Switching byte values between variables

**Chronom1** · 02-23-2005

Hey guys,
I thought some of you might find this trick useful.

say you have 2 integers
int a = 5;
int b = 3;
Say you need to switch these variables so that a = b and b=a.
Most people would od it such that

Code:

int a=5;
int b=3;
int c;
c=a;
a=b;
b=c;

However, there is a way to do this in which a third variable is not needed.

Code:

int a=5;
int b=3;
a ^= b;
b ^= a;
a ^= b;

That's it.
You just successfully switched the values of a and b without a third variable. Thank God for Boolean Logic :-D

Note: This was done using integers but it could also be done using any data type including characters and structs because this command does not really care. All it cares about is the binary data.

**MadCow257** · 02-23-2005

Interesting, however I don't think that will work with structs. If so, can you give me an example? I tried this

Code:

#include <iostream>

using std::cout;

int main()
{
	struct Test
	{
		int test1;
		int test2;
	}a, b;
	a.test1 = 15;
	a.test2 = 17;
	b.test1 = 16;
	b.test1 = 18;
	a ^= b;
	b ^= a;
	a ^= b;
	cout << a.test1 << "\n";
	cout << b.test1 << "\n";
	cout << a.test2 << "\n";
	cout << b.test2 << "\n";
	return 0;
}

However I made it in like 30 seconds so something may be wrong with it.
It gives three errors,
"error C2676: binary '^=' : 'struct main::Test' does not define this operator or a conversion to a type acceptable to the predefined operator"

**hk_mp5kpdw** · 02-23-2005

This keeps popping up every now and again on this board:

http://cboard.cprogramming.com/showthread.php?t=51193
http://cboard.cprogramming.com/showthread.php?t=26045
http://cboard.cprogramming.com/showthread.php?t=15891

Swapping using XOR should only be applied to integer types.

**Chronom1** · 02-23-2005

yea, im having difficulty with structs as well, but am looking into it by making the compiler thing of it as a different type. I've seen character types work. particularly unsigned characters.

**ssharish** · 02-23-2005

Code:

originally posted by MadCow257 

Interesting, however I don't think that will work with structs. If so, can you give me an example? I tried this

Code:
#include <iostream>

using std::cout;

int main()
{
    struct Test
    {
        int test1;
        int test2;
    }a, b;
    a.test1 = 15;
    a.test2 = 17;
    b.test1 = 16;
    b.test1 = 18;
    a ^= b;
    b ^= a;
    a ^= b;
    cout << a.test1 << "\n";
    cout << b.test1 << "\n";
    cout << a.test2 << "\n";
    cout << b.test2 << "\n";
    return 0;
}
However I made it in like 30 seconds so something may be wrong with it.
It gives three errors,
"error C2676: binary '^=' : 'struct main::Test' does not define this operator or a conversion to a type acceptable to the predefined operator"

you cant use the arithmatical operaters on structres or something like this a+b where a and b is a variable of type structures. but one thing u can do is overload the ^= this operator.

**swoopy** · 02-23-2005

>but it could also be done using any data type
How sure are you?

You might want to read Hk_mp5kpdw's post one more time.

**Salem** · 02-23-2005

> I thought some of you might find this trick useful.
Actually, it's useless.
http://cboard.cprogramming.com/showt...swap+variables

The obvious way
- is easier to explain
- works on any type for which = is defined
- is likely to produce quicker code.

**ssharish** · 02-23-2005

Code:

originally posted  by swoopy
>but it could also be done using any data type
How sure are you?

You might want to read Hk_mp5kpdw's post one more time.

i cant able to understand. do u think that this logic works for structres. it could work for structure data members.

**VirtualAce** · 02-23-2005

I have bigger fish to fry than worrying about swapping values between variables.

And there is a better way to do this with integers than using XOR. You can do SHR or SHL in assembly which rotates bits. But again it's all pointless.

**Dave Evans** · 02-23-2005

Originally Posted by Chronom1

Hey guys,
I thought some of you might find this trick useful.
Thank God for Boolean Logic :-D

Note: This was done using integers but it could also be done using any data type including characters and structs because this command does not really care. All it cares about is the binary data.

Of course, others have pointed out that it can't be done with "any data type" --- no need to go to structs or classes; try it with floats or doubles.

Now as to "Thanking God", Many mathematicians subscribe to the dictum of 19th century German mathematician Leopold Kronecker

"God gave us the integers; the rest is the work of Man,"

Now, even if you don't go all of the way with this, you still have to give some credit to British mathematician George Boole whose publications in the 1850s reported his invention of a new form of mathematics in which logical expressions are represented in mathematical form. (Why do you think we call it Boolean Algebra?)

This rather esoteric topic remained largely unknown to the general public until, in 1938, American mathematician Claude Shannon published his MIT master's thesis, in which he showed that Boole's expression values "TRUE" and "FALSE" could be applied to represent the states of switches in electronic circuits.

The rest, as they say, is history.

Now the little trick that you posted has been used to illustrate the concepts and actions of the exclusive-or functions on bits of binary numbers, and may, in fact serve some useful purpose in assembly language programs, but as a practical programming device in a language like C --- well, I don't think so.

(Just my opinion, of course; everyone has an opinion, and everyone has a right to their opinion, and everyone has a right to disagree with anyone else's opinion.)

Regards,

Dave

**pianorain** · 02-23-2005

Originally Posted by Chronom1

However, there is a way to do this in which a third variable is not needed.

Code:

int a=5;
int b=3;
a ^= b;
b ^= a;
a ^= b;

Don't try to write this up as a swap function, though. It doesn't even work with ints perfectly. Consider this:

Code:

void swap(int &a, int &b)
{
    a ^= b;
    b ^= a;
    a ^= b;
}

int main()
{
    int a = 5;
    int *b = &a;
    swap(a,*b);
    std::cout << a << " " << *b << std::endl; //outputs 0 0
    return 0;
}

It's a lot simpler to use the swap algorithm.

Code:

int main()
{
    int a = 5;
    int *b = &a;
    std::swap(a,*b);
    std::cout << a << " " << *b << std::endl;
    return 0;
}

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