reverse byte

[Kapil Khosla]

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

Thanks.
Kapil

[Allan Bruce]

"Kapil Khosla" <kh*********@ya hoo.com> wrote in message
news:91******** *************** ***@posting.goo gle.com...

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

Thanks.
Kapil

Create a temp Byte and AND the original byte with mask 0x1 to see if there
is a 1 in the LSB.
If there is a 1 then OR 0x80 to the temp byte

Now AND the original with 0x2 and if its a 1 then OR the temp with 0x4

And so on.

HTH
Allan

[Kevin Easton]

Kapil Khosla <kh*********@ya hoo.com> wrote:

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

The easiest, clearest and most efficient way is just to use a 256-entry
lookup table.

- Kevin.

[Jirka Klaue]

Kevin Easton wrote:

Kapil Khosla <kh*********@ya hoo.com> wrote: ....

I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

http://graphics.stanford.edu/~seander/bithacks.html
The easiest, clearest and most efficient way is just to use a 256-entry
lookup table.

That might be true. But it's not very helpful.
How is the lookup table generated? You wouldn't do this by hand, would you?

Jirka

[Robert Stankowic]

"Kapil Khosla" <kh*********@ya hoo.com> schrieb im Newsbeitrag
news:91******** *************** ***@posting.goo gle.com...

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

OK, fireproof suit and helmet on....

#include <stdlib.h>
#include <limits.h>
#include <stdio.h>

int main(void)
{
unsigned char in = 0;
unsigned char out;
int i;
int j;

for(j = 1; j <= UCHAR_MAX ; j++)
{
in = (unsigned char)j;
printf("in %x", (unsigned)in);
for(i = 0, out = 0; i < CHAR_BIT; i++)
{
in >>= (i != 0);
out <<= (i != 0);
out = (unsigned char)(out | (in & 1));
}
out = (unsigned char)(out | (in & 1));
printf(" out %x\n", (unsigned)out);
}
}

Critique and improvrment suggestions welcome :)
Especially about the casts...
Robert

[Robert Stankowic]

"Robert Stankowic" <pc******@netwa y.at> schrieb im Newsbeitrag
news:3f******** *************** @newsreader01.h ighway.telekom. at...

"Kapil Khosla" <kh*********@ya hoo.com> schrieb im Newsbeitrag
news:91******** *************** ***@posting.goo gle.com...

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

OK, fireproof suit and helmet on....

#include <stdlib.h>
#include <limits.h>
#include <stdio.h>

int main(void)
{
unsigned char in = 0;
unsigned char out;
int i;
int j;

for(j = 1; j <= UCHAR_MAX ; j++)
{
in = (unsigned char)j;
printf("in %x", (unsigned)in);
for(i = 0, out = 0; i < CHAR_BIT; i++)
{
in >>= (i != 0);
out <<= (i != 0);
out = (unsigned char)(out | (in & 1));
}
out = (unsigned char)(out | (in & 1));

Oops, drop the statement above - sorry
printf(" out %x\n", (unsigned)out);
}
}

Critique and improvrment suggestions welcome :)
Especially about the casts...
Robert

[pete]

Allan Bruce wrote:

"Kapil Khosla" <kh*********@ya hoo.com> wrote in message
news:91******** *************** ***@posting.goo gle.com...

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

Thanks.
Kapil

Create a temp Byte and AND the original byte
with mask 0x1 to see if there is a 1 in the LSB.
If there is a 1 then OR 0x80 to the temp byte

You don't start with 0x80 on clc.

step 1
Use unsigned char for mask variables.
One portable expression for the initial value of the high mask is:
(((unsigned char)-1 >> 1) + 1)
As stated, 1 is the initial value for the low mask.

step 2
Do a bitwise AND of the high mask and the byte, and
a bitwise AND of the low mask and the byte.

step 3
If the results of the bitwise ANDs are not either both zero
or both nonzero, then the values of the bits being tested
are different and they should be flipped.
One way to do that, would be to XOR the value of the byte
with the result of a bitwise OR of the two masks,
and change the value of the byte to the XOR result.

step 4
Change the values of the high and low mask.
Shift the high mask right by 1 and shift the low mask left by 1.

step 5
If the high mask is greater than the low mask,
then repeat, starting at step 2.

[Jirka Klaue]

Robert Stankowic wrote:
....

unsigned char out; .... out = (unsigned char)(out | (in & 1));

Oops, drop the statement above - sorry

And drop the cast, too. The result of (out | (in & 1)) is assigned
to an an unsigned char. The cast is redundant.

Jirka

[pete]

Robert Stankowic wrote:

"Robert Stankowic" <pc******@netwa y.at> schrieb im Newsbeitrag
news:3f******** *************** @newsreader01.h ighway.telekom. at...

"Kapil Khosla" <kh*********@ya hoo.com> schrieb im Newsbeitrag
news:91******** *************** ***@posting.goo gle.com...

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

OK, fireproof suit and helmet on....

#include <stdlib.h>
#include <limits.h>
#include <stdio.h>

int main(void)
{
unsigned char in = 0;
unsigned char out;
int i;
int j;

for(j = 1; j <= UCHAR_MAX ; j++)
{
in = (unsigned char)j;
printf("in %x", (unsigned)in);
for(i = 0, out = 0; i < CHAR_BIT; i++)
{
in >>= (i != 0);
out <<= (i != 0);
out = (unsigned char)(out | (in & 1));
}
out = (unsigned char)(out | (in & 1));

Oops, drop the statement above - sorry

printf(" out %x\n", (unsigned)out);
}
}

Critique and improvrment suggestions welcome :)
Especially about the casts...

#include <stdlib.h>
#include <limits.h>
#include <stdio.h>

int main(void)
{
unsigned in;
unsigned out;
int i;
unsigned char j;

j = 0;
while (++j != 0) {
in = j;
printf("in %x", in);
for (i = 0, out = 0; i < CHAR_BIT; i++) {
in >>= (i != 0);
out <<= (i != 0);
out |= in & 1;
}
printf(" out %x\n", out);
}
return 0;
}

[Joe Wright]

Kapil Khosla wrote:

Hi,
I am trying to reverse a byte eg.
11010000 should look like
00001011

Plz note, it is not a homework problem and I do not need the c code
for it.
Just give me an idea how should I proceed about it.

I know basic bit manipulation , shifting left, right and have done
simple problems like counting 1's etc but this one doesnt seem to
click to me.

Thanks.
Kapil

int revb(int n) {
n = ((n >> 1) & 0x55) | ((n << 1) & 0xaa);
n = ((n >> 2) & 0x33) | ((n << 2) & 0xcc);
n = ((n >> 4) & 0x0f) | ((n << 4) & 0xf0);
return n;
}

click?
--
Joe Wright mailto:jo****** **@earthlink.ne t
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---