# Bitwise And of Range

2017-05-13

Recently I had to solve a problem which asked you to determine the bitwise and of a range of nonnegative numbers. There is an obvious linear solution to this problem which simply computes the bitwise and of the range.

Bitwise and of [4, 10] = 4 & 5 & 6 & 7 & 8 & 9 & 10


However, after thinking about how the anding ends up “erasing” bits permanently I figured out the following logarithmic solution:

def bitwise_and_of_range(begin, end):
if begin == end:
return begin
else:
return bitwise_and_of_range(begin >> 1, end >> 1) << 1



Essentially, if you have at least two numbers in the range, the last bit will be zero, so you can compute the bitwise and of the prefixes and append a zero to the result (by shifting it to the left).