Practical uses of XOR

[cman]

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.

Tilak

[Mark McIntyre]

Re: Practical uses of XOR

On 12 Mar 2007 09:14:29 -0700, in comp.lang.c , "cman"
<tilakb@gmail.c omwrote:

>Could you point me to the practical uses of XOR in assembly and
>algorithms? I understand XOR to be "true if and only if p is true or q
>is true". Where is this used? I draw a blank on usage.

This isn't a C question. Ask again in comp.programmin g or a maybe
maths group, and in the meantime, think about how two-way light
switches work, like the ones in the stairwell of most houses.
--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

[Richard Bos]

Re: Practical uses of XOR

"cman" <tilakb@gmail.c omwrote:

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.

The very simplest of applications: to invert a B&W bitmap image, xor
each byte with ~0.
For more involved examples, one uses ^ in conjunction (NPI) with & and |
for reading, setting, and toggling flags within bytes.

Richard

[osmium]

Re: Practical uses of XOR

"cman" writes:

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.

The XOR is fundamental to cryptographic work. It is also used in binary
arithmetic.

http://en.wikipedia.org/wiki/Adder_(electronics)

[santosh]

Re: Practical uses of XOR


cman wrote:

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.
>
Tilak

Don't post all your questions to comp.lang.c. This group only deals
with ISO C. Post non-C questions to appropriate groups. For this one,
comp.programmin g, alt.lang.asm, comp.lang.asm.x 86, or sci.math might
be better candidates. The web is also a good resource for such simple
questions.

<http://en.wikipedia.or g/wiki/XOR>

[user923005]

Re: Practical uses of XOR

On Mar 12, 9:14 am, "cman" <til...@gmail.c omwrote:

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.
>
Tilak

You'll find some in here:

http://home.pipeline.com/~hbaker1/hakmem/hakmem.html

[Keith Thompson]

Re: Practical uses of XOR

rlb@hoekstra-uitgeverij.nl (Richard Bos) writes:

"cman" <tilakb@gmail.c omwrote:

>Could you point me to the practical uses of XOR in assembly and
>algorithms? I understand XOR to be "true if and only if p is true or q
>is true". Where is this used? I draw a blank on usage.

>
The very simplest of applications: to invert a B&W bitmap image, xor
each byte with ~0.

[...]

Surely it's simpler to invert the image itself, using the unary "~"
operator, rather than constructing another bitmap and xor'ing against
that.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Roberto Waltman]

Re: Practical uses of XOR

Richard Bos wrote:

>"cman" <tilakb@gmail.c omwrote:

>Could you point me to the practical uses of XOR in assembly and
>algorithms? I understand XOR to be "true if and only if p is true or q
>is true". Where is this used? I draw a blank on usage.

>
>The very simplest of applications: to invert a B&W bitmap image, xor
>each byte with ~0.

Too complicated ...

#include <stdio.h>
extern delay_until_nex t_second(void);

void clock_simulator (void)
{
int sound = 0;
while (1)
{
printf(sound ? "Tick\n" : "Tock\n");
delay_until_nex t_second();
sound ^= 1;
}
}

[CBFalconer]

Re: Practical uses of XOR

santosh wrote:

cman wrote:
>

>Could you point me to the practical uses of XOR in assembly and
>algorithms? I understand XOR to be "true if and only if p is true
>or q is true". Where is this used? I draw a blank on usage.

>
Don't post all your questions to comp.lang.c. This group only deals
with ISO C. Post non-C questions to appropriate groups. For this one,
comp.programmin g, alt.lang.asm, comp.lang.asm.x 86, or sci.math might
be better candidates. The web is also a good resource for such simple
questions.

However C has an XOR operator, spelled '^'. The OP should just
read his C book.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>



--
Posted via a free Usenet account from http://www.teranews.com

[Keith Thompson]

Re: Practical uses of XOR

Roberto Waltman <bookwormish@rw altman.comwrite s:

Richard Bos wrote:

>>"cman" <tilakb@gmail.c omwrote:

>>Could you point me to the practical uses of XOR in assembly and
>>algorithms? I understand XOR to be "true if and only if p is true or q
>>is true". Where is this used? I draw a blank on usage.

>>
>>The very simplest of applications: to invert a B&W bitmap image, xor
>>each byte with ~0.

