What is the best way to check if all bits are 1?

Question

I thought this would be easy due to ~, but ~ just returns a negative value of 2^x rather than 0.

Answer 1

You can modify a standard bithack for checking if a number is a power of 2 (numbers one less than a power of 2 are all 1s), with a special case check for zero:

def allOnes(n):
    return ((n+1) & n == 0) and (n!=0)

print allOnes(255)
print allOnes(158)
print allOnes(0)
print allOnes(-1)
print allOnes(-2)

Output:

True
False
False
True
False