Easy way to swap to bits in a Byte?

[Michael Menne]

I'm currently trying to figure out an easy way to swap to bits in a Byte variable.
So for example I want to swap the x and y in 0101 0x1y, so that it becomes 0101 0y1x. The other bits shouldn't be changed.

Currently I'm using if statements, but I just imagine that there is an easier (faster) way to do it with bit operations. (xor, &, |).
Or am I totally wrong and there is only the way with the if statments?

Pherhaps somebody does know it an is so friendly to help me.

Sincerely,
Michael Menne

[Doctor Luz]

supposing x is at i position (begining from right) and y is at j position and i<>j. Calling the byte b

swap=b-(b&(1<<i))-(b&(1<<j))+((1&(b>>i))<<j)+((1&(b>>j))<<i);

Test it please, because here is too late and my brain asks for sweet dreams.

[dude_1967]

I usually use templates and simple, easy-to-read algorithms for these types of operations. The templates allow for typesafe use with several data-types. You might find some interesting ideas in the implementation below.

Sincerely, Chris.

NOTE: Edited about 30 minutes after posting, code was corrected.

:)



typedef unsigned char BYTE;
typedef unsigned long int DWORD;

template <typename T> const T t_bit(const int bit)
{
return static_cast<T>(1) << bit;
}

template <typename T> const T swap(const T t, const int i, const int j)
{
T tt = t;

tt &= ~t_bit<T>(i); // mask out bit i
tt &= ~t_bit<T>(j); // mask out bit j

const T ti = t_bit<T>(i) & t; // isolate bit i
const T tj = t_bit<T>(j) & t; // isolate bit j

// Swap the bits
if(ti)
{
tt |= t_bit<T>(j);
}

if(tj)
{
tt |= t_bit<T>(i);
}

return tt;
}

int main(int argc, char* argv[])
{
BYTE b;

b = ::swap<BYTE>(0x0F, 0, 7);
b = ::swap<BYTE>(0xF0, 0, 7);

DWORD d;

d = ::swap<DWORD>(0x0000000F, 0, 31);
d = ::swap<DWORD>(0xF0000000, 0, 31);

return 1;
}

[Michael Menne]

Thanks for the help!

I now got a good and very fast way of swapping bits in bytes as well as in (unsigned) integers. Your solution is working, Doctor Luz, though the one I found is a little faster and in my opinion easier to understand. Nevertheless your's was a good starting point.
Basically I'm first checking if the two values are equal, if not I'm using a xor operation to swap them.

Thats the function I'm now using:


UINT&nbsp;u&nbsp;=&nbsp;279836;
int&nbsp;x&nbsp;=&nbsp;1;
int&nbsp;y&nbsp;=&nbsp;4;

if(((u&(1<<x))!=0)&nbsp;!=&nbsp;((u&(1<<y))!=0))
{
&nbsp;&nbsp;&nbsp;&nbsp;u&nbsp;^=&nbsp;((1<<x)&nbsp;|&nbsp;(1<<y));
}



-Menne