>
Too complicated ...
>
#include <stdio.h>
extern delay_until_nex t_second(void);
>
void clock_simulator (void)
{
int sound = 0;
while (1)
{
printf(sound ? "Tick\n" : "Tock\n");
delay_until_nex t_second();
sound ^= 1;
}
}

Simpler:

sound = 1 - sound;

Even simpler:

sound = !sound;

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Ben Pfaff]

Re: Practical uses of XOR

Keith Thompson <kst-u@mib.orgwrites :

Roberto Waltman <bookwormish@rw altman.comwrite s:

> #include <stdio.h>
> extern delay_until_nex t_second(void);
>>
> void clock_simulator (void)
> {
> int sound = 0;
> while (1)
> {
> printf(sound ? "Tick\n" : "Tock\n");
> delay_until_nex t_second();
> sound ^= 1;
> }
> }

>
Simpler:
>
sound = 1 - sound;
>
Even simpler:
>
sound = !sound;

I'd probably write it like this (modulo coding style):

void clock_simulator (void)
{
while (1)
{
printf("Tock\n" );
delay_until_nex t_second();

printf("Tick\n" );
delay_until_nex t_second();
}
}
--
int main(void){char p[]="ABCDEFGHIJKLM NOPQRSTUVWXYZab cdefghijklmnopq rstuvwxyz.\
\n",*q="kl BIcNBFr.NKEzjwC IxNJC";int i=sizeof p/2;char *strchr();int putchar(\
);while(*q){i+= strchr(p,*q++)-p;if(i>=(int)si zeof p)i-=sizeof p-1;putchar(p[i]\
);}return 0;}

[=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=]

Re: Practical uses of XOR

On Mar 12, 9:31 pm, Roberto Waltman <bookworm...@rw altman.comwrote :

Richard Bos wrote:

"cman" <til...@gmail.c omwrote:

Could you point me to the practical uses of XOR in assembly and
algorithms? I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.

>

The very simplest of applications: to invert a B&W bitmap image, xor
each byte with ~0.

>
Too complicated ...
>
#include <stdio.h>
extern delay_until_nex t_second(void);
>
void clock_simulator (void)
{
int sound = 0;
while (1)
{
printf(sound ? "Tick\n" : "Tock\n");
delay_until_nex t_second();
sound ^= 1;

Why an exclusive or with 1? Why not 8, or 42, or 197? And why not
simply

sound = !sound;

? This doesn't seem to me an obvious use of xor... which could just be
me, of course.

}
}

[Beej Jorgensen]

Re: Practical uses of XOR

Keith Thompson <kst-u@mib.orgwrote:

>Surely it's simpler to invert the image itself, using the unary "~"
>operator, rather than constructing another bitmap and xor'ing against
>that.

<OT>
Yes, but probably what he was getting at was more along the lines of the
famous Atari (was it?) XOR patent for drawing the cursor, namely, that
you can "stamp" arbitrary data on other data, and remove it again with
XOR, since:

(x ^ y) ^ y == x

(I'm sure you already knew this, Keith; this post is for posterity.)

Cursor on:

....XXX.. XXXXXXXX XXX...XX
...XX.XX. XXXXXXXX XX..X..X
..XX...XX XXXXXXXX X..XXX..
..XXXXXXX XXXXXXXX X.......
..XX...XX XOR XXXXXXXX == X..XXX..
..XX...XX XXXXXXXX X..XXX..
..XX...XX XXXXXXXX X..XXX..
......... XXXXXXXX XXXXXXXX

Then cursor off:

XXX...XX XXXXXXXX ...XXX..
XX..X..X XXXXXXXX ..XX.XX.
X..XXX.. XXXXXXXX .XX...XX
X....... XXXXXXXX .XXXXXXX
X..XXX.. XOR XXXXXXXX == .XX...XX
X..XXX.. XXXXXXXX .XX...XX
X..XXX.. XXXXXXXX .XX...XX
XXXXXXXX XXXXXXXX ........

Of course, the cursor could be anything--not just a solid block--and the
character beneath would be restored to its original state when the
cursor was turned off.

IIRC, old versions of Windows would draw the window frame in XOR mode
when you were moving or resizing it. Ugly but effective.

</OT>

-Beej, still looking for that elusive XNOT operation.

[mensanator@aol.com]

Re: Practical uses of XOR

On Mar 12, 11:14 am, "cman" <til...@gmail.c omwrote:

Could you point me to the practical uses of XOR in assembly and
algorithms?

Hamming Distance = popcount(a^b)

I understand XOR to be "true if and only if p is true or q
is true". Where is this used? I draw a blank on usage.
>
Tilak

[Coos Haak]

Re: Practical uses of XOR

Op Tue, 13 Mar 2007 00:21:51 +0100 (CET) schreef Beej Jorgensen:

<snip>

-Beej, still looking for that elusive XNOT operation.

<OTLike this?

http://mathworld.wolfram.com/XNOR.html

</OT>
--
Coos