Question

I have an arbitrary 8-bit binary number e.g., 11101101

I have to swap all the pair of bits like:

Before swapping: 11-10-11-01
After swapping: 11-01-11-10

I was asked this in an interview !

Answer 1

In pseudo-code:

x = ((x & 0b10101010) >> 1) | ((x & 0b01010101) << 1)

It works by handling the low bits and high bits of each bit-pair separately and then combining the result:

• The expression x & 0b10101010 extracts the high bit from each pair, and then >> 1 shifts it to the low bit position.
• Similarly the expression (x & 0b01010101) << 1 extracts the low bit from each pair and shifts it to the high bit position.
• The two parts are then combined using bitwise-OR.

Since not all languages allow you to write binary literals directly, you could write them in for example hexadecimal:

Binary        Hexadecimal  Decimal 
0b10101010    0xaa         170
0b01010101    0x55         85