Question

I am stymied at a part where I require to do the following:
Given a number X (Say 10101010) and a number Y (Say 1110) and two position variables i,j (Say i = 1, j = 4), what I need to do is to set all the bits in X from i to j to match the bits in Y.
For the example above, the answer should be 101|1110|0.

The solution what I had in mind was:

1. Right shift X >> i
3. Run loop from 0 to j-1
2. if(!(X (lsb) ^ Y(lsb)), then continue, else X(lsb) = ~X(lsb)

The thing here is I am not sure how to play with individual bits.

Answer 1

1. Create a mask where the only zeroes are in bit positions i to j, which is ~(2^j+1 – 2ⁱ)

2. Result = (mask & X) | (Y << i)

Example in Python:

def replace_bits(X, Y, i, j):
    mask = ~(2**(j+1) - 2**i)
    return (mask & X) | (Y << i)

>>> replace_bits(int('10101010', 2), int('1110', 2), 1, 4)
188
>>> bin(replace_bits(int('10101010', 2), int('1110', 2), 1, 4))
'0b10111100'