Negative numbers to binary system with Python

Question

I'm solving a task using Python. I need to count the number of non-zero bits in a given value. I don't have any problems with positive numbers as I use the function bin() and count the number of '1', but can't understand what's wrong with negative. In example, -1 is equal to 11111111111111111111111111111111, but when I use the function bin() it gives '-0b1'. Please, tell me how can I convert it right.

Python has arbitrary precision integers, so your expectation of twos-complement arithmetic is incorrect. Perhaps pack or struct can help you out. — President James K. Polk, Dec 15 '15 at 22:11
"In example, -1 is equal to ..." Incorrect. It is equal to `0b11...11`. But since there's no way to represent an infinite number of bits, we just use `-0b1` instead. — Ignacio Vazquez-Abrams, Dec 15 '15 at 22:11

Ali Nikneshan · Accepted Answer · 2020-03-01T09:06:56.020

11

Just need to add 2**32 (or 1 << 32) to the negative value.

bin(-1+(1<<32))
'0b11111111111111111111111111111111'

or use following command for all the values:

a = bin(val if val>0 else val+(1<<32))

edited Mar 01 '20 at 09:06

answered Dec 15 '15 at 22:13

Ali Nikneshan

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Or, in general, 2**k to simulate a k-bit "word" size. – President James K. Polk Dec 15 '15 at 22:14
remember to mark answer if answer is helpful and satisfying – Ali Nikneshan Dec 15 '15 at 22:22
'<' is missing in the 2nd code! be careful while posting answers – Nikhila Munipalli Feb 29 '20 at 08:11

score 2 · Answer 2 · edited Dec 12 '22 at 15:25

2

Use num % maxnum:

bin(-1 % 20) == bin(19)

The reason you are getting what you have, as explained in the comments, is because of how ints are represented in binary. I would not be able to tell you much, but it has to do with a variety of things including arbitrary byte size, unsigned/signed ints, etc etc.

So to achieve what you want (the int to wrap around some arbitrary max value, in this case 32 bit), just use modulo:

bin(num % 2*32)

Just a note that the difference between using % versus simply + the max num is that this will work for both positive and negative numbers.

edited Dec 12 '22 at 15:25

Błażej Michalik

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answered Dec 15 '15 at 22:17

R Nar

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Should it be 2 ** 32 instead of 2 * 32? – Myroslav Hryshyn Dec 29 '22 at 11:25

Negative numbers to binary system with Python

2 Answers2