bit operations

[agarwaga]

I want to rotate right a number by n bits. I don't have the width of the word of the machine and i wan't to solve my problem without finding out the width.(one can always find out the width of a word by counting the number of 1 bits in ~0). Ne ideas ?

[itsme86]

You can't rotate a number in C without knowing the width. You can shift, but not rotate.

[Dave_Sinkula]

Since ~0 is all bits set, shifting it right by one and adding one will be the high bit. So you could start with something like this.

#include <stdio.h>

unsigned int rotate(unsigned int value)
{
   unsigned int lsb = value & 1;
   value >>= 1;
   if ( lsb )
   {
      value |= (~0U >> 1) + 1;
   }
   return value;
}

int main()
{
   unsigned int num = 0xFFFF;
   printf("rotate(0x%X) = 0x%X\n", num, rotate(num));
   return 0;
}

/* my output
rotate(0xFFFF) = 0x80007FFF
*/

If you can get the high bit, you could get the n highest bits with a little more work.

[agarwaga]

thanks a lot dave

-gaurav