Converting hexstr to signed integer - Python

Question

I have this hexstr 0xfffffffffffffffffffffffffffffffffffffffffffffffffffdf05d84162877, in decimal terms it should give -580140491462537. However doing the below leads to bad answers. Can someone help?

In: test = '0xfffffffffffffffffffffffffffffffffffffffffffffffffffdf05d84162877'
In: int(test,16)
Out: 11579208923731619542357098500868790785326998466564056403945758342777263817739

Yeah i tried twos_complement and the other suggestions, they all led to that long numbered answer... I think it's related to how negative integers are in hexstr form — Kaleb
– Kaleb, Commented Mar 24, 2022 at 14:40
It's important to note that the answer you're getting isn't wrong, it's just that the default representation that Python is using isn't the same as the one you're trying to use. If you're doing this for educational purposes, one of the things that this lesson is trying to teach is the difference between the representation of a value and the value itself. — Patrick Haugh
– Patrick Haugh, Commented Mar 24, 2022 at 14:58

UnoriginalNick · Accepted Answer · 2022-03-24 14:55:33Z

3

First convert the string to bytes. You'll have to remove the leading "0x". Then use int.from_bytes and specify that it's signed and big-endian.

In: int.from_bytes(bytes.fromhex(test[2:]), byteorder="big", signed=True)
Out: -580140491462537

answered Mar 24, 2022 at 14:55

UnoriginalNick

3271 silver badge3 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

quamrana · Accepted Answer · 2022-03-24 14:52:13Z

1

I've adapted this answer

# hex string to signed integer
def htosi(val):
    uintval = int(val, 16)
    bits = 4 * len(val)
    indicative_binary = 2 ** (bits)
    indicative_binary_1 = 2 ** (bits-1)
    if uintval >= indicative_binary_1:
        uintval = int(0 - (indicative_binary - uintval))
    return uintval

This does require a pure hex string:

test1 = 'fffffffffffffffffffffffffffffffffffffffffffffffffffdf05d84162877'
print(htosi(test1))

-580140491462537

answered Mar 24, 2022 at 14:52

quamrana

39.5k13 gold badges57 silver badges77 bronze badges

1 Comment

Kaleb Over a year ago

thank you for your help, really appreciate it

Collectives™ on Stack Overflow

Converting hexstr to signed integer - Python

2 Answers 2

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related