@Rajeev and Piyush: Numbering the bits from the right starting with 0 as usual, you see that you need to move the even-numbered bits one bit to the left and the odd-numbered bits one bit to the right. You could do this one bit-pair at a time, but it would be more efficient if you could do all pairs simultaneously. So how can you pick out all of the even-numbered bits at the same time? By using the logical and of the data with a mask that has 0s in the odd bit positions and 1s in the even bit positions. I.e., anding x with 01010101 in binary, which is 0x55 in hexadecimal. Once you have isolated the even numbered bits, you shift them left one place. The result is (x & 0x55) << 1. Similarly, you want to isolate the odd-numbered bits and shift them right one place. You could do that by anding with the mask 10101010 (binary) = 0xAA, and then shifting right: (x & 0xAA) >> 1. This introduces another constant, which might not be as efficient as reusing the previous constant 0x55, which can be done by shifting right first and then anding: (x >> 1) & 0x55. Now that we have the two sets of bits in the correct positions, we simply need to combine them, and the logical or operator does that just fine. The overall result is (x & 0x55) << 1) | ((x >> 1) & 0x55).
Dave On Jun 27, 7:53 am, piyush kapoor <[email protected]> wrote: > Yep,I also want to know the same.. > > On Mon, Jun 27, 2011 at 6:23 PM, rajeev bharshetty > <[email protected]>wrote: > > > > > > > @ Dave How to think about the answer to the above question . I mean How do > > I tackle such problems ? > > > On Mon, Jun 27, 2011 at 6:17 PM, Dave <[email protected]> wrote: > > >> y = ((x & 0x55) << 1) | ((x >> 1) & 0x55). > > >> Note, 0x55 = 01010101 in binary. > > >> Dave > > >> On Jun 27, 7:18 am, rShetty <[email protected]> wrote: > >> > Given a byte, write a code to swap every two bits. [Using bit > >> > operators] > > >> > Eg: Input: 10 01 11 01 Output: 01 10 11 10 > > >> -- > >> You received this message because you are subscribed to the Google Groups > >> "Algorithm Geeks" group. > >> To post to this group, send email to [email protected]. > >> To unsubscribe from this group, send email to > >> [email protected]. > >> For more options, visit this group at > >>http://groups.google.com/group/algogeeks?hl=en. > > > -- > > You received this message because you are subscribed to the Google Groups > > "Algorithm Geeks" group. > > To post to this group, send email to [email protected]. > > To unsubscribe from this group, send email to > > [email protected]. > > For more options, visit this group at > >http://groups.google.com/group/algogeeks?hl=en. > > -- > *Regards,* > *Piyush Kapoor,* > *CSE-IT-BHU*- Hide quoted text - > > - Show quoted text - -- You received this message because you are subscribed to the Google Groups "Algorithm Geeks" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/algogeeks?hl=en.
