in all processors (old or new). Thanks to matsp for pointing out the differences.

regards
maverix

[cpjust]

These days, for educational purposes. In the past, or maybe when doing embedded programming, it would be to avoid a temporary when performing a swap. I note that matsp pretty much pointed this out in the post before your own

OK thanks.
I just tried it on paper with a=3, b=5 and now I see how it works. I guess you could swap CPU registers like that if you don't have a 3rd register available?

[matsp]

OK thanks.
I just tried it on paper with a=3, b=5 and now I see how it works. I guess you could swap CPU registers like that if you don't have a 3rd register available?

Yes, on a processor with 2 or 3 registers, it would make sense to do this.

--
Mats