How to modify bits in an integer?

Question

I have an integer with a value 7 (0b00000111) And I would like to replace it with a function to 13 (0b00001101). What is

Answer 1

Going by the examples provided, it sounds like you are looking to swap bits in an integer. For example in 7 (0b00000111), if you swap the bits in the 3rd and 1st positions you obtain 13 (0b00001101).

I would have the following as a function signature swap_bits(val, i, j)

What is the best algorithm? Well, the following algorithm takes constant time, O(1).

def swap_bits(val, i, j):
    """
    Given an integer val, swap bits in positions i and j if they differ
    by flipping their values, i.e, select the bits to flip with a mask.
    Since v ^ 1 = 0 when v = 1 and 1 when v = 0, perform the flip using an XOR.
    """
    if not (val >> i) & 1 == (val >> j) & 1:
        mask = (1 << i) | (1 << j)
        val ^= mask
    return val

Example:

 >>> swap_bits(7, 3, 1)
 13

The code leverage bit twiddling tricks and here is a good resource by Sean Anderson. I am working on providing the code snippets in Python here.