Question

Can someone please explain the purpose of the bitwise, Binary AND Operator( & ) and how to use it? I was looking at different ways of making a isprime function and came across this.

def isprime(n):
    # make sure n is a positive integer
    n = abs(int(n))
    # 0 and 1 are not primes
    if n < 2:
        return False
    # 2 is the only even prime number
    if n == 2: 
        return True    
    # all other even numbers are not primes
    if not n & 1: 
        return False
    # range starts with 3 and only needs to go up the squareroot of n
    # for all odd numbers (counts by 2's)
    for x in range(3, int(n**0.5)+1, 2):
        if n % x == 0:
            return False
    return True

I also looked at Python Bitwise Operators Example but couldn’t grasp it.

Answer 1

One number AND another is the bits of one number masked by the bits of another number. If a number AND 1 is 0 (not n & 1 would be True), that means it’s divisible by two, since all multiples of 2 have a 0 as the rightmost binary digit.

  11 = 00001011 (Not divisible by 2)      28 = 00011100 (Divisible by 2)
&  1 = 00000001                         &  1 = 00000001
---------------                         ---------------
       00000001                                00000000