Clearing python bits index through 0

Question

I want to know how to implement the following Java code into its Python equivalent.

In Java, I am taking a number and using bitwise operators clearing the bits from some index through to the 0th index(inclusive)

int clearBitsIthrough0(int num, int i) {
    int mask = (-1 << (i + 1));
    return num & mask;
}

My attempt at the equivalent python code is below

def clearBitsIThrough0(num, i):
    mask = format(0xf << 4, 'b')
    return num & mask

However, I get a

TypeError: unsupported operand type(s) for &: 'int' and 'str'

I know this happens because mask is a string here, but if I convert it to an int, I lose the underlying integer value and I still get the wrong result.

I tried it a different way doing the following

def clearBitsIThrough0(num, i):
    mask = int(format(0xf << 4, 'b'), 2)
    num = int(format(num, 'b'), 2)
    print(num, mask)
    return num & mask

But this returns 0, which is the wrong answer.

How do I do this?

Ned Batchelder · Accepted Answer · 2017-08-09 10:28:31Z

2

The same code works in Python:

def clearBitsIThrough0(num, i):
    mask = (-1 << (i + 1))
    return num & mask

edited Aug 9, 2017 at 10:28

answered Aug 9, 2017 at 10:18

Ned Batchelder

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2 Comments

redixhumayun Over a year ago

Just tried it myself. I was just stupid not to try that. I didn't try it because I figured since -1 in Python is not represented as 1111 (which it is in Java), this wouldn't work. Should I just delete the question, since it doesn't seem to serve any purpose?

merwok Over a year ago

There is value in showing that high-level Python also lets you do low-level bitstuff!

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